The UC1875 family of integrated circuits implements control of a bridge
power stage by phase-shifting the switching of one half-bridge with respect
to the other, allowing constant frequency pulse-width modulation in combi
nation with resonant, zero-voltage switching for high efficiency performance
at high frequencies. This family of circuits may be configured to provide
control in either voltage or current mode operation, with a separate
over-current shutdown for fast fault protection.
A programmable time delay is provided to insert a dead-time at the turn-on
of each output stage. This delay, providing time to allow the resonant
switching action, is independently controllable for each output pair (A-B,
C-D).
With the oscillator capable of operation at frequencies in excess of 2MHz,
overall switching frequencies to 1MHz are practical. In addition to the stan
dard free running mode, with the CLOCKSYNC pin, the user may configure
these devices to accept an external clock synchronization signal, or may
lock together up to 5 units with the operational frequency determined by the
fastest device.
Protective features include an undervoltage lockout which maintains all outputs in an active-low state until the supply reaches a 10.75V threshold.
1.5V hysteresis is built in for reliable, boot-strapped chip supply.
Over-current protection is provided, and will latch the outputs in the OFF
state within 70nsec of a fault. The current-fault circuitry implements
full-cycle restart operation.
application
INFO
available
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
-
-
07/99
UDG-95073
Page 2
DESCRIPTION (cont.)
Additional features include an error amplifier with band
width in excess of 7MHz, a 5V reference, provisions for
soft-starting, and flexible ramp generation and slope com
pensation circuitry.
These devices are available in 20-pin DIP, 28-pin
“bat-wing” SOIC and 28 lead power PLCC plastic pack
ages for operation over both 0°C to 70°C and –25°C to
+85°C temperature ranges; and in hermetically sealed
cerdip, and surface mount packages for –55°C to +125°C
operation.
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . +300°C
Note:Pin references are to 20 pin packages.All voltages are
with respect to ground.Currents are positive into, negative out of, device terminals. Consult Unitrode
databook for information regarding thermal specifications and limitations of packages.
SOIC-28, (Top View)
DWP Package
CONNECTION DIAGRAMS
Dil-20 (Top View)
J or N Package
PLCC-28 (Top View)
QP Package
2
Page 3
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
ELECTRICAL CHARACTERISTICS:
85°C for the UC2875/6/7/8 and 0°C < T
R
SLOPE
= 12kΩ, C
RAMP
= 200pF, C
DELAYSET A-B=CDELAYSET C-D
Unless otherwise stated, –55°C < TA< 125°C for the UC1875/6/7/8, –25°C < TA<
< 70°C for the UC3875/6/7/8, VC = VIN = 12V, R
A
= 0.01µF, I
DELAYSET A-B=IDELAYSET C-D
FREQSET
= 12kΩ,C
FREQSET
= –500µA, TA=TJ.
= 330pF,
PARAMETERTEST CONDITIONSMINTYPMAX UNITS
Undervoltage Lockout
Start ThresholdUC1875/UC187710.75 11.75V
UC1876/UC187815.25V
UVLO HysteresisUC1875/UC18770.51.252.0V
UC1876/UC18786.0V
Supply Current
I
StartupVIN = 8V, VC = 20V, R
IN
StartupVIN = 8V, VC = 20V, R
I
C
I
IN
I
C
SLOPE
SLOPE
open, I
open, I
= 0150600µA
DELAY
= 010100µA
DELAY
3040mA
1530mA
Voltage Reference
Output VoltageT
= +25°C4.9255.08V
J
Line Regulation11 < VIN < 20V110mV
Load RegulationI
= –10mA520mV
VREF
Total VariationLine, Load, Temperature4.95.1V
Noise Voltage10Hz to 10kHz50µVrms
Long Term StabilityT
Short Circuit CurrentVREF = 0V, T
= 125°C, 1000 hours2.5mV
J
= 25°C60mA
J
Error Amplifier
Offset Voltage515mV
Input Bias Current0.63µA
AVOL1V < V
CMRR1.5V < V
< 4V6090dB
E/AOUT
< 5.5V7595dB
CM
PSRR11V < VIN < 20V85100dB
Output Sink CurrentV
Output Source CurrentV
Output Voltage HighI
Output Voltage LowI
E/AOUT
E/AOUT
= 1V12.5mA
E/AOUT
= 4V–1.3–0.5mA
E/AOUT
= –0.5mA44.75V
= 1mA00.51V
Unity Gain BW711MHz
Slew Rate611V/µsec
3
Page 4
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
ELECTRICAL CHARACTERISTICS:
85°C for the UC2875/6/7/8 and 0°C < T
R
SLOPE
= 12kΩ, C
RAMP
= 200pF, C
DELAYSET A-B=CDELAYSET C-D
Unless otherwise stated, –55°C < TA< 125°C for the UC1875/6/7/8, –25°C < TA<
< 70°C for the UC3875/6/7/8, VC = VIN = 12V, R
A
= 0.01µF, I
DELAYSET A-B=IDELAYSET C-D
FREQSET
= 12kΩ,C
FREQSET
= –500µA, TA=TJ.
