UC2842/3/4/5

UC3842/3/4/5

October 1998

CURRENT MODE PWM CONTROLLER

.

OPTIMIZED FOR OFF-LINE AND DC TO DC

CONVERTERS

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LOW START-UP CURRENT (< 1 mA)

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AUTOMATIC FEED FORWARD COMPENSA-

TION

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PULSE-BY-PULSE CURRENT LIMITING

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ENHANCED LOAD RESPONSE CHARAC-

TERISTICS

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UNDER-VOLTAGE LOCKOUT WITH HYSTER-

ESIS

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DOUBLE PULSE SUPPRESSION

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HIGH CURRENT TOTEM POLE OUTPUT

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INTERNALLY TRIMMED BANDGAP REFER-

ENCE

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500 KHz OPERATION

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DESCRIPTION

The UC3842/3/4/5family 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. Internallyimple-

mentedcircuits include undervoltagelockout featur-

ing start-up current less than 1 mA, a precision ref-

erence trimmed for accuracy at the error amp input,

logic to insure latched operation, a PWM compara-

torwhich also providescurrent limit control,and a to-

tem 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 UC3842 and UC3844 have

UVLO thresholds of 16V (on) and 10V (off), ideally

suited off-line applications The corresponding

thresholds for the UC3843 and UC3845 are 8.5 V

and 7.9 V. The UC3842 and UC3843 can operate

to duty cycles approaching 100%. A range of the

zero to < 50 % is obtained by the UC3844 and

UC3845 by the additionof an internal toggle flip flop

which blanks the output off every other clock cycle.

BLOCK DIAGRAM (toggle flip flop used only in U3844 and UC3845)

Minidip

SO14

®

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