Page 1
®
ISL6568
Data Sheet
Two-Phase Buck PWM Controller with
Integrated MOSFET Drivers for VRM9,
VRM10, and AMD Hammer Applications
The ISL6568 two-phase PWM control IC provides a
precision voltage regulation system for advanced
microprocessors. The integration of power MOSFET drivers
into the controller IC marks a departure from the separate
PWM controller and driver configuration of previous multiphase product families. By reducing the number of external
parts, this integration is optimized for a cost and space
saving power management solution.
Outstanding features of this controller IC include
programmable VID codes compatible with Intel
VRM9,VRM10, as well as AMD Hammer microprocessors. A
unity gain, differential amplifier is provided for remote voltage
sensing, compensating for any potential difference between
remote and local grounds. The output voltage can also be
positively or negatively offset through the use of a single
external resistor.
A unique feature of the ISL6568 is the combined use of both
DCR and r
positioning (droop) and overcurrent protection are
accomplished through continuous inductor DCR current
sensing, while r
channel-current balance. Using both methods of current
sampling utilizes the best advantages of each technique.
current sensing. Load line voltage
DS(ON)
current sensing is used for accurate
DS(ON)
March 9, 2006
FN9187.4
Features
• Integrated Multi-Phase Power Conversion
- 1 or 2-Phase Operation
• Precision Core Voltage Regulation
- Differential Remote Voltage Sensing
- ±0.5% System Accuracy Over Temperature
- Adjustable Reference-Voltage Offset
• Precision Channel Current Sharing
- Uses Loss-Less r
DS(ON)
• Accurate Load Line Programming
- Uses Loss-Less Inductor DCR Current Sampling
• Variable Gate Drive Bias: 5V to 12V
• Microprocessor Voltage Identification Inputs
- Up to a 6-Bit DAC
- Selectable between Intel’s VRM9, VRM10, or AMD
Hammer DAC codes
- Dynamic VID-on-the-fly Technology
• Overcurrent Protection
• Multi-tiered Overvoltage Protection
• Digital Soft-Start
• Selectable Operation Frequency up to 1.5MHz Per Phase
• Pb-Free Plus Anneal Available (RoHS Compliant)
Current Sampling
Protection features of this controller IC include a set of
sophisticated overvoltage, undervoltage, and overcurrent
protection. Overvoltage results in the converter turning the
lower MOSFETs ON to clamp the rising output voltage and
protect the microprocessor. The overcurrent protection level
is set through a single external resistor. Furthermore, the
ISL6568 includes protection against an open circuit on the
remote sensing inputs. Combined, these features provide
advanced protection for the microprocessor and power
system.
Pinout
VID12.5
REF
OFS
VCC
COMP
FB
VDIFF
RGND
ISL6568 (QFN)
TOP VIEW
VID1
VID2
VID0
32 31 30 29 28 27 26 25
1
2
3
4
5
6
7
8
9 10111213141516
VSEN
OCSET
FS
GND
ISUM
ICOMP
LGATE1
PGOOD
33
IREF
LGATE2
ISEN1
UGATE1
24
BOOT1
23
PHASE1
22
VID4
21
VID3
20
ENLL
19
PHASE2
18
BOOT2
17
UGATE2
PVCC
ISEN2
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774
| Intersil (and design) is a registered trademark of Intersil Americas Inc.
All other trademarks mentioned are the property of their respective owners.
Copyright Intersil Americas Inc. 2004-2006. All Rights Reserved
Page 2
Ordering Information
ISL6568
PART
PAR T NUM B ER*
ISL6568CRR5184 ISL6568CR 0 to 70 32 Ld 5x5 QFN L32.5x5
ISL6568CR-TR5184 ISL6568CR 0 to 70 32 Ld 5x5 QFN (Pb-free) Tape and Reel L32.5x5
ISL6568CRZR5184 ISL6568CRZ 0 to 70 32 Ld 5x5 QFN (Pb-free) L32.5x5
ISL6568CRZ-TR5184 ISL6568CRZ 0 to 70 32 Ld 5x5 QFN (Pb-free) Tape and Reel L32.5x5
ISL6568CRZAR5184 ISL6568CRZ 0 to 70 32 Ld 5x5 QFN (Pb-free) L32.5x5
ISL6568CRZA-TR5184 ISL6568CRZ 0 to 70 32 Ld 5x5 QFN (Pb-free) Tape and Reel L32.5x5
ISL6568IR ISL6568IR -40 to 85 32 Ld 5x5 QFN L32.5x5
ISL6568IR-T ISL6568IR -40 to 85 32 Ld 5x5 QFN Tape and Reel L32.5x5
ISL6568IRZ ISL6568IRZ -40 to 85 32 Ld 5x5 QFN (Pb-free) L32.5x5
ISL6568IRZ-T ISL6568IRZ -40 to 85 32 Ld 5x5 QFN (Pb-free) Tape and Reel L32.5x5
ISL6568IRZA ISL6568IRZ -40 to 85 32 Ld 5x5 QFN (Pb-free) L32.5x5
ISL6568IRZA-T ISL6568IRZ -40 to 85 32 Ld 5x5 QFN (Pb-free) Tape and Reel L32.5x5
NOTE: Intersil Pb-free plus anneal products employ special Pb-free material sets; molding compounds/die attach materials and 100% matte tin plate
termination finish, which are RoHS compliant and compatible with both SnPb and Pb-free soldering operations. Intersil Pb-free products are MSL classified
at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020.
MARKING
TEMP.
(°C) PACKAGE
PKG.
DWG. #
2
FN9187.4
March 9, 2006
Page 3
Block Diagram
ISL6568ISL6568
ISUM
IREF
RGND
VSEN
VDIFF
ISEN AMP
x1
UVP
OVP
OVP
ICOMP
x1
OCSET
100µA
+1V
OC
PGOOD
SOFT START
AND
FAULT LOGIC
0.2V
CLOCK AND
SAWTOOTH
GENERATOR
0.66V
GATE
CONTROL
LOGIC
ENLL
POWER-ON
THROUGH
PROTECTION
RESET
SHOOT-
VCC
PVCC
BOOT1
UGATE1
PHASE1
LGATE1
FS
VID4
VID3
VID2
VID1
VID0
VID12.5
REF
FB
COMP
OFS
+150mV
x 0.82
DYNAMIC
VID
D/A
OFFSET
V
OVP
E/A
∑
∑
CHANNEL
CURRENT
BALANCE
PWM1
PWM2
BOOT2
UGATE2
GATE
CONTROL
LOGIC
PHASE 2
DETECT
1
N
SHOOT-
THROUGH
PROTECTION
PHASE2
LGATE2
∑
CHANNEL
CURRENT
SENSE
ISEN1
ISEN2
3
GND
FN9187.4
March 9, 2006
Page 4
Typical Application - ISL6568
ISL6568ISL6568
+5V
+12V
VDIFF
VSEN
RGND
VCC
OFS
FS
REF
VID4
VID3
VID2
VID1
VID0
VID12.5
PGOOD
GND
FB
ISL6568
COMP
PVCC1
BOOT1
UGATE1
PHASE1
ISEN1
LGATE1
PVCC2
BOOT2
UGATE2
PHASE2
ISEN2
LGATE2
+12V
+12V
LOAD
ENLL
IREF
OCSET
ICOMP
ISUM
4
FN9187.4
March 9, 2006
Page 5
ISL6568ISL6568
Typical Application - ISL6568 with NTC Thermal Compensation
+5V
+12V
VDIFF
VSEN
RGND
VCC
OFS
FS
REF
VID4
VID3
VID2
VID1
VID0
VID12.5
PGOOD
GND
FB
ISL6568
COMP
PVCC1
BOOT1
UGATE1
PHASE1
ISEN1
LGATE1
PVCC2
BOOT2
UGATE2
PHASE2
ISEN2
LGATE2
+12V
+12V
PLACE IN CLOSE
PROXIMITY
NTC
LOAD
ENLL
IREF
OCSET
ICOMP
ISUM
5
FN9187.4
March 9, 2006
Page 6
ISL6568ISL6568
Absolute Maximum Ratings
Supply Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6V
Supply Voltage, PVCC. . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +15V
Absolute Boot Voltage, V
Phase Voltage, V
GND - 8V (<400ns, 20µJ) to 24V (<200ns, V
PHASE
Upper Gate Voltage, V
V
- 3.5V (<100ns Pulse Width, 2µJ) to V
Lower Gate Voltage, V
PHASE
GND - 5V (<100ns Pulse Width, 2µJ) to PVCC+ 0.3V
. . . . . . . . GND - 0.3V to GND + 36V
BOOT
. . . . . . . . GND - 0.3V to 15V (PVCC = 12)
. . . . V
UGATE
LGATE
PHASE
. . . . . . . . GND - 0.3V to PVCC + 0.3V
BOOT-PHASE
- 0.3V to V
BOOT
BOOT
+ 0.3V
+ 0.3V
= 12V)
Thermal Information
Thermal Resistance θJA (°C/W) θJC (°C/W)
QFN Package (Notes 1, 2). . . . . . . . . . 35 5
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . 150°C
Maximum Storage Temperature Range. . . . . . . . . . . -65°C to 150°C
Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . . 300°C
Input, Output, or I/O Voltage . . . . . . . . . GND - 0.3V to VCC + 0.3V
ESD Classification . . . . . . . . . . . . . . . . . . . . . . . Class I JEDEC STD
Recommended Operating Conditions
VCC Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +5V ±5%
PVCC Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . +5V to 12V ±5%
Ambient Temperature
(ISL6568CRR5184, ISL6568CRZR5184) . . . . . . . . . 0°C to 70°C
Ambient Temperature (ISL6568IR, ISL6568IRZ) . . . . . -40°C to 85°C
CAUTION: Stress above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational section of this specification is not implied.
NOTES:
is measured in free air with the component mounted on a high effective thermal conductivity test board with “direct attach” features. See
1. θ
JA
Tech Brief TB379.
2. For θ
, the “case temp” location is the center of the exposed metal pad on the package underside.
JC
Electrical Specifications Recommended Operating Conditions, Unless Otherwise Specified.
