
UC1525A/27A
UC2525A/27A
UC3525A/27A
Regulating Pulse Width Modulators
FEATURES
8 to 35V Operation
5.1V Reference Trimmed to
±
1%
100Hz to 500kHz Oscillator Range
Separate Oscillator Sync Terminal
Adjustable Deadtime Control
Internal Soft-Start
Pulse-by-Pulse Shutdown
Input Undervoltage Lockout with
Hysteresis
Latching PWM to Prevent Multiple
Pulses
Dual Source/Sink Output Drivers
15
12
3
6
5
7
9
1
2
8
10
Reference
Regulator
16
UVLO
Lockout
4
OSC
3 k
5 k
50
μ
A
PWM
Latch
Flip
Flop
13
14
11
13
11
14
UC1527A Output Stage
UC1525A Output Stage
COMP
S
S
R
+VIN
GROUND
SYNC
RT
CT
DISCHARGE
COMPENSATION
INV INPUT
NI INPUT
SOFTSTART
SHUTDOWN
OUTPUT B
OUTPUT A
VC
OUTPUT B
OUTPUT A
VC
NOR
NOR
OR
OR
V
REF
Error
Amp
VREF
OSC
OUT
To internal
circuitry
BLOCK DIAGRAM
SLUS191B - February 1997 - Revised June 2005
DESCRIPTION
The UC1525A/1527A series of pulse width modulator integrated circuits are de-
signed to offer improved performance and lowered external parts count when
used in designing all types of switching power supplies. The on-chip +5.1V ref-
erence is trimmed to
±
1% and the input common-mode range of the error ampli-
fier includes the reference voltage, eliminating external resistors. A sync input to
the oscillator allows multiple units to be slaved or a single unit to be synchro-
nized to an external system clock. A single resistor between the C
T
and the dis-
charge terminals provides a wide range of dead-time adjustment. These
devices also feature built-in soft-start circuitry with only an external timing ca-
pacitor required. A shutdown terminal controls both the soft-start circuitry and
the output stages, providing instantaneous turn off through the PWM latch with
pulsed shutdown, as well as soft-start recycle with longer shutdown commands.
These functions are also controlled by an undervoltage lockout which keeps the
outputs off and the soft-start capacitor discharged for sub-normal input volt-
ages. This lockout circuitry includes approximately 500mV of hysteresis for jit-
ter-free operation. Another feature of these PWM circuits is a latch following the
comparator. Once a PWM pulse has been terminated for any reason, the out-
puts will remain off for the duration of the period. The latch is reset with each
clock pulse. The output stages are totem-pole designs capable of sourcing or
sinking in excess of 200mA. The UC1525A output stage features NOR logic,
giving a LOW output for an OFF state. The UC1527A utilizes OR logic which
results in a HIGH output level when OFF.