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1 FEATURES

■ TRIMMED OSCILLATOR DISCHARGE

CURRENT

■ CURRENT MODE OPERATION TO 500kHz

AUTOMATIC FEED FORWARD

COMPENSATION

■ LATCHING PWM FOR CYCLE-BY-CYCLE

CURRENT LIMITING

■ INTERNALLY TRIMMED REFERENCE WITH

UNDERVOLTAGE LOCKOUT

■ HIGH CURRENT TOTEM POLE OUTPUT

UNDERVOLTAGE LOCKOUT WITH

HYSTERESIS

■ LOW START-UP CURRENT (< 0.5mA)

DOUBLE PULSE SUPPRESSION

2 DESCRIPTION

The UC384xA family of control ICs provides the

necessary features to implement off-line or DC to

DC fixed frequency current mode control schemes

with a minimal external parts count. Internally implemented circuits include a trimmed oscillator for

precise DUTY CYCLE CONTROL under voltage

lockout featuring start-up current less than 0.5mA,

a precision reference trimmed for accuracy at the

error amp input, logic to insure latched operation,

a PWM comparator which also provides current

limit control, and a totem pole output stage designed to source or sink high peak current. The

output stage, suitable for driving N-Channel MOSFETs, is low in the off-state.

Differences between members of this family are

the under-voltage lockout thresholds and maximum duty cycle ranges. The UC3842A and

UC3844A have UVLO thresholds of 16V (on) and

10V (off), ideally suited off-line applications The

corresponding thresholds for the UC3843A and

UC3845A are 8.5 V and 7.9V. The UC3842A and

UC3843A can operate to duty cycles approaching

100. A range of the zero to < 50 is obtained by

the UC3844A and UC3845A by the addition of an

internal toggle flip flop which blanks the output off

every other clock cycle.