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UC3842T

HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

STMicroelectronics 

UC3842T UC3843T

®

UC3844T UC3845T

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 UC384XT family of control ICs provides the

necessary 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

Minidip

SO8

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 UC3842T and UC3844T have

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

suited to off-line applications The corresponding

thresholds for the UC3843T and UC3845T are 8.5 V

and 7.9 V. The UC3842T and UC3843T can operate

to duty cycles approaching 100%. A range of zero to

< 50 % is obtained by the UC3844T and UC3845T 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 UC3844T and UC3845T)

7

Vi

5

GROUND

4

RT/CT

2

VFB

1

COMP

CURRENT 3

SENSE

34V

UVLO

S/R

5V

REF

2.50V

VREF GOOD

LOGIC

INTERNAL

BIAS

OSC

ERROR AMP.

+

2R

-

R

T

S

R

1V

CURRENT

SENSE

COMPARATOR

PWM

LATCH

8 VREF

5V 50mA

6

OUTPUT

D95IN331

September 2001

1/15

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