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
BIAS SUPPLY AND INTERNAL OSCILLATOR
Input Bias Supply Current I
Gate Drive Bias Current I
VCC POR (Power-On Reset) Threshold VCC Rising 4.25 4.38 4.50 V
PVCC POR (Power-On Reset) Threshold PVCC Rising 4.25 4.38 4.50 V
Oscillator Ramp Amplitude (Note 3) V
Maximum Duty Cycle (Note 3) - 66.6 - %
Oscillator Frequency, F
SW
CONTROL THRESHOLDS
ENLL Rising Threshold -0.66-V
ENLL Hysteresis - 100 - mV
COMP Shutdown Threshold COMP Falling 0.2 0.3 0.4 V
REFERENCE AND DAC
System Accuracy (VID = 1.0V - 1.850V) -0.5 - 0.5 %
System Accuracy (VID = 0.8V - 1.0V) -0.8 - 0.8 %
DAC Input Low Voltage (VR9, VR10) --0.4V
DAC Input High Voltage (VR9, VR10) 0.8 - - V
DAC Input Low Voltage (AMD) --0.6V
DAC Input High Voltage (AMD) 1.0 - - V
OFS Sink Current Accuracy (Negative Offset) R
OFS Source Current Accuracy (Positive Offset) R
; ENLL = high - 15 20 mA
VCC
; ENLL = high - 1.5 - mA
PVCC
VCC Falling 3.75 3.88 4.00 V
PVCC Falling 3.60 3.88 4.00 V
PP
-1.50-V
RT = 100kΩ (± 0.1%) 225 250 275 kHz
= 30kΩ from OFS to VCC 47.5 50.0 52.5 µA
OFS
= 10kΩ from OFS to GND 47.5 50.0 52.5 µA
OFS
6
FN9187.4
March 9, 2006
Page 7
ISL6568ISL6568
Electrical Specifications Recommended Operating Conditions, Unless Otherwise Specified. (Continued)
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
ERROR AMPLIFIER
DC Gain (Note 3) R
Gain-Bandwidth Product (Note 3) C
Slew Rate (Note 3) C
Maximum Output Voltage Load = 1mA 3.90 4.20 - V
Minimum Output Voltage Load = -1mA - 0.85 1.0 V
OVERCURRENT PROTECTION
OCSET trip current 93 100 107 µA
OCSET Accuracy OCSET and ISUM Difference -5 0 5 mV
ICOMP Offset -5 0 5 mV
PROTECTION
Undervoltage Threshold VSEN falling 80 82 84 %VID
Undervoltage Hysteresis VSEN Rising - 3 - %VID
Overvoltage Threshold while IC Disabled V
Overvoltage Threshold VSEN Rising VID +
Overvoltage Hysteresis VSEN Falling - 50 - mV
Open Sense-Line Protection Threshold IREF Rising and Falling VDIFF
SWITCHING TIME (Note 3)
UGATE Rise Time t
LGATE Rise Time t
UGATE Fall Time t
LGATE Fall Time t
UGATE Turn-On Non-overlap t
LGATE Turn-On Non-overlap t
GATE DRIVE RESISTANCE (Note 3)
Upper Drive Source Resistance V
Upper Drive Sink Resistance V
Lower Drive Source Resistance V
Lower Drive Sink Resistance V
OVER TEMPERATURE SHUTDOWN
Thermal Shutdown Setpoint (Note 3) - 160 - °C
Thermal Recovery Setpoint (Note 3) - 100 - °C
NOTE:
3. Parameter magnitude guaranteed by design. Not 100% tested.
= 10K to ground - 96 - dB
L
= 100pF, RL = 10K to ground - 20 - MHz
L
= 100pF, Load = ±400µA-8-V/µs
L
, VRM9.0 Configuration 1.92 1.97 2.02 V
OVP
, Hammer and VRM10.0 Configurations 1.62 1.67 1.72 V
V
OVP
125mV
VID +
150mV
VID +
175mV
VDIFF + 1VVDIFF
+ 0.9V
RUGATE; VPVCC
RLGATE; VPVCC
FUGATE; VPVCC
FLGATE; VPVCC
PDHUGATE
PDHLGATE
= 12V, 15mA Source Current 1.25 2.0 3.0 Ω
PVCC
= 12V, 15mA Sink Current 0.9 1.65 3.0 Ω
PVCC
= 12V, 15mA Source Current 0.85 1.25 2.2 Ω
PVCC
= 12V, 15mA Sink Current 0.60 0.80 1.35 Ω
PVCC
= 12V, 3nF Load, 10% to 90% - 26 - ns
= 12V, 3nF Load, 10% to 90% - 18 - ns
= 12V, 3nF Load, 90% to 10% - 18 - ns
= 12V, 3nF Load, 90% to 10% - 12 - ns
; V
; V
= 12V, 3nF Load, Adaptive - 10 - ns
PVCC
= 12V, 3nF Load, Adaptive - 10 - ns
PVCC
+ 1.1V
V
V
7
FN9187.4
March 9, 2006
Page 8
Timing Diagram
UGATE
LGATE
t
PDHUGATE
t
RUGATE
ISL6568ISL6568
t
FUGATE
t
FLGATE
Simplified Power System Diagram
+12V
IN
+5V
IN
VID
5-6
ISL6568
DAC
CHANNEL1
CHANNEL2
t
PDHLGATE
Q1
Q2
Q3
Q4
t
RLGATE
V
OUT
Functional Pin Description
VCC
VCC is the bias supply for the ICs small-signal circuitry.
Connect this pin to a +5V supply and locally decouple using
a quality 1.0µF ceramic capacitor.
PVCC
This pin is the power supply pin for the MOSFET drivers.
This pin can be connected to any voltage from +5V to +12V,
depending on the desired MOSFET gate drive level.
GND
GND is the bias and reference ground for the IC.
ENLL
This pin is a threshold-sensitive (approximately 0.66V) enable
input for the controller. Held low, this pin disables controller
operation. Pulled high, the pin enables the controller for
operation. ENLL has a internal 1.0µA pull-up to 5V.
8
FS
A resistor, placed from FS to ground, will set the switching
frequency. Refer to Equation 34 for proper resistor
calculation.
VID4, VID3, VID2, VID1, VID0, and VID12.5
These are the inputs for the internal DAC that provides the
reference voltage for output regulation. These pins respond
to TTL logic thresholds. The ISL6568 decodes the VID
inputs to establish the output voltage; see VID Tables for
correspondence between DAC codes and output voltage
settings. These pins are internally pulled high, to
approximately 1.2V, by 40µA (typically) internal current
sources; the internal pull-up current decreases to 0 as the
VID voltage approaches the internal pull-up voltage. All VID
pins are compatible with external pull-up voltages not
exceeding the IC’s bias voltage (VCC).
The VID12.5 pin also serves as the internal DAC compliance
selector. The way this pin is connected selects which of the
three internal DAC codes will be used. For VRM10 codes
this pin must be less that 3V. To encode the DAC with Intel
FN9187.4
March 9, 2006
Page 9
ISL6568ISL6568
VRM9.0 codes, connect the VID12.5 pin to a +5V source
through a 50kΩ resistor. To encode the DAC with AMD
Hammer VID codes, connect this pin to a +5V source
through a 5kΩ resistor.
VSEN and RGND
VSEN and RGND are inputs to the precision differential
remote-sense amplifier and should be connected to the sense
pins of the remote load.
ICOMP, ISUM, and IREF
ISUM, IREF, and ICOMP are the DCR current sense
amplifier’s negative input, positive input, and output
respectively. For accurate DCR current sensing, connect a
resistor from each channel’s phase node to ISUM and
connect IREF to the summing point of the output inductors,
roughly Vout. A parallel R-C feedback circuit connected
between ISUM and ICOMP will then create a voltage from
IREF to ICOMP proportional to the voltage drop across the
inductor DCR. This voltage is referred to as the droop voltage
and is added to the differential remote-sense amplifier output.
NOTE: An optional 0.01µF ceramic capacitor can be placed from the
IREF pin to the ISUM pin to help reduce any noise affects that may
occur due to layout.
VDIFF
VDIFF is the output of the differential remote-sense amplifier.
The voltage on this pin is equal to the difference between
VSEN and RGND added to the difference between IREF and
ICOMP. VDIFF therefore represents the output voltage plus
the droop voltage.
FB and COMP
These pins are the internal error amplifier inverting input and
output respectively. FB, VDIFF, and COMP are tied together
through external R-C networks to compensate the regulator.
REF
The REF input pin is the positive input of the error amplifier. It
is internally connected to the DAC output through a 1kΩ
resistor. A capacitor is used between the REF pin and ground
to smooth the voltage transition during Dynamic VID
operations.
OFS
The OFS pin provides a means to program a dc current for
generating an offset voltage across the resistor between FB
and VDIFF. The offset current is generated via an external
resistor and precision internal voltage references. The polarity
of the offset is selected by connecting the resistor to GND or
VCC. For no offset, the OFS pin should be left unconnected.
OCSET
This is the overcurrent set pin. Placing a resistor from OCSET
to ICOMP allows a 100µA current to flow out this pin,
producing a voltage reference. Internal circuitry compares the
voltage at OCSET to the voltage at ISUM, and if ISUM ever
exceeds OCSET, the overcurrent protection activates.
ISEN1 and ISEN2
These pins are used for balancing the channel currents by
sensing the current through each channel’s lower MOSFET
when it is conducting. Connect a resistor between the ISEN1
and ISEN2 pins and their respective phase node. This
resistor sets a current proportional to the current in the lower
MOSFET during its conduction interval.
UGATE1 and UGATE2
Connect these pins to the corresponding upper MOSFET
gates. These pins are used to control the upper MOSFETs
and are monitored for shoot-through prevention purposes.
Maximum individual channel duty cycle is limited to 66%.
BOOT1 and BOOT2
These pins provide the bias voltage for the corresponding
upper MOSFET drives. Connect these pins to appropriatelychosen external bootstrap capacitors. Internal bootstrap
diodes connected to the PVCC pins provide the necessary
bootstrap charge.
PHASE1 and PHASE2
Connect these pins to the sources of the upper MOSFETs.
These pins are the return path for the upper MOSFET
drives.
LGATE1 and LGATE2
These pins are used to control the lower MOSFETs. Connect
these pins to the corresponding lower MOSFETs’ gates.
PGOOD
During normal operation PGOOD indicates whether the
output voltage is within specified overvoltage and
undervoltage limits. If the output voltage exceeds these limits
or a reset event occurs (such as an overcurrent event),
PGOOD is pulled low. PGOOD is always low prior to the end
of soft-start.
Operation
Multi-Phase Power Conversion
Microprocessor load current profiles have changed to the
point that the advantages of multi-phase power conversion
are impossible to ignore. The technical challenges
associated with producing a single-phase converter that is
both cost-effective and thermally viable have forced a
change to the cost-saving approach of multi-phase. The
ISL6568 controller helps simplify implementation by
integrating vital functions and requiring minimal external
components. The block diagram on page 2 provides a top
level view of multi-phase power conversion using the
ISL6568 controller.
9
FN9187.4
March 9, 2006
Page 10
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
IL2, 7A/DIV
PWM2, 5V/DIV
IL1, 7A/DIV
PWM1, 5V/DIV
1µs/DIV
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
Interleaving
The switching of each channel in a multi-phase converter is
timed to be symmetrically out of phase with each of the other
channels. In a 3-phase converter, each channel switches 1/3
cycle after the previous channel and 1/3 cycle before the
following channel. As a result, the three-phase converter has
a combined ripple frequency three times greater than the
ripple frequency of any one phase. In addition, the peak-topeak amplitude of the combined inductor currents is reduced
in proportion to the number of phases (Equations 1 and 2).
Increased ripple frequency and lower ripple amplitude mean
that the designer can use less per-channel inductance and
lower total output capacitance for any performance
specification.
Figure 1 illustrates the multiplicative effect on output ripple
frequency. The three channel currents (IL1, IL2, and IL3)
combine to form the AC ripple current and the DC load
current. The ripple component has three times the ripple
frequency of each individual channel current. Each PWM
pulse is terminated 1/3 of a cycle after the PWM pulse of the
previous phase. The peak-to-peak current for each phase is
about 7A, and the dc components of the inductor currents
combine to feed the load.
