®
UC2842/3/4/5
UC3842/3/4/5
CURRENT MODE PWM CONTROLLER
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OPTIMIZED FOR OFF-LINE AND DC TO DC
CONVERTERS
LOW START-UP CURRENT (< 1 mA)
AUTOMATIC FEED FORWARD COMPENSA-
TION
PULSE-BY-PULSE CURRENT LIMITING
ENHANCED LOAD RESPONSE CHARAC-
TERISTICS
UNDER-VOLTAGE LOCKOUT WITH HYSTER-
ESIS
DOUBLE PULSE SUPPRESSION
HIGH CURRENT TOTEM POLE OUTPUT
INTERNALLY TRIMMED BANDGAP REFER-
ENCE
500 KHz OPERATION
LOW R
O
ERROR AMP
Minidip
SO14
DESCRIPTION
TheUC3842/3/4/5family of controlICs provides the
necessary features to implement off-line or DC to
DC fixed frequency current mode control schemes
with a minimal externalparts 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 providescurrentlimit 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 theoff-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)
October 1998
1/11