©2000 Fairchild Semiconductor International

www.fairchildsemi.com

Rev. 5.0

Features

• Low Start Current 0.2mA (typ)

• Operating Range Up To 500KHz

• Cycle by Cycle Current Limiting

• Under Voltage Lock Out With Hysteresis

• Short Shutdown Delay Time: typ.100ns

• High Current Totem-pole Output

• Output Swing Limiting: 22V

Description

The UC3842A/UC3843A are fixed PWM controller for Off-

Line and DC to DC converter applications. The internal cir-

cuits include UVLO, low start up current circuit, tempera-

ture compensated reference, high gain error amplifier,

current sensing comparator, and high current totem-pole out-

put for driving a POWER MOSFET. Also UC3842A/

UC3843A provide low start up current below 0.3mA and

short shutdown delay time typ. 100ns. The UC3842A has

UVLO threshold of 16V(on) and 10V(off). The UC3843A is

8.4V(on) and 7.6V(off). The UC3842A and UC3843A can

operate within 100% duty cycle.

8-DIP

8-SOP

1

1

Internal Block Diagram

8

+

-

7

5

SET/

RESET

5V

Internal

Bias

Good LOGIC

2

R

S

OSCILLATOR

1

3

4

T

7

6

5

UVLO

29V

22V

1/3

Error Amp

1V

C.S

Comp.

PWM

LATCH

COMP

C.S

OUTPUT

GND

PWR

PWR

GND

UC3842A/UC3843A

SMPS Controller