To understand the reduction of ripple current amplitude in the
multi-phase circuit, examine the equation representing an
individual channel peak-to-peak inductor current.
VINV
–()V
OUT
IN
and V
IN
OUT
are the input and output
OUT
I
------------------------------------------------------=
PP
LfSV
In Equation 1, V
voltages respectively, L is the single-channel inductor value,
and f
is the switching frequency.
S
The output capacitors conduct the ripple component of the
inductor current. In the case of multi-phase converters, the
capacitor current is the sum of the ripple currents from each
of the individual channels. Compare Equation 1 to the
expression for the peak-to-peak current after the summation
(EQ. 1)
ISL6568ISL6568
of N symmetrically phase-shifted inductor currents in
Equation 2. Peak-to-peak ripple current decreases by an
amount proportional to the number of channels. Outputvoltage ripple is a function of capacitance, capacitor
equivalent series resistance (ESR), and inductor ripple
current. Reducing the inductor ripple current allows the
designer to use fewer or less costly output capacitors.
VINNV
–()V
OUT
I
------------------------------------------------------------=
CPP,
LfSV
OUT
IN
(EQ. 2)
Another benefit of interleaving is to reduce input ripple
current. Input capacitance is determined in part by the
maximum input ripple current. Multi-phase topologies can
improve overall system cost and size by lowering input ripple
current and allowing the designer to reduce the cost of input
capacitance. The example in Figure 2 illustrates input
currents from a three-phase converter combining to reduce
the total input ripple current.
The converter depicted in Figure 2 delivers 1.5V to a 36A load
from a 12V input. The RMS input capacitor current is 5.9A.
Compare this to a single-phase converter also stepping down
12V to 1.5V at 36A. The single-phase converter has 11.9A
RMS input capacitor current. The single-phase converter
must use an input capacitor bank with twice the RMS current
capacity as the equivalent three-phase converter.
INPUT-CAPACITOR CURRENT, 10A/DIV
CHANNEL 3
INPUT CURRENT
10A/DIV
CHANNEL 2
INPUT CURRENT
10A/DIV
CHANNEL 1
INPUT CURRENT
10A/DIV
1µs/DIV
FIGURE 2. CHANNEL INPUT CURRENTS AND INPUT-
CAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
Figures 22 and 23 in the section entitled Input Capacitor
Selection can be used to determine the input-capacitor RMS
current based on load current, duty cycle, and the number of
channels. They are provided as aids in determining the
optimal input capacitor solution.
PWM Operation
The timing of each converter leg is set by the number of
active channels. The default channel setting for the ISL6568
is two. One switching cycle is defined as the time between
the internal PWM1 pulse termination signals. The pulse
termination signal is the internally generated clock signal
that triggers the falling edge of PWM1. The cycle time of the
10
FN9187.4
March 9, 2006
Page 11
ISL6568ISL6568
pulse termination signal is the inverse of the switching
frequency set by the resistor between the FS pin and
ground. Each cycle begins when the clock signal commands
PWM1 to go low. The PWM1 transition signals the internal
channel-1 MOSFET driver to turn off the channel-1 upper
MOSFET and turn on the channel-1 synchronous MOSFET.
In the default channel configuration, the PWM2 pulse
terminates 1/2 of a cycle after the PWM1 pulse.
If the BOOT2 and PHASE2 pins are both connected to +12V
single channel operation is selected.
Once a PWM pulse transitions low, it is held low for a
minimum of 1/3 cycle. This forced off time is required to
ensure an accurate current sample. Current sensing is
described in the next section. After the forced off time
expires, the PWM output is enabled. The PWM output state
is driven by the position of the error amplifier output signal,
V
, minus the current correction signal relative to the
COMP
sawtooth ramp as illustrated in Figure 3. When the modified
V
voltage crosses the sawtooth ramp, the PWM output
COMP
transitions high. The internal MOSFET driver detects the
change in state of the PWM signal and turns off the
synchronous MOSFET and turns on the upper MOSFET.
The PWM signal will remain high until the pulse termination
signal marks the beginning of the next cycle by triggering the
PWM signal low.
Channel-Current Balance
One important benefit of multi-phase operation is the thermal
advantage gained by distributing the dissipated heat over
multiple devices and greater area. By doing this the designer
avoids the complexity of driving parallel MOSFETs and the
expense of using expensive heat sinks and exotic magnetic
materials.
In order to realize the thermal advantage, it is important that
each channel in a multi-phase converter be controlled to
carry about the same amount of current at any load level. To
achieve this, the currents through each channel must be
sampled every switching cycle. The sampled currents, I
from each active channel are summed together and divided
by the number of active channels. The resulting cycle
average current, I
, provides a measure of the total load-
AVG
current demand on the converter during each switching
cycle. Channel-current balance is achieved by comparing
the sampled current of each channel to the cycle average
current, and making the proper adjustment to each channel
pulse width based on the error. Intersil’s patented currentbalance method is illustrated in Figure 3, with error
correction for channel 1 represented. In the figure, the cycle
average current, I
sample, I
, to create an error signal IER.
1
, is compared with the channel 1
AVG
The filtered error signal modifies the pulse width
commanded by V
I
toward zero. The same method for error signal
ER
to correct any unbalance and force
COMP
correction is applied to each active channel.
,
n
V
COMP
NOTE: Channel 2 is optional.
FIGURE 3. CHANNEL-1 PWM FUNCTION AND CURRENT-
+
-
SAWTOOTH SIGNAL
FILTER
f(s)
I
ER
I
AVG
-
+
I
1
BALANCE ADJUSTMENT
÷ N
PWM1
+
-
TO GATE
CONTROL
LOGIC
Σ
I
2
Current Sampling
In order to realize proper current-balance, the currents in
each channel must be sampled every switching cycle. This
sampling occurs during the forced off-time, following a PWM
transition low. During this time the current-sense amplifier
uses the ISEN inputs to reproduce a signal proportional to
the inductor current, I
a scaled version of the inductor current. The sample window
opens exactly 1/6 of the switching period, t
PWM transitions low. The sample window then stays open
the rest of the switching cycle until PWM transitions high
again, as illustrated in Figure 4.
The sampled current, at the end of the t
proportional to the inductor current and is held until the next
switching period sample. The sampled current is used only
for channel-current balance.
OLD SAMPLE
CURRENT
FIGURE 4. SAMPLE AND HOLD TIMING
The ISL6568 supports MOSFET r
sample each channel’s current for channel-current balance.
The internal circuitry, shown in Figure 5 represents channel
n of an N-channel converter. This circuitry is repeated for
each channel in the converter, but may not be active
. This sensed current, I
L
SAMPLE
I
L
PWM
SWITCHING PERIOD
I
SEN
SAMPLING PERIOD
TIME
DS(ON)
, is simply
SEN
, after the
SW
, is
NEW SAMPLE
CURRENT
current sensing to
11
FN9187.4
March 9, 2006
Page 12
ISL6568
depending on the status of the BOOT2 and PHASE2 pins,
as described in the PWM Operation section.
V
IN
r
I
SEN
In
SAMPLE
&
HOLD
ISL6565A INTERNAL CIRCUIT EXTERNAL CIRCUIT
DS ON()
--------------------------=
I
L
R
ISEN
+
ISEN(n)
R
ISEN
CHANNEL N
LOWER MOSFET
CHANNEL N
UPPER MOSFET
I
L
-
ILr
DS ON()
+
FIGURE 5. ISL6568 INTERNAL AND EXTERNAL CURRENT-
SENSING CIRCUITRY FOR CURRENT BALANCE
The ISL6568 senses the channel load current by sampling
the voltage across the lower MOSFET r
DS(ON)
, as shown in
Figure 5. A ground-referenced operational amplifier, internal
to the ISL6568, is connected to the PHASE node through a
resistor, R
the voltage drop across the r
. The voltage across R
ISEN
ISEN
of the lower MOSFET
DS(ON)
is equivalent to
while it is conducting. The resulting current into the ISEN pin
is proportional to the channel current, I
. The ISEN current is
L
TABLE 2. AMD HAMMER VOLTAGE IDENTIFICATION CODES
VID4 VID3 VID2 VID1 VID0 VDAC
11111Off
111100.800
111010.825
111000.850
110110.875
110100.900
110010.925
110000.950
101110.975
101101.000
101011.025
101001.050
100111.075
100101.100
100011.125
100001.150
011111.175
011101.200
sampled and held as described in the Current Sampling
section. From Figure 5, the following equation for I
derived where I
r
InI
DS ON()
----------------------=(EQ. 3)
L
R
is the channel current.
L
ISEN
is
n
Output Voltage Setting
The ISL6568 uses a digital to analog converter (DAC) to
generate a reference voltage based on the logic signals at the
VID pins. The DAC decodes the 5 or 6-bit logic signals into
one of the discrete voltages shown in Tables 2, 3, and 4.
Each VID pin is pulled up to an internal 1.2V voltage by a
weak current source (40µA current), which decreases to 0 as
the voltage at the VID pin varies from 0 to the internal 1.2V
pull-up voltage. External pull-up resistors or active-high
output stages can augment the pull-up current sources, up to
a voltage of 5V.
.
The ISL6568 accommodates three different DAC ranges:
Intel VRM9.0, AMD Hammer, or Intel VRM10.0. The state of
the VID12.5 pin decides which DAC version is active. Refer
011011.225
011001.250
010111.275
010101.300
010011.325
010001.350
001111.375
001101.400
001011.425
001001.450
000111.475
000101.500
000011.525
000001.550
to Table 1 for a description of how to select the desired DAC
version.
