ST L6911C User Manual
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5 BIT PROGRAMMABLE STEP DOWN CONTROLLER
■
OPERATING SUPPL Y IC VOLTAGE FROM 5V
TO 12V BUSES
■
UP TO 1.3A GATE CURRENT CAPABILITY
■
TTL-COMP A T I BLE 5 BIT P ROGR AMMABLE
OUTPUT CO MPLIANT WITH VRM 8.4 :
1.3V TO 2.05V WITH 0.05V BINARY STEPS
2.1V TO 3.5V WITH 0.1V BINARY STEPS
■
VOLTAGE MODE PWM CONTROL
■
EXCELLENT OUTPUT ACCURACY: ±1%
OVER LINE AND TEMPERATURE
VARIATIONS
■
VERY FAST LOAD TRANSIENT RESPONSE:
FROM 0% TO 100% DUTY CYCLE
■
POWER GOOD OU T PUT VO LTA GE
■
OVERVOLTAGE PROTECTION AND
MONITOR
■
OVERCURRENT PROTECTION REALIZED
USING THE UPPER MOSFET'S R
■
200KHz INTERNAL OSCILLATOR
■
OSCILLATOR EXTERNALLY ADJUST ABLE
FROM 50KHz TO 1MHz
■
SOFT START AND INHIBIT FUNCTIONS
APPLICATIONS
■
POWER SUPPLY FOR ADVANCED
MICROPROCESSOR CORE
■
DISTRIBUTED PO WE R SUPP LY
■
HIGH POWER DC-DC REGULATORS
L6911C
WITH SYNCHRONOUS RECTIFICATION
SO-20
dsON
ORDERING NUMB ERS : L6911C
DESCRIPTION
The device is a power supply controller specifically designed to provide a high performance DC/DC conversion for high current microprocessors. A precise 5-bit
digital to analog converter (DAC) allows adjusting the
output voltage from 1.30V to 2.05V with 50mV binary
steps and from 2.1 0V to 3.50V with 100 mV binary steps.
The high precision internal r eference ass ures the selected output voltage to be within ±1%. The high peak
current gate drive affords to have fast switching to the
external power mos providing low switching losses.
The device assures a fast protection against load
overcurrent and load overvoltage. An ex ternal SCR is
triggered to crowbar the input supply in case of hard
over-voltage. An internal crowbar is also provided
turning on the low side mosfet as long as the overvoltage is detected. In case of ove r-current detection,
the soft start capacitor is discharged and the system
works in HICCUP mode.
L6911CTR
(Tape and Reel)
BLOCK DIAGRAM
November 2001
PGOOD
OVP
VD0
VD1
VD2
VD3
VD4
D01IN1260
Vcc 5 to 12V
VCC OCSET
SS
RT
D/A
MONITOR and
PROTECTION
OSC
+
E/A
COMP
+
PWM
BOOT
UGATE
PHASE
LGATE
PGND
GND
VSEN
VFB
Vin 5V to12V
1.300V to 3.500V
Vo
1/20
L6911C
ABSOLUTE MAXIMUM RATINGS
Symbol Parameter Value Unit
V
CC
V
BOOT-VPHASE
V
HGATE-VPHASE
VCC to GND, PGND 15 V
Boot Voltage 15 V
15 V
OCSET, LGATE, PHASE -0.3 to Vcc+0.3 V
RT, SS, FB, PGOOD, VSEN, VID0-4 7 V
OVP, COMP 6.5 V
THERMAL DATA
Symbol Parameter Value Unit
R
th j-amb
T
PIN CONNECTION
Thermal Resistance Junction to Ambient 110 °
T
Maximum junction temperature 150 °
j
Storage temperature range -40 to 150 °
stg
T
Junction temperature range 0 to 125 °
J
(Top view)
C/W
C
C
C
2/20
VSEN
OCSET
SS/INH
VID0
VID1
VID2
VID3
VID4
COMP PGOOD
FB GND
2
3
4
5
6
7
8
9
10
D98IN958
20
19
18
17
16
15
14
13
12
11
RT1
OVP
VCC
LGATE
PGND
BOOT
UGATE
PHASE
PIN FUNCTION
g
g
g
g
g
g
Pin
Num.
1 VSEN Connected to the output voltage is able to manage over-voltage conditions and the PGOOD
2 OCSET A resistor connected from this pin and the upper Mos Drain sets the current limit protection.
Name Description
signal.
The internal 200µA current
The Over-Current threshold is due to the followin
enerator sinks a current from the drain through the external resistor.
equation:
I
I
--------------------------------------------- -=
P
⋅
OCSETROCSET
R
DSon
L6911C
3 SS/INH
The soft start time is pro
internal current
enerator forces through the capacitor 10µA.
