电源pwm控制ic uc3842b

.optimized for off-line and dc to dc
.converters .lowstart-up current (< 1 ma) automatic feed forward compensa.
tion .pulse-by-pulsecurrent limiting enhanced load response charac.
teristics under-voltagelockoutwithhyster.
esis .double pulse suppression .high current totempoleoutput internally trimmed bandgap refer.
ence .500 khz operation lowro erroramp
description
theuc3842/3/4/5family of controlics providesthe
necessary features to implement off-line or dc to
dc fixed frequency current mode controlschemes
witha minimal externalparts count.internallyimplementedcircuitsincludeundervoltagelockout
featuring
start-up current less than 1 ma, a precision reference
trimmed for accuracy at the erroramp input,
logic to insure latched operation, a pwmcomparatorwhichalso
providescurrentlimit control,anda totem
pole output stage designed to source or sink
high peak current. the output stage, suitable for
driving n-channelmosfets, is low in theoff-state.
differencesbetweenmembers 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
uc3845by the additionof aninternal toggleflip flop
which blanks the output off every other clock cycle.