
UCC1570
UCC2570
UCC3570
Low Power Pulse Width Modulator
DESCRIPTION
The UCC1570 family of pulse width modulators is intended for application
in isolated switching supplies using primary side control and a voltage
mode feedback loop. Made with a BiCMOS process, these devices feature
low startup current for efficient off-line starting with a bootstrapped low volt-
age supply. Operating current is also very low; yet these devices maintain
the ability to drive a power MOSFET gate at frequencies above 500kHz.
Voltage feedforward provides fast and accurate response to wide line volt-
age variation without the noise sensitivity of current mode control. Fast cur-
rent limiting is included with the ability to latch off after a programmable
number of repetitive faults has occurred. This allows the power supply to
ride through a temporary overload, while still shutting down in the event of
a permanent fault. Additional versatility is provided with a maximum duty
cycle clamp programmable within a 20% to 80% range and line voltage
sensing with a programmable window of allowable operation.
10 I3
10 I4
I3
I4
I4
I4
10
7
4
11
9
VFWD
FREQ
SLOPE
RAMP
ISET
CLOCK
GENERATOR
1V
4V
HIGH
LINE
LOW
LINE
1V
FEEDBK
SOFTST
CURLIM
CURRENT
LIMIT
CLK
RAMP
VALLEY
RAMP
PEAK
RAMP
LATCH
4V
1V
S
R
5V
GENERATOR
4.5V
VREF
GND
15V
13/9V
VCC
OUT
PGND
PWM
LATCH
PWM
R
D
S
R
0.2V
CLK
COUNT
8
14
2
1
4V
SHUTDOWN
LATCH
SHUTDOWN
0.6V
R
S
D
6
12
13
3
5
S
D
BLOCK DIAGRAM
04/99
FEATURES
Low Power BiCMOS Process
85
μ
A Start-up Current
1mA Run Current
1A Peak Gate Drive Output
Voltage Feed Forward
Programmable Duty Cycle Clamp
Optocoupler Interface
500kHz Operation
Soft Start
Fault Counting Shutdown
Fault Latch Off or Automatic Restart