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UC284XA

UC384XA

May 2004

1

FEATURES

■

TRIMMED OSCILLATOR DISCHARGE

CURRENT

■

CURRENT MODE OPERATION TO 500kHz

AUTOMATIC FEED FORWARD

COMPENSATION

■

LATCHING PWM FOR CYCLE-BY-CYCLE

CURRENT LIMITING

■

INTERNALLY TRIMMED REFERENCE WITH

UNDERVOLTAGE LOCKOUT

■

HIGH CURRENT TOTEM POLE OUTPUT

UNDERVOLTAGE LOCKOUT WITH

HYSTERESIS

■

LOW START-UP CURRENT (< 0.5mA)

DOUBLE PULSE SUPPRESSION

2

DESCRIPTION

The UC384xA 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 im-

plemented circuits include a trimmed oscillator for

precise DUTY CYCLE CONTROL under voltage

lockout featuring start-up current less than 0.5mA,

a precision 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 de-

signed to source or sink high peak current. The

output stage, suitable for driving N-Channel MOS-

FETs, is low in the off-state.

Differences between members of this family are

the under-voltage lockout thresholds and maxi-

mum duty cycle ranges. The UC3842A and

UC3844A have UVLO thresholds of 16V (on) and

10V (off), ideally suited off-line applications The

corresponding thresholds for the UC3843A and

UC3845A are 8.5 V and 7.9V. The UC3842A and

UC3843A can operate to duty cycles approaching

100%. A range of the zero to < 50 % is obtained by

the UC3844A and UC3845A by the addition of an

internal toggle flip flop which blanks the output off

every other clock cycle.

NOT FOR NEW DESIGN

HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

Figure 2. 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

REV. 5

Figure 1. Package

Table 1. Order Codes

Part Number

Package

UC2842AD1; UC3842AD1;

UC2843AD1; UC3843AD1;

UC2844AD1; UC3844AD1;

UC2845AD1; UC3845AD1

SO-8

UC2842AN; UC3842AN;

UC2843AN; UC3843AN;

UC2844AN; UC3844AN;

UC2845AN; UC3845AN

DIP-8

SO-8

DIP-8