= 330pF,
PARAMETERTEST CONDITIONSMINTYPMAX UNITS
PWM Comparator
Ramp Offset VoltageT
= 25°C (Note 3)1.3V
J
Zero Phase Shift Voltage(Note 4)0.550.9V
PWM Phase Shift (Note1)V
V
Output Skew (Note 1)V
> (Ramp Peak + Ramp Offset)9899.5102%
E/AOUT
< Zero Phase Shift Voltage00.52%
E/AOUT
< 1V5±20nsec
E/AOUT
Ramp to Output DelayUC3875/6/7/8 (Note 6)65100nsec
UC1875/6/7/8, UC2875/6/7/8 (Note 6)65125nsec
Oscillator
Initial AccuracyT
= 25°C0.8511.15MHz
J
Voltage Stability11V < VIN < 20V0.22%
Total VariationLine, Temperature0.801.20MHz
Sync Pin ThresholdT
Clock Out PeakT
Clock Out LowT
Unless otherwise stated, –55°C < TA< 125°C for the UC1875/6/7/8, –25°C < TA<
< 70°C for the UC3875/6/7/8, VC = VIN = 12V, R
A
= 0.01µF, I
DELAYSET A-B=IDELAYSET C-D
FREQSET
PARAMETERTEST CONDITIONSMINTYPMAX UNITS
Output Drivers
Output Low LevelI
Output High LevelI
= 50mA0.20.4V
OUT
= 500mA1.22.6V
I
OUT
= –50mA1.52.5V
OUT
= –500mA1.72.6V
I
OUT
Delay Set (UC1875 and UC1876 only)
Delay Set VoltageI
Delay TimeI
= –500µA2.32.42.6V
DELAY
= –250µA (Note 5) (UC3875/6/7/8,
DELAY
UC2875/6/7/8)
= –250µA (Note 5) (UC1875/6/7/8)150250600nsec
I
DELAY
Note 1: Phase shift percentage (0% = 0°, 100% = 180°) is defined as
fined in Figure 1. At 0% phase shift,
is the output skew.
θ=
200
T
, whereis the phase shift, andand T are de
Φ%
Note 2: Delay time is defined as delay = T (1/2–(duty cycle)), where T is defined in Fig. 1.
Note 3: Ramp offset voltage has a temperature coefficient of about –4mV/°C.
Note 4: Zero phase shift voltage has a temperature coefficient of about –2mV/°C.
Note 5: Delay time can be programmed via resistors from the delay set pins to ground.Delay time
I
DELAY
=
R
DELAY
The recommended range for I
DELAY
is 25 A I
DELAY
1mA
Delay set voltage
Note 6: Ramp delay to output time is defined in Fig.2.
= 12kΩ,C
FREQSET
= 330pF,
= –500µA, TA=TJ.
150250400nsec
12.–
•62 5 10
≅
I
DELAY
sec. Where
-
Duty Cycle = t/T
Period = T
T
(A to C) = T
DHL
(B to D) = Φ
DHL
Phase Shift, Output Skew & Delay Time Definitions
Figure 1
UDG-95074
UDG-95075
Figure 2
5
Page 6
PIN FUNCTIONAL DESCRIPTIONS
CLOCKSYNC (bi-directional clock and synchroniza
tion pin): Used as an output, this pin provides a clock
signal. As an input, this pin provides a synchronization
point. In its simplest usage, multiple devices, each with
their own local oscillator frequency, may be connected to
gether by the CLOCKSYNC pin and will synchronize on
the fastest oscillator. This pin may also be used to syn
chronize the device to an external clock, provided the ex
ternal signal is of higher frequency than the local
oscillator. A resistor load may be needed on this pin to
minimize the clock pulse width.
