LM3570

Low Noise White LED Driver System

General Description

The LM3570 device provides a complete LED driver solution

for wireless handsets and other portable devices using a

display and keypad. With three constant current sources, up

to three white LEDs can be used for display lighting with

excellent current matching (0.3% typ.). The regulated 4.35V

output voltage is perfect for driving auxiliary keypad LEDs in

voltage mode.

The LM3570 utilizes a high efficiency 3/2 CMOS charge-

pump with a pre-regulation loop that minimizes conducted

noise on the input. It accepts an input voltage range from

2.7V to 5.5V and maintains a constant current determined by

the current through an external R

The device supplies up to 80mA of total load current to

accommodate any combination of up to three white LEDs,

and additional current from V

set at 500kHz. (typ.) to keep the conducted noise spectrum

away from sensitive frequencies within portable RF devices.

By applying a pulse width modulated (PWM) signal to the

PWM pin, the user has the ability to independently control

the brightness of the regulated current source outputs with-

out shutting down the regulated output voltage.

Features

n

2.7V to 5.5V Input Voltage

n

Regulated Output Voltage (V

n

Regulated I

current outputs

n

High Efficiency 3/2 Boost function

n

Drives one, two, or three white LEDs with no bias

resistor losses

n

Drives auxiliary keypad LEDS in voltage mode

n

Up to 80mA total output current

n

Active-High Enable

n

Active-High PWM Control Pin for independent control of

current sources

n

Very small solution size

n

1µA(max) shutdown current

n

500kHz switching frequency (typ.)

n

Linear regulation generates predictable noise spectrum

n

LLP-14 package: 4.0mm X 3.0mm X 0.8mm

Applications

n

Portable devices using white or blue LEDs with display

and backlight or frontlight

n

1-Cell LiIon battery-operated equipment including PDAs,

hand-held PCs and cellular phones

Typical Application Circuit

20085021

July 2004

© 2004 National Semiconductor Corporation

DS200850

www.national.com