UC2842B/3B/4B/5B
® UC3842B/3B/4B/5B
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
. 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-UP AND OPERATING CURRENT
Minidip
SO8
comparatorwhich alsoprovidescurrent limit control,
and a totem pole output stage designed to source
or sink high peakcurrent. The outputstage, suitable
for driving N-Channel MOSFETs, is low in the off-
state.
DESCRIPTION
Differences between members of this family are the
under-voltagelockout thresholds and maximum duty
TheUC384xB family ofcontrolICs providesthe nec- cycle ranges. The UC3842B and UC3844B have
essary features to implement off-line or DC to DC UVLO thresholds of 16V (on) and 10V (off), ideally
fixed frequency current mode control schemes wwwwit.DhataSheet4Us.cuomitedoff-lineapplicationsThecorrespondingthresh-
a minimal external parts count. Internally imple- oldsforthe UC3843BandUC3845Bare8.5 V and7.9
mented circuits include a trimmed oscillator for pre- V. The UC3842B and UC3843B can operate to duty
cise DUTY CYCLE CONTROL under voltage lock- cycles approaching 100%. A range of the zero to <
outfeaturingstart-up current less than0.5mA,a pre- 50 % is obtained by the UC3844B and UC3845B by
cision reference trimmed for accuracy at the error the addition of an internal toggle flip flopwhich blanks
amp input, logicto insure latched operation,a PWM the output off every otherclock cycle.
BLOCK DIAGRAM (toggle flip flop used only in UC3844B and UC3845B)
7
Vi
5
GROUND
4
RT/CT
VFB
COMP
CURRENT
SENSE
2
1
3
34V UVLO
S/R
5V
REF
2.50V
OSC
VREF GOOD
LOGIC
INTERNAL
BIAS
T
8 VREF
5V 50mA
6
OUTPUT
ERROR AMP.
+
-
2R
R
S
R
1V
CURRENT
SENSE
COMPARATOR
PWM
LATCH
UC3842B
D95IN331
March 1999
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