JULY 2000

LCD Desktop Backlight Lamp Driver

1

GENERAL DESCRIPTION

The ML4877 is an ideal solution for driving multiple cold

cathode fluorescent tubes (CCFL) used in liquid crystal

display (LCD) backlight applications. It provides dimming

ballast control for the LCD display.

By utilizing differential drive the ML4877 can deliver the

same light output with significantly less input power

compared to existing single ended drive schemes.

Improvements as high as 30% can be realized when using

low power lamps and advanced LCD screen housings. This

increased light output is achieved because the differential

drive configuration is much less sensitive, and therefore

less power is wasted in the capacitive parasitics that exist

in the backlight housing. An additional benefit of this

configuration is an even distribution of light.

The IC includes an adjustable lamp out detect circuit that

latches the IC off when a lamp fault is detected. Also, the

unique architecture of the ML4877 allows the

development of a backlight system that will inherently

meet the UL requirements for safety.

The ML4877 is optimized for large LCD applications

applications where high efficiency is critical to maximize

battery life. The high efficiency is achieved by a resonant

scheme with zero voltage switching.

BLOCK DIAGRAM

FEATURES

s

Ideal for 30W inverter designs, 1 to 8 lamp design

s

PWM dimming capability

s

Backlight lamp driver with differential drive

s

Up to 30% lower power for same light output

s

Low standby current (<10µA)

s

Improved efficiency (

»95%)

s

Allows all N-channel MOSFET drive

s

Adjustable lamp out detect with latch

s

Resonant threshold detection and synchronous

rectification

s

Positive input for dimming control

LINEAR

REGULATOR

MASTER

BIAS

&

UVLO

12

5

13

VDD

VDD

AZR

15

+

–

VDD

10

B ON

19

B OFF

20

11

DR3

DR1

DR1

Q

QR

S

OSCILLATOR

CLK

9

–

+

–

+

3

7

T

Q

Q

14

16

DR2

DR2

L RTD

L GATE 1

L GATE 2

LEA–

SS CAP

ON/OFF

VREF

HVDD

2

17

PGND

18

GND

1

4

8

LEA OUT

0.5V

6

L ILIM

B SYNC OUT

–

+

RESONANT

THRESHOLD

DETECTOR

NEG

EDGE

DELAY

S

RQ

Q

ONE

SHOT

LEA+

SS

SS

* THIS PART IS END OF LIFE AS OF J

ULY 1, 2000

NEW!

30W CCFL

Inverter

Design