LED Backlighting Solution

with LM3430 and LM3432

National Semiconductor

Application Note 1679

TK Man

September 2007

Introduction

Since the release of high brightness White LED (HB-WLED)

in the middle of 1990's, tremendous research efforts have

been undergoing to improve the emission efficiency, reliability

and thermal packaging technologies in order to expand its

range of applications. In the last few years, improvements in

HB-WLED performance and price structure attracts more po-

tential applications considering this new lighting source tech-

nology as an alternative to conventional one. One area of

applications, LCD display backlighting, consider this new

backlighting method as the ultimate solution to replace the

existing Cold Cathode Fluorescent Lamps (CCFL). Backlight

LED-driver solutions need to exhibit following characteristics:

•

Instead of voltage control, LED-driver need direct LED

current control to ensure consistent color and brightness;

•

High conversion efficiency under various conditions;

•

Support PWM dimming;

•

Limit LED's rail voltage while LEDs open circuited;

•

Ability to shutdown individual channel(s) when short or

open circuited LED(s) detected;

•

Device over temperature detect and shutdown;

•

Low profile, small size and ease to use.

The LM3432 is a 6-channel high voltage current regulator

which provides a simple solution for LED backlighting appli-

cations and the LM3430 is a companion device to supply high

voltage required to drive serially connected LED strings. The

LM3430 and the LM3432 provide a complete solution to most

HB-WLED backlighting applications for notebook and PC

monitor. In this application note, a typical example for a solu-

tion to drive six strings of twelve LEDs in series running at

20mA per string is described in details.

The Demonstration Board

The LM3432 is used to drive 6-channel LED strings with 12

LEDs in series per string running at 20mA. This example pro-

vides a simple solution for HB-WLED backlight applications

in notebooks and monitors. The LM3432 is powered with its

companion device, LM3430, the Dynamic Headroom Control

(DHC) feature helps to provide the optimal system efficiency.

The design specification for this demonstration board is

shown in below:

Design Specifications:

•

Supply Voltage, VIN

6V to 21V

•

Boost Converter Switching Frequency, F

SW

1 MHz

•

Boost Converter Output Voltage, V

LED

50V

(With load disconnected)

•

Number of LED String

6

•

Number of LED per String

12

•

LED Current per String, I

LED

20 mA

PWM Dimming Function:

•

Analog Dimming Mode

Control Voltage Input, V

PWM

1V to 3V (0% to 100%)

Dimming Frequency, F

PWM

•

Digital Dimming Mode

External PWM Signal

20 kHz to 25 kHz, 2.5V

peak

© 2007 National Semiconductor Corporation

300289

www.national.com