TABLE 1. ISL6568 DAC SELECT TABLE
DAC VERSION VID12.5 PIN CONDITION
VRM10.0 Less then 3V
VRM9.0 50kΩ resistor to +5V
AMD HAMMER 5kΩ resistor to +5V
12
FN9187.4
March 9, 2006
Page 13
ISL6568
TABLE 3. VRM9 VOLTAGE IDENTIFICATION CODES
VID4 VID3 VID2 VID1 VID0 VDAC
11111Off
111101.100
111011.125
111001.150
110111.175
110101.200
110011.225
110001.250
101111.275
101101.300
101011.325
101001.350
100111.375
100101.400
100011.425
100001.450
011111.475
011101.500
011011.525
011001.550
010111.575
010101.600
010011.625
010001.650
001111.675
001101.700
001011.725
001001.750
0 0 0 12.75 1 1.775
000101.800
000011.825
000001.850
TABLE 4. VRM10 VOLTAGE IDENTIFICATION CODES
VID4 VID3 VID2 VID1 VID0 VID12.5 VDAC
111111Off
111110Off
0101000.8375
0100110.8500
0100100.8625
0100010.8750
0100000.8875
0011110.9000
0011100.9125
0011010.9250
0011000.9375
0010110.9500
0010100.9625
0010010.9750
0010000.9875
0001111.0000
0001101.0125
0001011.0250
0001001.0375
0000111.0500
0000101.0625
0000011.0750
0000001.0875
1111011.1000
1111001.1125
1110111.1250
1110101.1375
1110011.1500
1110001.1625
1101111.1750
1101101.1875
1101011.2000
1101001.2125
1100111.2250
1100101.2375
13
1100011.2500
1100001.2625
FN9187.4
March 9, 2006
Page 14
ISL6568
TABLE 4. VRM10 VOLTAGE IDENTIFICATION CODES (Continued)
VID4 VID3 VID2 VID1 VID0 VID12.5 VDAC
1 0 1 1 1 1 1.2750
1 0 1 1 1 0 1.2875
1011011.300
1 0 1 1 0 0 1.3125
1 0 1 0 1 1 1.3250
1 0 1 0 1 0 1.3375
1 0 1 0 0 1 1.3500
1 0 1 0 0 0 1.3625
1 0 0 1 1 1 1.3750
1 0 0 1 1 0 1.3875
1 0 0 1 0 1 1.4000
1 0 0 1 0 0 1.4125
1 0 0 0 1 1 1.4250
1 0 0 0 1 0 1.4375
1 0 0 0 0 1 1.4500
1 0 0 0 0 0 1.4625
0 1 1 1 1 1 1.4750
0 1 1 1 1 0 1.4875
0 1 1 1 0 1 1.5000
0 1 1 1 0 0 1.5125
0 1 1 0 1 1 1.5250
0 1 1 0 1 0 1.5375
0 1 1 0 0 1 1.5500
0 1 1 0 0 0 1.5625
0 1 0 1 1 1 1.5750
0 1 0 1 1 0 1.5875
0 1 0 1 0 1 1.6000
Voltage Regulation
In order to regulate the output voltage to a specified level,
the ISL6568 uses the integrating compensation network
shown in Figure 6. This compensation network insures that
the steady-state error in the output voltage is limited only to
the error in the reference voltage (output of the DAC) and
offset errors in the OFS current source, remote-sense and
error amplifiers. Intersil specifies the guaranteed tolerance of
the ISL6568 to include the combined tolerances of each of
these elements.
EXTERNAL CIRCUIT ISL6568 INTERNAL CIRCUIT
R
C
C
C
+
R
V
FB
OFS
-
+
V
OUT
-
+
V
DROOP
-
COMP
C
REF
VDIFF
VSEN
RGND
ICOMP
REF
FB
IREF
VID DAC
1k
+
-
ERROR AMPLIFIER
I
OFS
+
+
-
-
DIFFERENTIAL
REMOTE-SENSE
AMPLIFIER
V
COMP
FIGURE 6. OUTPUT VOLTAGE AND LOAD-LINE
REGULATION WITH OFFSET ADJUSTMENT
The ISL6568 incorporates an internal differential remotesense amplifier in the feedback path. The amplifier removes
the voltage error encountered when measuring the output
voltage relative to the controller ground reference point,
resulting in a more accurate means of sensing output
voltage. Connect the microprocessor sense pins to the noninverting input, VSEN, and inverting input, RGND, of the
remote-sense amplifier. The droop voltage, V
DROOP
, also
feeds into the remote-sense amplifier. The remote-sense
output, V
voltage, V
, is therefore equal to the sum of the output
DIFF
, and the droop voltage. V
OUT
is connected to
DIFF
the inverting input of the error amplifier through an external
resistor.
The output of the error amplifier, V
, is compared to the
COMP
sawtooth waveform to generate the PWM signals. The PWM
signals control the timing of the Internal MOSFET drivers
and regulate the converter output so that the voltage at FB is
equal to the voltage at REF. This will regulate the output
voltage to be equal to Equation 4. The internal and external
circuitry that controls voltage regulation is illustrated in
Figure 6.
V
OUTVREFVOFS
– V
–=
DROOP
(EQ. 4)
14
FN9187.4
March 9, 2006
Page 15
ISL6568
Load-Line (Droop) Regulation
Some microprocessor manufacturers require a preciselycontrolled output impedance. This dependence of output
voltage on load current is often termed “droop” or “load line”
regulation.
As shown in Figure 6, a voltage, V
total current in all active channels, I
differential remote-sense amplifier. The resulting voltage at
the output of the remote-sense amplifier is the sum of the
output voltage and the droop voltage. As Equation 4 shows,
feeding this voltage into the compensation network causes
the regulator to adjust the output voltage so that it’s equal to
the reference voltage minus the droop voltage.
The droop voltage, V
, is created by sensing the
DROOP
current through the output inductors. This is accomplished
by using a continuous DCR current sensing method.
Inductor windings have a characteristic distributed
resistance or DCR (Direct Current Resistance). For
simplicity, the inductor DCR is considered as a separate
lumped quantity, as shown in Figure 7. The channel current,
I
, flowing through the inductor, passes through the DCR.
L
Equation 5 shows the s-domain equivalent voltage, V
across the inductor.
VLs() ILsL DCR+⋅()⋅=
The inductor DCR is important because the voltage dropped
across it is proportional to the channel current. By using a
simple R-C network and a current sense amplifier, as shown
in Figure 7, the voltage drop across all of the inductors’
DCRs can be extracted. The output of the current sense
amplifier, V
channel currents I
V
DROOP
s()
, can be shown to be proportional to the
DROOP
-------------------------------- ----------------------------------------- -
and IL2, shown in Equation 6.
L1
sL⋅
------------- 1+
DCR
sR
⋅⋅ 1+()
COMPCCOMP
R
-----------------------
If the R-C network components are selected such that the
R-C time constant matches the inductor L/DCR time
constant, then V
is equal to the sum of the voltage
DROOP
drops across the individual DCRs, multiplied by a gain. As
Equation 7 shows, V
total output current, I
R
COMP
V
DROOP
---------------------
R
S
is therefore proportional to the
DROOP
.
OUT
DCR⋅⋅=
I
OUT
, proportional to the
DROOP
, feeds into the
OUT
COMP
IL1IL2+()DCR⋅⋅⋅=
R
S
,
L
(EQ. 5)
(EQ. 6)
(EQ. 7)
-
VL(s)
PHASE1
PHASE2
R
S
ISUM
-
+
C
ICOMP
-
V
DROOP
IREF
+
ISL6568
FIGURE 7. DCR SENSING CONFIGURATION
COMP
(optional)
By simply adjusting the value of R
+
L
DCR
INDUCTOR
I
L1
R
S
L
DCR
INDUCTOR
I
L2
R
COMP
, the load line can be set
S
I
C
OUT
V
OUT
OUT
to any level, giving the converter the right amount of droop at
all load currents. It may also be necessary to compensate for
any changes in DCR due to temperature. These changes
cause the load line to be skewed, and cause the R-C time
constant to not match the L/DCR time constant. If this
becomes a problem a simple negative temperature
coefficient resistor network can be used in the place of
R
to compensate for the rise in DCR due to
COMP
temperature.
Note: An optional 10nF ceramic capacitor from the ISUM pin
to the IREF pin is recommended to help reduce any noise
affects on the current sense amplifier due to layout.
Output-Voltage Offset Programming
The ISL6568 allows the designer to accurately adjust the
offset voltage by connecting a resistor, R
pin to VCC or GND. When R
is connected between OFS
OFS
and VCC, the voltage across it is regulated to 1.5V. This
causes a proportional current (I
and out of the FB pin. If R
OFS
) to flow into the OFS pin
OFS
is connected to ground, the
voltage across it is regulated to 0.5V, and I
FB pin and out of the OFS pin. The offset current flowing
through the resistor between VDIFF and FB will generate the
desired offset voltage which is equal to the product (I
R
). These functions are shown in Figures 8 and 9.
FB
, from the OFS
OFS
flows into the
OFS
OFS
x
15
FN9187.4
March 9, 2006
Page 16
ISL6568
VDIFF
+
V
R
OFS
FB
-
FB
I
OFS
OFS
-
OFS
+
VDIFF
OFS
GND
PROGRAMMING
R
FB
FB
I
OFS
R
FIGURE 8. POSITIVE OFFSET OUTPUT VOLTAGE
V
ISL6568
VREF
VREF
E/A
GND
E/A
+
0.5V
-
VCC
+
-
1.5V
For Negative Offset (connect R
1.5 RFB×
--------------------------=
OFS
V
OFFSET
OFS
to VCC):
(EQ. 9)R
Dynamic VID
Modern microprocessors need to make changes to their core
voltage as part of normal operation. They direct the corevoltage regulator to do this by making changes to the VID
inputs. The core-voltage regulator is required to monitor the
DAC inputs and respond to on-the-fly VID changes in a
controlled manner, supervising a safe output voltage transition
without discontinuity or disruption.
The DAC mode the ISL6568 is operating in determines how
the controller responds to a dynamic VID change. When in
VRM10 mode the ISL6568 checks the VID inputs six times
every switching cycle. If a new code is established and it
stays the same for 3 consecutive readings, the ISL6568
recognizes the change and increments the reference.
Specific to VRM10, the processor controls the VID
transitions and is responsible for incrementing or
decrementing one VID step at a time. In VRM10 setting, the
ISL6568 will immediately change the reference to the new
requested value as soon as the request is validated; in
cases where the reference step is too large, the sudden
change can trigger overcurrent or overvoltage events.
In order to ensure the smooth transition of output voltage
during a VRM10 VID change, a VID step change smoothing
network is required for an ISL6568 based voltage regulator.
This network is composed of a 1kΩ internal resistor between
the output of DAC and the capacitor C
pin and ground. The selection of C
duration for 1 bit VID change and the allowable delay time.
, between the REF
REF
is based on the time
REF
VCC
VCC
+
-
1.5V
R
OFS
+
GND
0.5V
-
OFS
ISL6568
FIGURE 9. NEGATIVE OFFSET OUTPUT VOLTAGE
PROGRAMMING
Once the desired output offset voltage has been determined,
use the following formulas to set R
For Positive Offset (connect R
0.5 RFB×
OFS
--------------------------=
V
OFFSET
R
OFS
:
OFS
to GND):
(EQ. 8)
16
Assuming the microprocessor controls the VID change at 1
bit every T
, the relationship between C
VID
REF
and T
VID
is
given by Equation 10.
C
REF
VID
(EQ. 10)
0.004X T
=
As an example, for a VID step change rate of 5µs per bit, the
value of C
is 22nF based on Equation 10.
REF
When running in VRM9 or AMD Hammer operation, the
ISL6568 responds slightly differently to a dynamic VID change
than when in VRM10 mode. In these modes the VID code can
be changed by more than a 1-bit step at a time. Once the
controller receives the new VID code it waits half of a phase
cycle and then begins slewing the DAC 12.5mV every phase
cycle, until the VID and DAC are equal. Thus, the total time
required for a VID change, t
switching frequency (f
), the size of the change (∆V
S
, is dependent on the
DVID
VID
), and
the time required to register the VID change. The one-cycle
addition in the t
equation is due to the possibility that the
DVID
VID code change may occur up to one full switching cycle
before being recognized. The approximate time required for a
ISL6568-based converter in AMD Hammer configuration
FN9187.4
March 9, 2006
Page 17
ISL6568
running at fS= 335kHz to make a 1.1V to 1.5V reference
voltage change is about 100µs, as calculated using the
following equation.
t
DVID
---- -
------------------ 1.5+
=
f
0.0125
S
(EQ. 11)
∆
V
1
VID
Advanced Adaptive Zero Shoot-Through Deadtime
Control (Patent Pending)
The integrated drivers incorporate a unique adaptive deadtime
control technique to minimize deadtime, resulting in high
efficiency from the reduced freewheeling time of the lower
MOSFET body-diode conduction, and to prevent the upper and
lower MOSFETs from conducting simultaneously. This is
accomplished by ensuring either rising gate turns on its
MOSFET with minimum and sufficient delay after the other has
turned off.