E/AOUT (error amplifier output): This is is the gain
stage for overall feedback control. Error amplifier output
voltage levels below 1 volt will force 0° phase shift. Since
the error amplifier has a relatively low current drive capa
bility, the output may be overridden by driving with a suffi
ciently low impedance source.
CS+ (current sense): The non-inverting input to the cur
rent-fault comparator whose reference is set internally to
a fixed 2.5V (separate from VREF). When the voltage at
this pin exceeds 2.5V the current-fault latch is set, the
outputs are forced OFF and a SOFT-START cycle is initiated. If a constant voltage above 2.5V is applied to this
pin the outputs are disabled from switching and held in a
low state until the CS+ pin is brought below 2.5V. The
outputs may begin switching at 0 degrees phase shift before the SOFTSTART pin begins to rise -- this condition
will not prematurely deliver power to the load.
FREQSET (oscillator frequency set pin): A resistor
and a capacitor from FREQSET to GND will set the oscil
lator frequency.
DELAYSET A-B, DELAYSET C-D (output delay con
trol): The user programmed current flowing from these
pins to GND set the turn-on delay for the corresponding
output pair. This delay is introduced between turn-off of
one switch and turn-on of another in the same leg of the
bridge to provide a dead time in which the resonant
switching of the external power switches takes place.
Separate delays are provided for the two half-bridges to
accommodate differences in the resonant capacitor
charging currents.
EA– (error amplifier inverting input): This is normally
connected to the voltage divider resistors which sense
the power supply output voltage level.
EA+ (error amplifier non-inverting input): This is nor
mally connected to a reference voltage used for compari
son with the sensed power supply output voltage level at
the EA+ pin.
GND (signal ground): All voltages are measured with
respect to GND. The timing capacitor, on the FREQSET
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
pin, any bypass capacitor on the VREF pin, bypass ca
-
pacitors on VIN and the ramp capacitor, on the RAMP
pin, should be connected directly to the ground plane
near the signal ground pin.
-
OUTA-OUTD (outputs A-D): The outputs are 2A to
tem-pole drivers optimized for both MOSFET gates and
-
level-shifting transformers. The outputs operate as pairs
-
with a nominal 50% duty-cycle. The A-B pair is intended
to drive one half-bridge in the external power stage and
is syncronized with the clock waveform. The C-D pair will
drive the other half-bridge with switching phase shifted
with respect to the A-B outputs.
PWRGND (power ground): VC should be bypassed with
a ceramic capacitor from the VC pin to the section of the
-
ground plane that is connected to PWRGND. Any re
-
quired bulk reservoir capacitor should parallel this one.
Power ground and signal ground may be joined at a sin
gle point to optimize noise rejection and minimize DC
drops.
RAMP (voltage ramp): This pin is the input to the PWM
comparator. Connect a capacitor from here to GND. A
voltage ramp is developed at this pin with a slope:
SenseVoltage
dV
=
dT
RC
SLOPERAMP
Current mode control may be achieved with a minimum
amount of external circuitry, in which case this pin provides slope compensation.
Because of the 1.3V offset between the ramp input and
-
the PWM comparator, the error amplifier output voltage
can not exceed the effective ramp peak voltage and duty
cycle clamping is easily achievable with appropriate val
-
ues of R
SLOPE
SLOPE (set ramp slope/slope compensation): A resis
tor from this pin to VCC will set the current used to gen
erate the ramp. Connecting this resistor to the DC input
line voltage will provide voltage feed-forward.
SOFTSTART (soft start): SOFTSTART will remain at
GND as long as VIN is below the UVLO threshold.
SOFTSTART will be pulled up to about 4.8V by an inter
nal 9µA current source when VIN becomes valid (assum
ing a non-fault condition). In the event of a current-fault
(CS+ voltage exceeding 2.5V), SOFTSTART will be
pulled to GND and them ramp to 4.8V. If a fault occurs
during the SOFTSTART cycle, the outputs will be imme
diately disabled and SOFTSTART must charge fully prior
to resetting the fault latch.
