UC2842A/3A/4A/5A
UC3842A/3A/4A/5A
March1999
HIGH PERFORMANCE CURRENTMODE PWM CONTROLLER
.TRIMMED OSCILLATOR DISCHARGE CUR-
RENT
.CURRENT MODE OPERATIONTO 500kHz
.AUTOMATIC FEED FORWARD COMPENSA-
TION
.LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
.INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGELOCKOUT
.HIGHCURRENT TOTEMPOLEOUTPUT
.UNDERVOLTAGE LOCKOUT WITH HYSTER-
ESIS
.LOWSTART-UP CURRENT (< 0.5mA)
.DOUBLEPULSESUPPRESSION
DESCRIPTION
TheUC384xAfamilyofcontrolICsprovidesthenec-
essaryfeatures to implement off-line or DC to DC
fixedfrequencycurrent modecontrolschemeswith
a minimal external parts count. Internally imple-
mentedcircuits includea trimmedoscillatorfor pre-
cise DUTY CYCLE CONTROL under voltagelock-
outfeaturingstart-upcurrentlessthan0.5mA,a pre-
cision reference trimmed for accuracy at the error
ampinput,logicto insurelatchedoperation,a PWM
comparatorwhichalsoprovidescurrentlimitcontrol,
and a totempole output stage designed to source
orsinkhighpeakcurrent.Theoutputstage,suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differencesbetween members of thisfamily are the
under-voltagelockoutthresholdsandmaximumduty
cycle ranges. The UC3842A and UC3844A have
UVLO thresholds of 16V (on) and 10V (off), ideally
suitedoff-lineapplicationsThecorrespondingthresh-
olds for the UC3843A and UC3845A are 8.5 V and
7.9V. The UC3842A and UC3843A can operate to
dutycyclesapproaching100%.Arangeofthezeroto
<50%isobtainedbytheUC3844AandUC3845Aby
theadditionofaninternaltoggleflipflopwhichblanks
theoutputoffeveryotherclockcycle.
BLOCK DIAGRAM (toggle flip flop used only in UC3844A and UC3845A)
UVLO
S/R 5V
REF
34V
INTERNAL
BIAS
VREF GOOD
LOGIC
2.50V
T
S
R
OSC
R1V
CURRENT
SENSE
COMPARATOR
2R
+
- PWM
LATCH
7
5
4
2
1
3
8
6
ERROR AMP.
Vi
GROUND
RT/CT
VFB
COMP
CURRENT
SENSE
VREF
5V 50mA
OUTPUT
D95IN331
Minidip
SO8
UC3842A
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