During turn-off of the lower MOSFET, the PHASE voltage is
monitored until it reaches a -0.3V/+0.8V trip point for a
forward/reverse current, at which time the UGATE is released
to rise. An auto-zero comparator is used to correct the r
DS(ON)
drop in the phase voltage preventing false detection of the
-0.3V phase level during r
DS(ON)
conduction period. In the
case of zero current, the UGATE is released after 35ns delay of
the LGATE dropping below 0.5V. During the phase detection,
the disturbance of LGATE falling transition on the PHASE node
is blanked out to prevent falsely tripping. Once the PHASE is
high, the advanced adaptive shoot-through circuitry monitors
the PHASE and UGATE voltages during a PWM falling edge
and the subsequent UGATE turn-off. If either the UGATE falls
to less than 1.75V above the PHASE or the PHASE falls to less
than +0.8V, the LGATE is released to turn on.
Internal Bootstrap Device
Both integrated drivers feature an internal bootstrap schottky
diode. Simply adding an external capacitor across the BOOT
and PHASE pins completes the bootstrap circuit. The
bootstrap function is also designed to prevent the bootstrap
capacitor from overcharging due to the large negative swing
at the PHASE node. This reduces voltage stress on the boot
to phase pins.
1.6
1.4
1.2
1.
(µF)
0.8
0.6
BOOT_CAP
C
0.4
0.2
20nC
0.0
FIGURE 10. BOOTSTRAP CAPACITANCE vs BOOT RIPPLE
Gate Drive Voltage Versatility
Q
50nC
VOLTAGE
= 100nC
GATE
0.30.0 0.1 0.2 0.4 0.5 0.6 0.90.7 0.8 1.0
∆V
BOOT_CAP
(V)
The ISL6568 provides the user flexibility in choosing the
gate drive voltage for efficiency optimization. The controller
ties the upper and lower drive rails together. Simply applying
a voltage from 5V up to 12V on PVCC sets both gate drive
rail voltages simultaneously.
Initialization
Prior to initialization, proper conditions must exist on the
ENLL, VCC, PVCC and the VID pins. When the conditions are
met, the controller begins soft-start. Once the output voltage is
within the proper window of operation, the controller asserts
PGOOD.
Enable and Disable
While in shutdown mode, the PWM outputs are held in a
high-impedance state. This forces the drivers to short gateto-source of the upper and lower MOSFET’s to assure the
MOSFETs remain off. The following input conditions must be
met before the ISL6566 is released from this shutdown
mode.
The bootstrap capacitor must have a maximum voltage
rating above PVCC + 5V and its capacitance value can be
chosen from the following equation:
Q
GATE
--------------------------------------
C
BOOT_CAP
Q
GATE
where Q
at V
GS1
control MOSFETs. The ∆V
≥
∆V
BOOT_CAP
QG1PVCC•
----------------------------------- -
V
GS1
is the amount of gate charge per upper MOSFET
G1
•=
N
Q1
(EQ. 12)
gate-source voltage and NQ1 is the number of
BOOT_CAP
term is defined as the
allowable droop in the rail of the upper gate drive.
17
1. The bias voltage applied at VCC must reach the internal
power-on reset (POR) rising threshold. Once this
threshold is reached, proper operation of all aspects of
the ISL6568 is guaranteed. Hysteresis between the rising
and falling thresholds assure that once enabled, the
ISL6568 will not inadvertently turn off unless the bias
voltage drops substantially (see Electrical
Specifications).
FN9187.4
March 9, 2006
Page 18
ISL6568
ISL6568 INTERNAL CIRCUIT
POR
CIRCUIT
SOFT-START
AND
FAULT LOGIC
FIGURE 11. POWER SEQUENCING USING THRESHOLD-
SENSITIVE ENABLE (ENLL) FUNCTION
ENABLE
COMPARATOR
+
-
0.66V
EXTERNAL CIRCUIT
VCC
PVCC1
+12V
10.7kΩ
ENLL
1.40kΩ
2. The voltage on ENLL must be above 0.66V. The EN input
allows for power sequencing between the controller bias
voltage and another voltage rail. The enable comparator
holds the ISL6568 in shutdown until the voltage at ENLL
rises above 0.66V. The enable comparator has 60mV of
hysteresis to prevent bounce.
3. The driver bias voltage applied at the PVCC pin must
reach the internal power-on reset (POR) rising threshold.
Hysteresis between the rising and falling thresholds
assure that once enabled, the ISL6568 will not
inadvertently turn off unless the PVCC bias voltage drops
substantially (see Electrical Specifications).
4. T h e VID code m u s t not be 111111 o r 11111 0 in VRM10
mode or 11111 in A M D H ammer or V R M 9 modes. Th e s e
codes signal the controller that no load is present. The
controller will enter shut-down mode after receiving either
of these codes and will execute soft-start upon receiving
any other code. These codes can be used to enable or
disable the controller but it is not recommended. After
receiving one of these codes, the controller executes a
2-cycle delay before changing the overvoltage trip level to
the shut-down level and disabling PWM. Overvoltage
shutdown cannot be reset using one of these codes.
When each of these conditions is true, the controller
immediately begins the soft-start sequence.
SOFT-START
The soft-start function allows the converter to bring up the
output voltage in a controlled fashion, resulting in a linear
ramp-up. Following a delay of 16 PHASE clock cycles
between enabling the chip and the start of the ramp, the
output voltage progresses at a fixed rate of 12.5mV per each
16 PHASE clock cycles.
Thus, the soft-start period (not including the 16 PHASE clock
cycle delay) up to a given voltage, V
DAC
, can be
approximated by the following equation
V
1280⋅
T
SS
where V
DAC
---------------------------------=
f
S
is the DAC-set VID voltage, and fS is the
DAC
(EQ. 13)
switching frequency.
The ISL6568 also has the ability to start up into a precharged output, without causing any unnecessary
disturbance. The FB pin is monitored during soft-start, and
should it be higher than the equivalent internal ramping
reference voltage, the output drives hold both MOSFETs off.
Once the internal ramping reference exceeds the FB pin
potential, the output drives are enabled, allowing the output
to ramp from the pre-charged level to the final level dictated
by the DAC setting. Should the output be pre-charged to a
level exceeding the DAC setting, the output drives are
enabled at the end of the soft-start period, leading to an
abrupt correction in the output voltage down to the DAC-set
level.
OUTPUT PRECHARGED
ABOVE DAC LEVEL
OUTPUT PRECHARGED
BELOW DAC LEVEL
V
GND>
GND>
T1
T2
FIGURE 12. SOFT-START WAVEFORMS FOR ISL6568-BASED
MULTI-PHASE CONVERTER
T3
(0.5V/DIV)
OUT
ENLL (5V/DIV)
Fault Monitoring and Protection
The ISL6568 actively monitors output voltage and current to
detect fault conditions. Fault monitors trigger protective
measures to prevent damage to a microprocessor load. One
common power good indicator is provided for linking to
external system monitors. The schematic in Figure 13
outlines the interaction between the fault monitors and the
power good signal.
18
FN9187.4
March 9, 2006
Page 19
ISL6568
R
OCSET
OCSET
+
IREF
ICOMP
-
V
OCSET
+
ISUM
VDIFF
ISEN
-
+1V
VID + 150mV
V
OVP
-
V
DROOP
+
+
-
SOFT-START, FAULT
AND CONTROL LOGIC
100uA
+
-
OC
-
VSEN
RGND
+
x1
-
OV
+
PGOOD
-
UV
+
0.82 x DAC
FIGURE 13. POWER GOOD AND PROTECTION CIRCUITRY
ISL6568 INTERNAL CIRCUITRY
overvoltage trip level is the higher of DAC plus 150mV or a
fixed voltage, V
. The fixed voltage, V
OVP
, is 1.67V when
OVP
running in AMD Hammer, or VRM10 modes, and 1.97V for
VRM9 mode. Upon successful soft-start, the overvoltage trip
level is only DAC plus 150mV. OVP releases 50mV below its
trip point if it was “DAC plus 150mV” that tripped it, and
releases 100mV below its trip point if it was the fixed voltage,
V
, that tripped it. Actions are taken by the ISL6568 to
OVP
protect the microprocessor load when an overvoltage
condition occurs, until the output voltage falls back within set
limits.
At the inception of an overvoltage event, all LGATE signals
are commanded high, and the PGOOD signal is driven low.
This causes the controller to turn on the lower MOSFETs
and pull the output voltage below a level that might cause
damage to the load. The LGATE outputs remain high until
VDIFF falls to within the overvoltage limits explained above.
The ISL6568 will continue to protect the load in this fashion
as long as the overvoltage condition recurs.
Once an overvoltage condition ends the ISL6568 continues
normal operation and PGOOD returns high.
Pre-POR Overvoltage Protection
Prior to PVCC and VCC exceeding their POR levels, the
ISL6568 is designed to protect the load from any overvoltage
events that may occur. This is accomplished by means of an
internal 10kΩ resistor tied from PHASE to LGATE, which
turns on the lower MOSFET to control the output voltage
until the overvoltage event ceases or the input power supply
cuts off. For complete protection, the low side MOSFET
should have a gate threshold well below the maximum
voltage rating of the load/microprocessor.
Power Good Signal
The power good pin (PGOOD) is an open-drain logic output
that transitions high when the converter is operating after
soft-start. PGOOD pulls low during shutdown and releases
high after a successful soft-start. PGOOD transitions low
when an undervoltage, overvoltage, or overcurrent condition
is detected or when the controller is disabled by a reset from
ENLL, POR, or one of the no-CPU VID codes. If after an
undervoltage or overvoltage event occurs the output returns
to within under and overvoltage limits, PGOOD will return
high.
Undervoltage Detection
The undervoltage threshold is set at 82% of the VID code.
When the output voltage (VSEN-RGND) is below the
undervoltage threshold, PGOOD gets pulled low. No other
action is taken by the controller. PGOOD will return high if
the output voltage rises above 85% of the VID code.
Overvoltage Protection
The ISL6568 constantly monitors the difference between the
VSEN and RGND voltages to detect if an overvoltage event
occurs. During soft-start, while the DAC is ramping up, the
19
In the event that during normal operation the PVCC or VCC
voltage falls back below the POR threshold, the pre-POR
overvoltage protection circuitry reactivates to protect from
any more pre-POR overvoltage events.
