© 2009 Semtech Corporation

POWER MANAGEMENT



SC656

Backlight Driver for 7 LEDs with

Charge Pump and PWM Control

Features

Input supply voltage range — 2.9V to 5.5V

Charge pump modes — x, .5x and 2x

PWM dimming control with low pass filter provides

DC backlight current (not pulsed)

Two independently configurable backlight banks

PWM frequency range — 200Hz to 50kHz

Seven adjustable current sinks — 500µA to 25mA

Backlight current accuracy ±.5% typical

Backlight current matching ±0.5% typical

LED float detection

Charge pump frequency — 250kHz

Low shutdown current — 0.µA typical

Ultra-thin package — 3 x 3 x 0.6(mm)

Fully WEEE and RoHS compliant, and halogen free.

Applications

Cellular phones, smart phones, and PDAs

LCD display modules

Portable media players

Digital cameras

Personal navigation devices

Display/keypad backlighting and LED indicators

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Description

The SC656 is a high efficiency charge pump LED driver

using Semtech’s proprietary charge pump technology.

Performance is optimized for use in single-cell Li-ion

battery applications.

The device provides backlight current using up to seven

matched current sinks. The load and supply conditions

determine whether the charge pump operates in x, .5x,

or 2x mode. The seven backlights can be configured as a

single group or split into two independent banks by

setting the state of the BANK2 and BANK pins. If only one

bank is needed, the BANK2, BANK, ENS and ISETS pins

must be grounded.

The maximum current per LED in each bank is set by a

resistor connected to ISETM or ISETS. LED current can be

set between 500µA and 25mA. Backlight current is varied

by applying a pulse-width modulated (PWM) signal to the

ENM pin for the main LED bank and the ENS pin for the

sub LED bank. The resulting DC current in each LED (I

equal to the maximum current setting multiplied by the

duty cycle of the PWM signal. During PWM operation, a

low-pass filter is used to develop a DC current through the

LED. The resulting power conversion is more efficient than

comparable pulsed current solutions. Backlight fading is

initiated when the duty cycle is changed.

The 3 x 3 (mm) package and minimal number of small

external components make the SC656 an ideal backlight

driver solution for space-limited designs.

IN

ENM

GND

OUT

BL1

ISETM

2.2µF

C1+

PWM Signal

2.2µF

BL2

BL3

BL4

C2+

C1-

C2-

BL5

C2

2.2µF

C1

2.2µF

ENS

ISETS

BL6

BL7

SC656

PWM Signal

BANK2

BANK1

US Patents: 6,504,422; 6,794,926

Typical Application Circuit

September 24, 2009