UCC2974

UCC3974

SLUS485A – JULY 2001 – REVISED NOVEMBER 2001

6

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APPLICATION INFORMATION

functional block diagram

UDG–01021

3

+

+

+

VIN

OPEN LAMP

DETECTOR

ZERO DETECTOR

VOLTAGE CLAMP

OSCILLATOR

4–BIT

COUNTER

AND LOGIC

SYNC

VDRV

1

S

Q

QR

R DOMINANT

LFD–PWM

UVLO

OPEN LAMP

OE

VINT

5

+

+

EA1

UVLO

SQ

Q

R

SD

OPEN LAMP

OVERFLOW

INTERNAL REGULATOR

VREG UVLO

1.5

1.0

VIN

13

2

VIN

CBP

BUCK1

OUT1

FB1

SAME

AS

OTHER

CHANNEL

MODE SELECT

10

MODE

5

µA

VINT

14 BUCK2

16 OUT2

11 FB2

+

+

1.5

LFD OSCILLATOR

& LOGIC

LFD–PWM

4

COMP1

PRE–CON

6

RT1

7

RT2

9

DIM

8

LFDSYNC

1.0

LFD SECTION

12 COMP2

40 kHz TO 200 kHZ

LFD–EN

general description

The UCC3974 extends the capabilities of the UCC3972 and UCC3973 backlight controllers. The basic

functionality is the same as that for the UCC3972; a buck controlled current source feeding a royer oscillator

CCFL circuit. As such the application information for the UCC3972 that pertains to the royer oscillator, buck

controller and CCFL circuit in general apply to the UCC3974. Also, this device implements a voltage clamping

scheme, similar to that of the UCC3973, using an internal current source to bias up the FB pin and thereby

limiting the current available to the royer stage. This limits the voltage to which the secondary side of the

transformer is exposed.

The extensions this device provides are two separately controlled channels to be used with two separate royer

stages, and integrated low-frequency dimming (LFD) control .