Open Sense Line Protection
In the case that either of the remote sense lines, VSEN or
GND, become open, the ISL6568 is designed to detect this
and shut down the controller. This event is detected by
monitoring the voltage on the IREF pin, which is a local
version of V
sensed at the outputs of the inductors.
OUT
If VSEN or RGND become opened, VDIFF falls, causing the
duty cycle to increase and the output voltage on IREF to
increase. If the voltage on IREF exceeds “VDIFF+1V”, the
controller will shut down. Once the voltage on IREF falls
below “VDIFF+1V”, the ISL6568 will restart at the beginning
of soft-start.
Overcurrent Protection
The ISL6568 detects overcurrent events by comparing the
droop voltage, V
shown in Figure 13. The droop voltage, set by the external
current sensing circuitry, is proportional to the output current
, to the OCSET voltage, V
DROOP
, as
OCSET
FN9187.4
March 9, 2006
Page 20
ISL6568
as shown in Equation 7. A constant 100µA flows through
R
, creating the OCSET voltage. When the droop
OCSET
voltage exceeds the OCSET voltage, the overcurrent
protection circuitry activates. Since the droop voltage is
proportional to the output current, the overcurrent trip level,
I
, can be set by selecting the proper value for R
MAX
OCSET
as shown in Equation 14.
R
OCSET
I
MAXRCOMP
----------------------------------------------------------=
100µ R
⋅
DCR⋅⋅
S
(EQ. 14)
Once the output current exceeds the overcurrent trip level,
V
DROOP
will exceed V
, and a comparator will trigger
OCSET
the converter to begin overcurrent protection procedures. At
the beginning of overcurrent shutdown, the controller turns
off both upper and lower MOSFETs. The system remains in
this state for a period of 4096 switching cycles. If the
controller is still enabled at the end of this wait period, it will
attempt a soft-start (as shown in Figure 14). If the fault
remains, the trip-retry cycles will continue indefinitely until
either the controller is disabled or the fault is cleared. Note
that the energy delivered during trip-retry cycling is much
less than during full-load operation, so there is no thermal
hazard.
OUTPUT CURRENT, 50A/DIV
0A
OUTPUT VOLTAGE,
500mV/DIV
0V
FIGURE 14. OVERCURRENT BEHAVIOR IN HICCUP MODE
F
SW
= 500kHz
2ms/DIV
General Design Guide
This design guide is intended to provide a high-level
explanation of the steps necessary to create a multi-phase
power converter. It is assumed that the reader is familiar with
many of the basic skills and techniques referenced below. In
addition to this guide, Intersil provides complete reference
designs that include schematics, bills of materials, and example
board layouts for all common microprocessor applications.
Power Stages
The first step in designing a multi-phase converter is to
determine the number of phases. This determination
depends heavily on the cost analysis which in turn depends
on system constraints that differ from one design to the next.
Principally, the designer will be concerned with whether
components can be mounted on both sides of the circuit
board, whether through-hole components are permitted, the
total board space available for power-supply circuitry, and
the maximum amount of load current. Generally speaking,
,
the most economical solutions are those in which each
phase handles between 25 and 30A. All surface-mount
designs will tend toward the lower end of this current range.
If through-hole MOSFETs and inductors can be used, higher
per-phase currents are possible. In cases where board
space is the limiting constraint, current can be pushed as
high as 40A per phase, but these designs require heat sinks
and forced air to cool the MOSFETs, inductors and heatdissipating surfaces.
MOSFETS
The choice of MOSFETs depends on the current each
MOSFET will be required to conduct, the switching frequency,
the capability of the MOSFETs to dissipate heat, and the
availability and nature of heat sinking and air flow.
LOWER MOSFET POWER CALCULATION
The calculation for power loss in the lower MOSFET is
simple, since virtually all of the loss in the lower MOSFET is
due to current conducted through the channel resistance
(r
output current, I
Equation 1), and d is the duty cycle (V
P
). In Equation 15, IM is the maximum continuous
DS(ON)
LOW 1,
r
is the peak-to-peak inductor current (see
PP
OUT/VIN
I
LPP,
--------------------------------+=
2
1d–()
12
DS ON()
I
M
-----N
2
1d–()
).
(EQ. 15)
An additional term can be added to the lower-MOSFET loss
equation to account for additional loss accrued during the
dead time when inductor current is flowing through the
lower-MOSFET body diode. This term is dependent on the
diode forward voltage at I
frequency, f
, and the length of dead times, td1 and td2, at
S
, V
M
, the switching
D(ON)
the beginning and the end of the lower-MOSFET conduction
interval respectively.
P
LOW 2,
=
V
DON()fS
I
I
M
PP
------
---------+
N
2
I
M
t
+
------
d1
–
N
I
PP
--------2
(EQ. 16)
t
d2
The total maximum power dissipated in each lower MOSFET
is approximated by the summation of P
LOW,1
and P
LOW,2
.
UPPER MOSFET POWER CALCULATION
In addition to r
losses, a large portion of the upper-
DS(ON)
MOSFET losses are due to currents conducted across the
input voltage (V
) during switching. Since a substantially
IN
higher portion of the upper-MOSFET losses are dependent
on switching frequency, the power calculation is more
complex. Upper MOSFET losses can be divided into
separate components involving the upper-MOSFET
switching times, the lower-MOSFET body-diode reverse-
20
FN9187.4
March 9, 2006
Page 21
ISL6568
recovery charge, Qrr, and the upper MOSFET r
DS(ON)
conduction loss.
When the upper MOSFET turns off, the lower MOSFET does
not conduct any portion of the inductor current until the
voltage at the phase node falls below ground. Once the
lower MOSFET begins conducting, the current in the upper
MOSFET falls to zero as the current in the lower MOSFET
ramps up to assume the full inductor current. In Equation 17,
the required time for this commutation is t
approximated associated power loss is P
t
P
≈
UP 1,VIN
I
M
------
N
I
---------+
PP
2
1
----
2
f
S
and the
1
.
UP,1
(EQ. 17)
At turn-on, the upper MOSFET begins to conduct and this
transition occurs over a time t
approximate power loss is P
t
t
≈
P
≈
P
UP 2,VIN
UP 2,VIN
I
I
M
M
------
------
N
N
I
I
2
2
PP
PP
----
----
–
–
---------
--------2
2
2
2
. In Equation 18, the
2
.
UP,2
f
f
S
S
(EQ. 18)
A third component involves the lower MOSFET reverserecovery charge, Q
. Since the inductor current has fully
rr
commutated to the upper MOSFET before the lowerMOSFET body diode can recover all of Q
, it is conducted
rr
through the upper MOSFET across VIN. The power
dissipated as a result is P
VINQrrf
=
P
UP 3,
S
UP,3
.
(EQ. 19)
Calculating the power dissipation in the drivers for a desired
application is critical to ensure safe operation. Exceeding the
maximum allowable power dissipation level will push the IC
beyond the maximum recommended operating junction
temperature of 125°C. The maximum allowable IC power
dissipation for the 5x5 QFN package is approximately 4W at
room temperature. See Layout Considerations paragraph for
thermal transfer improvement suggestions.
When designing the ISL6568 into an application, it is
recommended that the following calculation is used to
ensure safe operation at the desired frequency for the
selected MOSFETs. The total gate drive power losses,
P
Qg_TOT
, due to the gate charge of MOSFETs and the
integrated driver’s internal circuitry and their corresponding
average driver current can be estimated with Equations 21
and 22, respectively.
P
Qg_TOTPQg_Q1PQg_Q2IQ
3
P
Qg_Q1
P
Qg_Q2QG2
I
DR
-- -
2
3
-- -
• QG2NQ2•+
Q
2
PVCC•• FSW• NQ1• N
Q
G1
PVCC• FSW• NQ2N
N•
G1
Q1
VCC•++=
•=
PHASE
••=
PHASE
N
• F
PHASE
+•=
SWIQ
(EQ. 21)
(EQ. 22)
Finally, the resistive part of the upper MOSFET is given in
Equation 20 as P
P
UP 4,rDS ON()
UP,4
2
I
M
-----N
.
2
I
PP
+≈
d
---------12
(EQ. 20)
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 17, 18, 19 and 20. Since the power
equations depend on MOSFET parameters, choosing the
correct MOSFETs can be an iterative process involving
repetitive solutions to the loss equations for different
MOSFETs and different switching frequencies.
Package Power Dissipation
When choosing MOSFETs it is important to consider the
amount of power being dissipated in the integrated drivers
located in the controller. Since there are a total of two drivers
in the controller package, the total power dissipated by both
drivers must be less than the maximum allowable power
dissipation for the QFN package.
In Equations 21 and 22, P
power loss and P
the gate charge (Q
is the total lower gate drive power loss;
Qg_Q2
and QG2) is defined at the particular gate
G1
is the total upper gate drive
Qg_Q1
to source drive voltage PVCC in the corresponding MOSFET
data sheet; I
at both drive outputs; N
and lower MOSFETs per phase, respectively; N
number of active phases. The I
is the driver total quiescent current with no load
Q
Q1
and N
are the number of upper
Q2
VCC product is the quiescent
Q*
PHASE
is the
power of the controller without capacitive load and is typically
75mW at 300kHz.
PVCC
FIGURE 15. TYPICAL UPPER-GATE DRIVE TURN-ON PATH
BOOT
PHASE
D
C
GD
R
HI1
R
LO1
UGATE
G
R
GI1
R
G1
C
GS
S
Q1
C
DS
21
FN9187.4
March 9, 2006
Page 22
ISL6568
PVCC
D
C
GD
R
HI2
R
LO2
LGATE
G
R
GI2
R
G2
C
GS
S
Q2
C
DS
FIGURE 16. TYPICAL LOWER-GATE DRIVE TURN-ON PATH
The total gate drive power losses are dissipated among the
resistive components along the transition path and in the
bootstrap diode. The portion of the total power dissipated in the
controller itself is the power dissipated in the upper drive path
resistance, P
and in the boot strap diode, P
be dissipated by the external gate resistors (R
the internal gate resistors (R
, the lower drive path resistance, P
DR_UP
. The rest of the power will
BOOT
and R
GI1
GI2
DR_UP
and RG2) and
G1
) of the MOSFETs.
,
Figures 15 and 16 show the typical upper and lower gate
drives’ turn-on transition path. The total power dissipation in the
controller itself, P
P
DRPDR_UPPDR _LOW PBOOTIQ
P
BOOT
P
DR_UP
P
DR _LOW
R
EXT1RG1
P
---------------------=
, can be roughly estimated as:
DR
VCC•()+++=
Qg_Q1
(EQ. 23)
3
R
HI1
--------------------------------------
R
+
HI1REXT1
R
HI2
--------------------------------------
R
+
HI2REXT2
R
GI1
-------------+=
N
Q1
R
LO1
----------------------------------------+
R
+
LO1REXT1
R
LO2
----------------------------------------+
R
+
LO2REXT2
R
EXT2RG2
P
Qg_Q1
---------------------
•=
P
---------------------
•=
R
-------------+=
N
3
Qg_Q2
2
GI2
Q2
Current Balancing Component Selection
The ISL6568 senses the channel load current by sampling
the voltage across the lower MOSFET r
Figure 17. The ISEN pins are denoted ISEN1 and ISEN2.