This pin can be used to disable the device forcin
rammed connecting an external capacitor from this pin and GND. The
a voltage lower than 0.4V
4 - 8 VID0 - 4 Voltage Identification Code pins. These input are internally pulled-up and TTL compatible. They
are used to program the output voltage as specified in Table 1 and to set the overvoltage and
power good thresholds.
Connect to GND to program a ‘0’ while leave floating to program a ‘1’.
9 COMP This pin is connected to the error amplifier output and is used to compensate the voltage control
feedback loop.
10 FB This pin is connected to the error amplifier inverting input and is used to compensate the voltage
control feedback loop.
11 GND All the internal references are referred to this pin. Connect it to the PCB signal ground.
12 PGOOD This pin is an open collector output and is pulled low if the output voltage is not within the above
specified thresholds.
If not used may be left floating.
13 PHASE This pin is connected to the source of the upper mosfet and provides the return path for the high
side driver. This pin monitors the drop across the upper mosfet for the current limit
14 UGATE High side gate driver output.
15 BOOT Bootstrap capacitor pin. Through this pin is supplied the high side driver and the upper mosfet.
Connect through a capacitor to the PHASE pin and through a diode to Vcc (cathode vs. boot).
16 PGND Power ground pin. This pin has to be connected closely to the low side mosfet source in order to
reduce the noise injection into the device
17 LGATE This pin is the lower mosfet gate driver output
18 VCC Device supply voltage. The operative nominal supply voltage ranges from 5 to 12V.
DO NOT CONNECT V
TO A VOLTAGE GREATER THAN VCC.
IN
19 OVP Over voltage protection. If the output voltage reaches the 17% above the programmed voltage
this pin is driven high and can be used to drive an external SCR that crowbar the supply voltage.
If not used, it may be left floating.
20 RT Oscillator switching frequency pin. Connecting an external resistor from this pin to GND, the
external frequency is increased according to the equation:
f
S
200kHz
4.94 10
-------------------------+=
R
⋅
kΩ()
T
6
Connecting a resistor from this pin to Vcc (12V), the switching frequency is reduced according to
the equation:
f
S
200kHz
4.306 10
---------------------------- -–=
R
⋅
kΩ()
T
7
If the pin is not connected, the switching frequency is 200KHz.
The volta
e at this pin is fixed at 1.23V (typ). Forcing a 50µA current into this pin, the built in
oscillator stops to switch.
3/20
L6911C
ELECTRICAL CHARACTERISTCS
(VCC = 12V, T
= 25°C unless otherwise specified)
amb
Symbol Parameter Test Condition Min. Typ. Max. Unit
V
SUPPLY CURRENT
CC
Icc Vcc Supply current UGATE and LGATE open 5 mA
POWER-ON
Turn-On Vcc threshold VOCSET=4.5V 4.6 V
Turn-Off Vcc threshold VOCSET=4.5V 3.6 V
Rising V
I
Soft start Current 10 µ
SS
threshold 1.24 V
OCSET
OSCILLATOR
Free running frequency R
Total Variation
∆
Ramp amplitude RT = OPEN 1.9 Vp-p
V
osc
= OPEN 180 200 220 KHz
T
6 KΩ < RT to GND < 200 K
Ω -15 15 %
REFERENCE AND DAC
DACOUT Voltage
Accuracy
VID0, VID1, VID2, VID3, VID4
see Table1; Tamb = 0 to 70°C
-1 1 %
VID Pull-Up voltage 4 V
ERROR AMPLIFIER
DC Gain 88 dB
GBWP Gain-Bandwidth Produ ct 10 MHz
SR Slew-Rate COMP=10pF 10
GATE DRIVERS
I
UGATE
R
UGATE
I
LGATE
High Side Source
Current
High Side Sink
Resistance
Low Side Source
- V
V
BOOT
V
V
I
- V
UGATE
BOOT-VPHASE
= 300mA
UGATE
Vcc=12V, V
PHASE
PHASE
LGATE
=12V,
= 6V
=12V,
= 6V
1 1.3 A
24Ω
0.9 1.1 A
Current
R
LGATE
Low Side Sink
Vcc=12V, I
LGATE
= 300mA
1.5 3 Ω
Resistance
Output Driver Dead Time PHASE connected to GND 120 ns
PROTECTIONS
V
Rising 117 120 %
SEN
= 4.5V 170 200 230 µ
OCSET
> OVP Trip, V
SEN
=0V 60 mA
OVP
I
OCSET
I
OVP
Over Voltage Trip
/DACOUT)
(V
SEN
OCSET Current Source V
OVP Sourcing Current V
POWER GOOD
V
Rising 110 112 114 %
SEN
V
Falling 86 88 90 %
SEN
Upper and Lower threshold 2 %
= -5mA 0.5 V
PGOOD
V
PGOOD
Upper Threshold
/DACOUT)
(V
SEN
Lower Threshold
(V
/DACOUT)
SEN
Hysteresis
(V
/DACOUT)
SEN
PGOOD Voltage Low I
A
V/µS
A
4/20
Table 1. VID Settings
VID4 VID3 VID2 VID1 VID0
01111
01110
01101
01100
01011
01010
01001
01000
00111
00110
00101
00100
00011
00010
00001
00000
Output
Voltage (V)
1.30
1.35
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
1.95
2.00
2.05
L6911C
VID4 VID3 VID2 VID1 VID0
11111Output Off
11110 2.1
