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廬

UC2842B/3B/4B/5B

UC3842B/3B/4B/5B

HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

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.

.

.

.

.

.

.

.

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

DESCRIPTION

TheUC384xB family ofcontrolICs providesthe 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, logicto insure latched operation,a PWM

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.

Differences between members of this family are the

under-voltagelockout thresholds and maximum duty

cycle ranges. The UC3842B and UC3844B have

UVLO thresholds of 16V (on) and 10V (off), ideally

suitedoff-lineapplicationsThecorrespondingthresh-

oldsforthe UC3843BandUC3845Bare8.5 V and7.9

V. The UC3842B and UC3843B can operate to duty

cycles approaching 100%. A range of the zero to <

50 % is obtained by the UC3844B and UC3845B by

the addition of an internal toggle flip flopwhich blanks

the output off every otherclock cycle.

BLOCK DIAGRAM

(toggle flip flop used only in UC3844B and UC3845B)

Vi

7

34V

GROUND

5

UVLO

S/R

5V

REF

INTERNAL

BIAS

VREF GOOD

LOGIC

RT/CT

4

OSC

ERROR AMP.

2R

R

1V

T

8

VREF

5V 50mA

2.50V

6

OUTPUT

VFB

COMP

CURRENT

SENSE

2

1

3

+

-

S

R

CURRENT

SENSE

COMPARATOR

PWM

LATCH

UC3842B

D95IN331

March 1999

1/15

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