The resistors connected between these pins and the
respective phase nodes determine the gains in the channelcurrent balance loop.
DS(ON)
, as shown in
V
IN
CHANNEL N
UPPER MOSFET
I
L
ISEN(n)
R
ISEN
ISL6568
CHANNEL N
LOWER MOSFET
-
ILr
DS ON()
+
FIGURE 17. ISL6568 INTERNAL AND EXTERNAL CURRENT-
Select values for these resistors based on the room
temperature r
operating current, I
SENSING CIRCUITRY
of the lower MOSFETs; the full-load
DS(ON)
; and the number of phases, N using
FL
Equation 24.
R
ISEN
r
DS ON()
----------------------50 10
I
FL
--------=
–
6
N
×
(EQ. 24)
In certain circumstances, it may be necessary to adjust the
value of one or more ISEN resistors. When the components of
one or more channels are inhibited from effectively dissipating
their heat so that the affected channels run hotter than
desired, choose new, smaller values of R
for the affected
ISEN
phases (see the section entitled Channel-Current Balance).
Choose R
in proportion to the desired decrease in
ISEN,2
temperature rise in order to cause proportionally less current
to flow in the hotter phase.
∆T
R
ISEN 2,
R
ISEN
In Equation 25, make sure that ∆T
----------=
∆T
2
1
is the desired temperature
2
rise above the ambient temperature, and ∆T
is the measured
1
(EQ. 25)
temperature rise above the ambient temperature. While a
single adjustment according to Equation 25 is usually
sufficient, it may occasionally be necessary to adjust R
ISEN
two or more times to achieve optimal thermal balance
between all channels.
Load Line Regulation Component Selection (DCR
Current Sensing)
For accurate load line regulation, the ISL6568 senses the
total output current by detecting the voltage across the
output inductor DCR of each channel (As described in the
Load Line Regulation section). As Figure 18 illustrates, an
R-C network is required to accurately sense the inductor
DCR voltage and convert this information into a “droop”
voltage, which is proportional to the total output current.
22
FN9187.4
March 9, 2006
Page 23
ISL6568
Choosing the components for this current sense network is a
two step process. First, R
chosen so that the time constant of this R
COMP
and C
must be
COMP
COMP-CCOMP
network matches the time constant of the inductor L/DCR.
Then the resistor R
must be chosen to set the current
S
sense network gain, obtaining the desired full load droop
voltage. Follow the steps below to choose the component
values for this R-C network.
1. Choose an arbitrary value for C
. The recommended
COMP
value is 0.01µF.
2. Plug the inductor L and DCR component values, and the
values for C
calculate the value for R
COMP
=
R
3. Use the new value for R
chosen in steps 1, into Equation 26 to
COMP
L
---------------------------------------
⋅
DCR C
COMP
.
COMP
obtained from Equation
COMP
(EQ. 26)
26, as well as the desired full load current, IFL, full load
droop voltage, V
27 to calculate the value for R
I
FL
------------------------ -
R
S
V
DROOP
PHASE1
PHASE2
ISUM
-
+
ICOMP
-
V
DROOP
IREF
+
ISL6568
FIGURE 18. DCR SENSING CONFIGURATION
, and inductor DCR in Equation
DROOP
R
COMP
(optional)
C
COMP
.
S
DCR⋅⋅=
VL(s)
+
L
INDUCTOR
I
L1
R
S
L
INDUCTOR
I
R
L2
S
R
COMP
DCR
DCR
(EQ. 27)
-
I
OUT
C
V
OUT
OUT
Due to errors in the inductance or DCR it may be necessary
to adjust the value of R
to match the time constants
COMP
correctly. The effects of time constant mismatch can be seen
in the form of droop overshoot or undershoot during the
initial load transient spike, as shown in Figure 19. Follow the
steps below to ensure the R-C and inductor L/DCR time
constants are matched accurately.
1. Capture a transient event with the oscilloscope set to
about L/DCR/2 (sec/div). For example, with L = 1µH and
DCR = 1mΩ, set the oscilloscope to 500µs/div.
2. Record ∆V1 and ∆V2 as shown in Figure 19.
3. Select a new value, R
resistor based on the original value, R
, for the time constant
COMP,2
COMP,1
, using the
following equation.
R
COMP 2,
4. Replace R
V1∆
R
COMP 1,
COMP
----------
⋅=
∆
V
2
with the new value and check to see that
(EQ. 28)
the error is corrected. Repeat the procedure if necessary.
After choosing a new value for R
necessary to adjust the value of R
, it will most likely be
COMP
to obtain the desired full
S
load droop voltage. Use Equation 27 to obtain the new value
for R
.
S
∆V
∆V
1
FIGURE 19. TIME CONSTANT MISMATCH BEHAVIOR
2
V
OUT
I
TRAN
∆I
Compensation
The two opposing goals of compensating the voltage
regulator are stability and speed.
The load-line regulated converter behaves in a similar
manner to a peak current mode controller because the two
poles at the output filter L-C resonant frequency split with the
introduction of current information into the control loop. The
final location of these poles is determined by the system
function, the gain of the current signal, and the value of the
compensation components, R
and CC.
C
23
FN9187.4
March 9, 2006
Page 24
ISL6568
C2 (OPTIONAL)
C
C
R
C
R
FB
FIGURE 20. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6568 CIRCUIT
COMP
FB
ISL6568
VDIFF
Since the system poles and zero are affected by the values
of the components that are meant to compensate them, the
solution to the system equation becomes fairly complicated.
Fortunately, there is a simple approximation that comes very
close to an optimal solution. Treating the system as though it
were a voltage-mode regulator, by compensating the L-C
poles and the ESR zero of the voltage mode approximation,
yields a solution that is always stable with very close to ideal
transient performance.
Select a target bandwidth for the compensated system, f
.
0
The target bandwidth must be large enough to assure
adequate transient performance, but smaller than 1/3 of the
per-channel switching frequency. The values of the
compensation components depend on the relationships of f
to the L-C pole frequency and the ESR zero frequency. For
each of the following three, there is a separate set of
equations for the compensation components.
1
>
Case 1:
------------------- f
2π LC
R
CRFB
------------------------------------=
C
C
2π VPPRFBf
0
2π f0VppLC
------------------------------------=
0.66V
0.66V
IN
IN
0
In Equations 29, L is the per-channel filter inductance
divided by the number of active channels; C is the sum total
of all output capacitors; ESR is the equivalent series
resistance of the bulk output filter capacitance; and V
PP
is
the peak-to-peak sawtooth signal amplitude as described in
the Electrical Specifications.
Once selected, the compensation values in Equations 29
assure a stable converter with reasonable transient
performance. In most cases, transient performance can be
improved by making adjustments to R
value of R
while observing the transient performance on an
C
. Slowly increase the
C
oscilloscope until no further improvement is noted. Normally,
C
will not need adjustment. Keep the value of CC from
C
Equations 29 unless some performance issue is noted.
The optional capacitor C
, is sometimes needed to bypass
2
noise away from the PWM comparator (See Figure 20).
Keep a position available for C
, and be prepared to install a
2
high-frequency capacitor of between 22pF and 150pF in
case any leading edge jitter problem is noted.
Output Filter Design
The output inductors and the output capacitor bank together
to form a low-pass filter responsible for smoothing the
pulsating voltage at the phase nodes. The output filter also
must provide the transient energy until the regulator can
respond. Because it has a low bandwidth compared to the
switching frequency, the output filter limits the system
transient response. The output capacitors must supply or
0
sink load current while the current in the output inductors
increases or decreases to meet the demand.
In high-speed converters, the output capacitor bank is usually
the most costly (and often the largest) part of the circuit.
Output filter design begins with minimizing the cost of this part
of the circuit. The critical load parameters in choosing the
output capacitors are the maximum size of the load step, ∆I,
the load-current slew rate, di/dt, and the maximum allowable
output-voltage deviation under transient loading, ∆V
MAX
.
Capacitors are characterized according to their capacitance,
ESR, and ESL (equivalent series inductance).
Case 2:
Case 3:
1
------------------2π LC
R
CRFB
-------------------------------------------------------------=
C
C
2π()2f
------------------------------>
f
0
2π C ESR()
R
CRFB
0.66VINESR()C
-------------------------------------------------=
C
C
f
------------------------------<≤
0
2π C ESR()
VPP2π()2f
--------------------------------------------=
0.66 V
0.66V
2
VPPRFBLC
0
1
2π f0VppL
------------------------------------------=
0.66 V
2π V
PPRFBf0
24
1
2
0
IN
IN
ESR()
IN
L
LC
(EQ. 29)
At the beginning of the load transient, the output capacitors
supply all of the transient current. The output voltage will
initially deviate by an amount approximated by the voltage
drop across the ESL. As the load current increases, the
voltage drop across the ESR increases linearly until the load
current reaches its final value. The capacitors selected must
have sufficiently low ESL and ESR so that the total outputvoltage deviation is less than the allowable maximum.
Neglecting the contribution of inductor current and regulator
response, the output voltage initially deviates by an amount
∆V ESL()
di
----- ESR()∆I+≈
dt
(EQ. 30)
The filter capacitor must have sufficiently low ESL and ESR
so that ∆V < ∆V
MAX
.
FN9187.4
March 9, 2006
Page 25
ISL6568
Most capacitor solutions rely on a mixture of high frequency
capacitors with relatively low capacitance in combination
with bulk capacitors having high capacitance but limited
high-frequency performance. Minimizing the ESL of the
high-frequency capacitors allows them to support the output
voltage as the current increases. Minimizing the ESR of the
bulk capacitors allows them to supply the increased current
with less output voltage deviation.
The ESR of the bulk capacitors also creates the majority of
the output-voltage ripple. As the bulk capacitors sink and
source the inductor ac ripple current (See Interleaving and
Equation 2), a voltage develops across the bulk capacitor
ESR equal to I
(ESR). Thus, once the output capacitors
C,PP
are selected, the maximum allowable ripple voltage,
V
PP(MAX)
L ESR()
, determines the lower limit on the inductance.
V
NV
–
IN
------------------------------------------------------------≥
fSVINV
V
OUT
OUT
PP MAX()
(EQ. 31)
Since the capacitors are supplying a decreasing portion of
the load current while the regulator recovers from the
transient, the capacitor voltage becomes slightly depleted.
The output inductors must be capable of assuming the entire
load current before the output voltage decreases more than
∆V
. This places an upper limit on inductance.
MAX
Equation 32 gives the upper limit on L for the cases when
the trailing edge of the current transient causes a greater
output-voltage deviation than the leading edge. Equation 33
addresses the leading edge. Normally, the trailing edge
dictates the selection of L because duty cycles are usually
less than 50%. Nevertheless, both inequalities should be
evaluated, and L should be selected based on the lower of
the two results. In each equation, L is the per-channel
inductance, C is the total output capacitance, and N is the
number of active channels.
