UC2842B/3B/4B/5B

UC3842B/3B/4B/5B

March 1999

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

DESCRIPTION

The UC384xB 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-

out featuring start-up current less than 0.5mA, a pre-

cision reference trimmed for accuracy at the error

amp input, logic to insure latched operation, a PWM

comparator which also provides current limit control,

and a totem pole output stage designed to source

or sink high peak current. The output stage, suitable

for driving N-Channel MOSFETs, is low in the off-

state.

Differences between members of this family are the

under-voltage lockout thresholds and maximum duty

cycle ranges. The UC3842B and UC3844B have

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

suited off-line applications The corresponding thresh-

olds for the UC3843B and UC3845B are 8.5 V and 7.9

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 flop which blanks

the output off every other clock cycle.

BLOCK DIAGRAM (toggle flip flop used only in UC3844B and UC3845B)

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

UC3842B

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