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MC68HC705BD7B
UC3843B
TRIMMED OSCILLATOR FOR PRECISE FRE-
QUENCY CONTROL
OSCILLATOR FREQUENCY GUARANTEED
AT 250kHz
CURRENT MODE OPERATION TO 500kHz
AUTOMATIC FEED FORWARD COMPENSA-
TION
LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGE LOCKOUT
HIGH CURRENT TOTEM POLE OUTPUT
UNDERVOLTAGE LOCKOUT WITH HYSTER-
ESIS
LOW START-UPAND OPERATINGCURRENT
DESCRIPTION
TheUC384xBfamilyofcontrolICsprovidesthenec-
essary features to implement off-line or DC to DC
fixedfrequencycurrent modecontrol schemes with
a minimal external parts count. Internally imple-
mentedcircuits includea trimmed oscillator for pre-
cise DUTY CYCLE CONTROL under voltage lock-
outfeaturingstart-upcurrentlessthan0.5mA,a pre-
cision reference trimmed for accuracy at the error
ampinput,logicto insurelatchedoperation,a PWM
comparatorwhichalsoprovidescurrentlimitcontrol,
and a totem pole output stage designed to source
orsinkhighpeakcurrent.Theoutputstage,suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differencesbetween members of this family are the
under-voltagelockoutthresholdsand maximum duty
cycle ranges. The UC3842B and UC3844B have
UVLO thresholds of 16V (on) and 10V (off), ideally
suitedoff-lineapplicationsThecorrespondingthresh-
oldsfortheUC3843BandUC3845Bare8.5Vand7.9
V. The UC3842B and UC3843B can operateto duty
cycles approaching100%. A range of the zero to <
50 % is obtainedby the UC3844Band UC3845Bby
theadditionofan internaltoggleflip flopwhichblanks
theoutputoffeveryotherclock cycle.
BLOCK DIAGRAM
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
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