Switching frequency is determined by the selection of the
frequency-setting resistor, R
are provided to assist in selecting the correct value for R
[]
R
10.61 1.035 fS()log–
10
=
T
1000
)
Ω
100
(k
T
R
10
10 100 1000 10000
SWITCHING FREQUENCY (kHz)
FIGURE 21. RT vs SWITCHING FREQUENCY
. Figure 21 and Equation 34
T
(EQ. 34)
.
T
Input Capacitor Selection
The input capacitors are responsible for sourcing the ac
component of the input current flowing into the upper
MOSFETs. Their RMS current capacity must be sufficient to
handle the ac component of the current drawn by the upper
MOSFETs which is related to duty cycle and the number of
active phases.
0.3
)
O
I
RMS/
0.2
⋅⋅⋅
2NCV
--------------------------------- ∆V
L
()
1.25
L
---------------------------------- ∆V
≤
2
()
∆I
NC⋅⋅
2
()
∆I
O
MAX
MAX
∆I ESR⋅()–≤
∆I ESR⋅()– VINVO–
(EQ. 32)
(EQ. 33)
Switching Frequency
There are a number of variables to consider when choosing
the switching frequency, as there are considerable effects on
the upper MOSFET loss calculation. These effects are
outlined in MOSFETs, and they establish the upper limit for
the switching frequency. The lower limit is established by the
requirement for fast transient response and small outputvoltage ripple as outlined in Output Filter Design. Choose the
lowest switching frequency that allows the regulator to meet
the transient-response requirements.
25
0.1
I
= 0
L,PP
= 0.5 I
I
L,PP
INPUT-CAPACITOR CURRENT (I
I
L,PP
0
00.4 1.00.2 0.6 0.8
FIGURE 22. NORMALIZED INPUT-CAPACITOR RMS
O
= 0.75 I
O
DUTY CYCLE (V
CURRENT FOR 2-PHASE CONVERTER
IN/VO
)
For a two-phase design, use Figure 22 to determine the
input-capacitor RMS current requirement set by the duty
cycle, maximum sustained output current (I
of the peak-to-peak inductor current (I
), and the ratio
O
) to IO. Select a
L,PP
bulk capacitor with a ripple current rating which will minimize
FN9187.4
March 9, 2006
Page 26
ISL6568
the total number of input capacitors required to support the
RMS current calculated. The voltage rating of the capacitors
should also be at least 1.25 times greater than the maximum
input voltage. Figure 23 provides the same input RMS
current information for single-phase designs. Use the same
approach for selecting the bulk capacitor type and number.
0.6
)
O
/I
RMS
0.4
0.2
I
= 0
L,PP
= 0.5 I
I
L,PP
INPUT-CAPACITOR CURRENT (I
0
FIGURE 23. NORMALIZED INPUT-CAPACITOR RMS
I
L,PP
00.4 1.00.2 0.6 0.8
CURRENT FOR SINGLE-PHASE CONVERTER
O
= 0.75 I
DUTY CYCLE (V
O
)
IN/VO
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the input bulk capacitors to suppress
leading and falling edge voltage spikes. The spikes result from
the high current slew rate produced by the upper MOSFET
turn on and off. Select low ESL ceramic capacitors and place
one as close as possible to each upper MOSFET drain to
minimize board parasitics and maximize suppression.
Layout Considerations
MOSFETs switch very fast and efficiently. The speed with
which the current transitions from one device to another
causes voltage spikes across the interconnecting
impedances and parasitic circuit elements. These voltage
spikes can degrade efficiency, radiate noise into the circuit
and lead to device overvoltage stress. Careful component
selection, layout, and placement minimizes these voltage
spikes. Consider, as an example, the turnoff transition of the
upper PWM MOSFET. Prior to turnoff, the upper MOSFET
was carrying channel current. During the turnoff, current
stops flowing in the upper MOSFET and is picked up by the
lower MOSFET. Any inductance in the switched current path
generates a large voltage spike during the switching interval.
Careful component selection, tight layout of the critical
components, and short, wide circuit traces minimize the
magnitude of voltage spikes.
There are two sets of critical components in a DC-DC
converter using a ISL6566 controller. The power
components are the most critical because they switch large
amounts of energy. Next are small signal components that
connect to sensitive nodes or supply critical bypassing
current and signal coupling.
The power components should be placed first, which include
the MOSFETs, input and output capacitors, and the inductors. It
is important to have a symmetrical layout for each power train,
preferably with the controller located equidistant from each.
Symmetrical layout allows heat to be dissipated equally
across all three power trains. Equidistant placement of the
controller to the three power trains also helps keep the gate
drive traces equally short, resulting in equal trace impedances
and similar drive capability of all sets of MOSFETs.
When placing the MOSFETs try to keep the source of the
upper FETs and the drain of the lower FETs as close as
thermally possible. Input Bulk capacitors should be placed
close to the drain of the upper FETs and the source of the lower
FETs. Locate the output inductors and output capacitors
between the MOSFETs and the load. The high-frequency input
and output decoupling capacitors (ceramic) should be placed
as close as practicable to the decoupling target, making use of
the shortest connection paths to any internal planes, such as
vias to GND next or on the capacitor solder pad.
The critical small components include the bypass capacitors
for VCC and PVCC, and many of the components
surrounding the controller including the feedback network
and current sense components. Locate the VCC/PVCC
bypass capacitors as close to the ISL6566 as possible. It is
especially important to locate the components associated
with the feedback circuit close to their respective controller
pins, since they belong to a high-impedance circuit loop,
sensitive to EMI pick-up. It is also important to place the
current sense components close to their respective pins on
the ISL6566, including R
ISEN
, RS, R
COMP
, and C
COMP
.
A multi-layer printed circuit board is recommended. Figure 25
shows the connections of the critical components for the
converter. Note that capacitors C
xxIN
and C
xxOUT
could each
represent numerous physical capacitors. Dedicate one solid
layer, usually the one underneath the component side of the
board, for a ground plane and make all critical component
ground connections with vias to this layer. Dedicate another
solid layer as a power plane and break this plane into smaller
islands of common voltage levels. Keep the metal runs from the
PHASE terminal to output inductors short. The power plane
should support the input power and output power nodes. Use
copper filled polygons on the top and bottom circuit layers for
the phase nodes. Use the remaining printed circuit layers for
small signal wiring.
Routing UGATE, LGATE, and PHASE traces
Great attention should be paid to routing the UGATE, LGATE,
and PHASE traces since they drive the power train MOSFETs
using short, high current pulses. It is important to size them as
large and as short as possible to reduce their overall
impedance and inductance. They should be sized to carry at
least one ampere of current (0.02” to 0.05”). Going between
26
FN9187.4
March 9, 2006
Page 27
layers with vias should also be avoided, but if so, use two vias
for interconnection when possible.
Extra care should be given to the LGATE traces in particular
since keeping their impedance and inductance low helps to
significantly reduce the possibility of shoot-through. It is also
important to route each channels UGATE and PHASE traces
in as close proximity as possible to reduce their inductances.
Thermal Management
For maximum thermal performance in high current, high
switching frequency applications, connecting the thermal
GND pad of the ISL6566 to the ground plane with multiple
vias is reco mmen ded. This heat spreading allows the part to
achieve its full thermal potential. It is also recommended
that the controller be placed in a direct path of airflow if
possible to help thermally manage the part.
Suppressing MOSFET Gate Leakage
With VCC at ground potential, UGATE is high impedance. In
this state, any stray leakage has the potential to deliver
charge to the gate of the upper MOSFET. If UGATE receives
sufficient charge to bias the device on, a low impedance path
will be connected between the upper MOSFET drain and
PHASE. If this occurs and the input power supply is present
and active, the system could see potentially damaging
current. Worst-case leakage currents are on the order of
pico-amps; therefore, a 10kΩ resistor, connected from
UGATE to PHASE, is more than sufficient to bleed off any
stray leakage current. This resistor will not affect the normal
performance of the driver or reduce its efficiency.
ISL6568
27
FN9187.4
March 9, 2006
Page 28
ISL6568
C
R
FB
VDIFF
VSEN
+5V
(CF1)
R
OFS
R
T
RGND
VCC
OFS
FS
REF
C
REF
FB
2
R
C
1
1
LOCATE CLOSE TO IC
(MINIMIZE CONNECTION PATH)
COMP
PVCC
BOOT1
UGATE1
PHASE1
LGATE1
ISEN1
C
BOOT1
R
ISEN1
+12V
(CF2)
KEY
HEAVY TRACE ON CIRCUIT PLANE LAYER
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT PLANE LAYER
VIA CONNECTION TO GROUND PLANE
LOCATE NEAR SWITCHING TRANSISTORS;
(MINIMIZE CONNECTION PATH)
C
BIN1
ISL6568
VID4
+12V
VID3
VID2
VID1
VID0
VID12.5
PGOOD
GND
BOOT2
UGATE2
PHASE2
ISEN2
LGATE2
C
BOOT2
R
ISEN2
+12V
C
BOUT
BIN2
LOCATE NEAR LOAD;
(MINIMIZE CONNECTION PATH)
(C
HFOUT
)C
LOAD
ENLL
IREF
OCSET
R
OCSET
ICOMP
R
COMP
C
COMP
ISUM
R
S
R
S
FIGURE 24. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS
28
FN9187.4
March 9, 2006
Page 29
ISL6568
Quad Flat No-Lead Plastic Package (QFN)
Micro Lead Frame Plastic Package (MLFP)
L32.5x5
32 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE
(COMPLIANT TO JEDEC MO-220VHHD-2 ISSUE C
MILLIMETERS
SYMBOL
A 0.80 0.90 1.00 A1 - - 0.05 A2 - - 1.00 9
A3 0.20 REF 9
b 0.18 0.23 0.30 5,8
D 5.00 BSC D1 4.75 BSC 9
D2 2.95 3.10 3.25 7,8
E 5.00 BSC E1 4.75 BSC 9
E2 2.95 3.10 3.25 7,8
e 0.50 BSC -
k0.25 - - -
L 0.30 0.40 0.50 8
L1 - - 0.15 10
N322
Nd 8 3
Ne 8 8 3
P- -0.609
θ --129
NOTES:
1. Dimensioning and tolerancing conform to ASME Y14.5-1994.
2. N is the number of terminals.
3. Nd and Ne refer to the number of terminals on each D and E.
4. All dimensions are in millimeters. Angles are in degrees.
5. Dimension b applies to the metallized terminal and is measured
between 0.15mm and 0.30mm from the terminal tip.
6. The configuration of the pin #1 identifier is optional, but must be
located within the zone indicated. The pin #1 identifier may be
either a mold or mark feature.
7. Dimensions D2 and E2 are for the exposed pads which provide
improved electrical and thermal performance.
8. Nominal dimensions are provided to assist with PCB Land Pattern
Design efforts, see Intersil Technical Brief TB389.
9. Features and dimensions A2, A3, D1, E1, P & θ are present when
Anvil singulation method is used and not present for saw
singulation.
10. Depending on the method of lead termination at the edge of the
package, a maximum 0.15mm pull back (L1) maybe present. L
minus L1 to be equal to or greater than 0.3mm.
NOTESMIN NOMINAL MAX
Rev. 1 10/02
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
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29
FN9187.4
March 9, 2006
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