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UC3842A Datasheet(PDF) 1 Page - STMicroelectronics

Part No. UC3842A
Description  HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

UC3842A Datasheet(HTML) 1 Page - STMicroelectronics

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UC2842A/3A/4A/5A
UC3842A/3A/4A/5A
March 1999
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
.
TRIMMED OSCILLATOR DISCHARGE CUR-
RENT
.
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 CURRENT (< 0.5mA)
.
DOUBLE PULSE SUPPRESSION
DESCRIPTION
The UC384xA family of control ICs provides the nec-
essary features to implement off-line or DC to DC
fixed frequency current mode control schemes with
a minimal external parts count. Internally imple-
mented circuits include a trimmed oscillator for pre-
cise DUTY CYCLE CONTROL under voltage lock-
outfeaturingstart-up current less than0.5mA,a pre-
cision reference trimmed for accuracy at the error
amp input, logic to insure latched operation, a PWM
comparatorwhich alsoprovidescurrent limit control,
and a totem pole output stage designed to source
or sink high peakcurrent. The output stage, suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differences between members of this family are the
under-voltagelockout thresholds and maximum duty
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
dutycycles approaching100%.A range of the zeroto
< 50 % is obtainedby theUC3844Aand UC3845Aby
the addition of an internal toggle flip flop which blanks
the output off every other clock cycle.
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
Mini dip
®
SO8
UC3842A
1/15


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