For paralleled controllers, the SOFTSTART pins may be
paralled to a single capacitor, but the charge currents will
be additive.
6
•
and C
RAMP
.
-
-
-
-
-
-
-
-
-
-
Page 7
PIN FUNCTIONAL DESCRIPTIONS (cont.)
VC (output switch supply voltage): This pin supplies
power to the output drivers and their associated bias cir
cuitry. Connect VC to a stable source above 3V for nor
mal operation, above 12V for best performance. This
supply should be bypassed directly to the PWRGND pin
with low ESR, low ESL capacitors.
VIN (primary chip supply voltage): This pin supplies
power to the logic and analog circuitry on the integrated
circuit that is not directly associated with driving the out
put stages. Connect VIN to a stable source above 12V
for normal operation. To ensure proper chip functionality,
these devices will be inactive until VIN exceeds the upper
undervoltage lockout threshold. This pin should by by
passed directly to the GND pin with low ESR, low ESL
capacitors.
APPLICATIONS INFORMATION
UNDERVOLTAGE LOCKOUT SECTION
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
NOTE: When VIN exceeds the UVLO threshold the sup
-
ply current (I
-
in excess of 20µA. If the UC1875 is not connected to a
well bypassed supply, it may immediately enter UVLO
again.
VREF: This pin is an accurate 5V voltage reference. This
output is capable of delivering about 60mA to peripheral
circuitry and is internally short circuit current limited.
-
VREF is disabled while VIN is low enough to force the
chip into UVLO. The circuit is also in UVLO until VREF
reaches approximately 4.75V. For best results bypass
VREF with a 0.1µF, low ESR, low ESL, capacitor to the
-
GND pin.
) will jump from about 100µA to a current
IN
-
When power is applied to the circuit and VIN is below
the upper UVLO threshold, I
will be below 600µA, the
IN
reference generator will be off, the fault latch is reset,
the soft-start pin is discharged, and the outputs are actively
held low. When VIN exceeds the upper UVLO thresh
old, the reference generator turns on. All else remains
in the shut-down mode until the output of the reference,
VREF, exceeds 4.75V.
UDG-95076
free-running operation, the frequency is set via an ex
ternal resistor and capacitor to ground from the
FREQSET pin.
-
-
UDG-95077
UDG-95079
UDG-95078
7
Page 8
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
APPLICATIONS INFORMATION (cont.)
SYNCHRONIZING THE OSCILLATOR
The CLOCKSYNC pin of the oscillator may be used to synchronize multiple UC1875 devices simply by connecting
the CLOCKSYNC of each UC1875 to the others:
1875/6/7/8s only
UDG-95080
All ICs will sync to chip with the fastest local oscillator.
R1 & RN
R1 & RN
may
be needed to keep sync pulse narrow due to capacitance on line.
may
also be needed to properly terminate R
SYNC
line.
Syncing to external TTL/CMOS
ICs will sync to fastest chip or TTL clock if it is higher frequency.
may
R & RN
Although each UC1875/6/7/8 has a local oscillator fre
quency, the group of devices will synchronize to the
fastest oscillator driving the CLOCKSYNC pin. This ar
rangement allows the synchronizing connection be
tween ICs to be broken without any local loss of
functionality.
Synchronizing the device to an external clock signal
may be accomplished with a minimum of external cir
cuitry, as shown in the previous figure.
be needed for same reasons as above
-
Capacitive loading on the CLOCKSYNC pin will in
crease the clock pulse width, and may adversely effect
-
system performance. Therefore, a resistor to ground
-
from the CLOCKSYNC pin is optional, but may be re
quired to offset capacitive loading on this pin. These re
sistors are shown in the oscillator schematics as R1,
RN.
-
UDG-95081
-
-
-
8
Page 9
APPLICATIONS INFORMATION (cont.)
