©2002 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1

Features

• Low Start up Current

• Maximum Duty Clamp

• UVLO With Hysteresis

• Operating Frequency up to 500KHz

Description

The UC3842/UC3843/UC3844/UC3845 are fixed

frequencycurrent-mode PWM controller. They are specially

designed for Off-Line and DC to DC converter applications

with minimum external components. These integrated

circuits feature a trimmed oscillator for precise duty cycle

control, a temperature compensated reference, high gain

error amplifier, current sensing comparator and a high

current totempole output for driving a Power MOSFET. The

UC3842 and UC3844 have UVLO thresholds of 16V (on)

and 10V (off). The UC3843 and UC3845 are 8.5V(on) and

7.9V (off). The UC3842 and UC3843 can operate within

100% duty cycle. The UC3844 and UC3845 can operate

with 50% duty cycle.

8-DIP

14-SOP

1

1

8-SOP

1

Internal Block Diagram

UC3842/UC3843/UC3844/UC3845

SMPS Controller

* NORMALLY 8DIP/8SOP PIN NO.

* ( ) IS 14SOP PINNO.

* TOGGLE FLIP FLOP USED ONLY IN UC3844, UC3845