11101 2.2
11100 2.3
11011 2.4
11010 2.5
11001 2.6
11000 2.7
10111 2.8
10110 2.9
10101 3.0
10100 3.1
10011 3.2
10010 3.3
10001 3.4
10000 3.5
Output
Voltage (V)
Device Description
The device is an i ntegrated circuit r ealized in BCD technol ogy. It provides c omplete control logic and protections
for a high performance step-down DC-DC converter optimized for microprocessor power supply. It is designed
to drive N-Channel Mosfets in a synchronous-rectified buck topology. The device works properly with Vcc ranging from 5V to 12V and regulates the output voltage starting from a 1.26V power stage s upply voltage (Vin). The
output voltage of the converter can be precisely regulated, programming the VID pins, from 1.3V to 2.05V with
50mV binary steps and from 2.1V to 3.5V with 100mV binary steps, with a maximum tol erance of ±1% over temperature and line voltage variations. The device provides voltage-mode control with fast transient response. It
includes a 200kHz free-running oscillator that is adjustable from 50kHz to 1MHz. The error amplifier features a
15MHz gain-bandwidth product and 10V/
µ
sec slew rate which permits high converter bandwidth for fast transient performance. The resulting PWM duty cycle ranges from 0% to 100%. The device protects against overcurrent conditions enter ing i n HICCUP mode. The devi ce moni tors the cur rent by using the r
DS(ON)
of the upper
MOSFET which eliminates the need for a current sensing resistor.
The device is available in SO20 package.
Oscillator
The switching frequency is internally fixed to 200kHz. The internal oscillator generates the triangular waveform
for the PWM charging and discharging with a constant c urrent an internal capacit or. The current deliver ed to the
oscillator is ty pically 50
µ
A (Fsw=200KHz) and may be varied using an external resistor (RT) connected between
RT pin and GND or VCC. Since the RT pin is maintained at fixed voltage (typ. 1.235V), the frequency is varied
proportionally to the current sunk (forced) from (into) the pin.
In particular connecting it to GND the frequency is increased (current is sunk from the pin), according to the
following relationship:
6
⋅
4.94 10
f
200kHz
S
-------------------------+=
Ω()
R
k
T
Connecting RT to VCC=12V or to VCC=5V the frequency is reduced (current is forced into the pin), according
to the following relationships:
5/20
L6911C
f
S
f
200kHz
S
200kHz
⋅
4.306 10
---------------------------- -+=
R
k
T
⋅
15 10
--------------------+=
R
T
Ω()
Ω()
k
7
V
7
V
CC
CC
= 12V
= 5V
Switching frequency variations vs. R
µ
Note that forcing a 50
A current into this pin, the device stops switching because no current is delivered to the
are reported in Fig.1.
T
oscillator.
Figure 1.
10000
1000
100
Resistance [kOhm]
10
10 100 1000
RT to GN D
RT to VCC =12 V
RT to VCC =5V
Frequency [kHz]
Digital to Analog Converter
The built-in digital to analog converter allows the adjustment of the output voltage from 1.30V to 2.05V with
50mV binary steps and from 2.10V to 3.50V with 100mV binary steps as shown in the previous table 1. The
internal reference is trimmed to ensure the precision of 1%.
The internal reference voltage for the regulation is programmed by the voltage identification (VID) pins. These
are TTL compatible inputs of an internal DAC that is realized by means of a series of resistors providing a partition of the internal voltage reference. The VID code drives a multiplexer that selects a voltage on a precise
point of the divider. The DAC output is delivered to an amplifier obtaining the V
set-point of the error amplifier). Internal pull-ups are provided (realized with a 5
voltage reference (i.e. the
PROG
µ
A current generator); in this
way, to program a logic "1" it is enough to leave the pin floating, while to program a logic "0" it is enough to short
the pin to GND.
The voltage identification (VID) pin configuration also sets the power-good thresholds (PGOOD) and the overvoltage protection (OVP) thresholds.
The VID code "11111" disable the device (as a short on the SS pin) and no output voltage is regulated.
Soft Start and Inhibit
At start-up a ramp is generated charging the external capacitor CSS by means of a 10µA constant current, as
shown in figure 1.
When the voltage across the soft start capacitor (V
6/20
) reaches 0.5V the lower power MOS is turned on to dis-
SS
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