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

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