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

15V

GND

13/9V

VCC

OUT

PGND

PWM

PWM

LATCH

S

R

0.2V

CLK

COUNT

8

14

2

1

4V

SHUTDOWN

LATCH

SHUTDOWN

0.6V

R

6

12

13

3

5

BLOCK DIAGRAM

04/99

FEATURES

• Low Power BiCMOS Process

• 85µA 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