DELAY BLOCKS AND OUTPUT STAGES
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
In each of the output stages, transistors Q3 through Q6
form a high-speed totem-pole driver which will source
or sink more than one amp peak with a total delay of
approximately 30 nanoseconds. To ensure a low output
level prior to turn-on, transistors Q7 through Q9 form a
self-biased driver to hold Q6 on prior to the supply
reaching its turn-on threshold. This circuit is operable
when the chip supply is zero. Q6 is also turned on and
held low with a signal from the fault logic portion of the
chip.
The delay providing the dead-time is accomplished with
C1 which must discharge to V
before the output can
TH
go high. The time is defined by the current sources, I1,
which is programmed by an external resistor, R
TD
. The
2.5V and the range of dead time control is
from 50 to 200 nanoseconds. NOTE: There is no way
to disable the delay circuitry, and the delay time must
be programmed.
voltage on the Delay Set pins is internally regulated to
OUTPUT SWITCH ORIENTATION
The four outputs of the UC1875/6/7/8 interface to the full bridge converter switches as shown below:
UDG-95082
3 Winding Bifilar, AWG 30 Kynar Insulation
9
UDG-95083
Page 10
APPLICATIONS INFORMATION (cont.)
FAULT/SOFT-START
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
The fault control circuitry provides two forms of power
shutdown:
• Complete turn-off of all four output power stages.
• Clamping the phase shift command to zero.
Complete turn-off is ordered for an over-current fault or
a low supply voltage. When the SOFTSTART pin
reaches its low threshold, switching is allowed to pro
ceed while the phase-shift is advanced from zero to its
nominalvaluewith thetimeconstantofthe
SOFT-START capacitor.
The fault logic insures that a continuous fault will insti
tute a low frequency “hiccup” retry cycle by forcing the
SOFT-START capacitor to charge through its full cycle
between each restart attempt.
-
-
UDG-95084
UDG-95085
10
Page 11
APPLICATIONS INFORMATION (cont.)
SLOPE/RAMP PINS
UC1875/6/7/8
UC2875/6/7/8
UC3875/6/7/8
The ramp generator may be configured for the following
control methods:
• Voltage Mode
• Voltage Feedforward
• Current Mode
• Current Mode with Slope Compensation
Voltage Mode Operation
The figure below shows a voltage-mode configuration.
With R
SLOPE
form on C
conventional voltage-mode control. If R
tied to a stable voltage source, the wave
will be a constant-slope ramp, providing
RAMP
SLOPE
is con
nected to the power supply input voltage, a vari
able-slope ramp will provide voltage feedforward.
1.Simplevoltagemodeoperation
achieved by placing R
SLOPE
between VIN
and SLOPE.
2. Voltage Feedforward achieved by placing R
SLOPE
between supply voltage and
SLOPE pin of UC1875.
RAMP
dV
≈
dTVRC
Rslope
•
SLOPERAMP
-
-
-
UDG-95086
For current-mode control the ramp generator may be disabled by grounding the slope pin and using the ramp pin
as a direct current sense input to the PWM comparator.
T exas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue
any product or service without notice, and advise customers to obtain the latest version of relevant information
to verify, before placing orders, that information being relied on is current and complete. All products are sold
subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those
pertaining to warranty, patent infringement, and limitation of liability.
TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent
TI deems necessary to support this warranty . Specific testing of all parameters of each device is not necessarily
performed, except those mandated by government requirements.
CERTAIN APPLICA TIONS USING SEMICONDUCT OR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF
DEATH, PERSONAL INJURY, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE (“CRITICAL
APPLICATIONS”). TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED, AUTHORIZED, OR
WARRANTED TO BE SUITABLE FOR USE IN LIFE-SUPPORT DEVICES OR SYSTEMS OR OTHER
CRITICAL APPLICA TIONS. INCLUSION OF TI PRODUCTS IN SUCH APPLICATIONS IS UNDERST OOD TO
BE FULLY AT THE CUSTOMER’S RISK.
In order to minimize risks associated with the customer’s applications, adequate design and operating
safeguards must be provided by the customer to minimize inherent or procedural hazards.
TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent
that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other
intellectual property right of TI covering or relating to any combination, machine, or process in which such
semiconductor products or services might be or are used. TI’s publication of information regarding any third
party’s products or services does not constitute TI’s approval, warranty or endorsement thereof.
Copyright 1999, Texas Instruments Incorporated
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