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SC4509

Highly Integrated Step-Up Converter for White LED Applications in MLPD-W8-2x2

Semtech Corporation 

POWER MANAGEMENT

Block Diagram

FB

VIN

VREF

1.25V

199mV

EN/SS

SC4509

PRELIMINARY

∑S

Ramp

Generator

1.2MHz

Oscillator

PWM

RQ

S

SW

Internal Schottky

OVP

VOUT

GND

Theory of Operation

The SC4509 is a constant frequency PWM step-up regu-

lator IC designed to operate with an input voltage range

from 2.7V to 10V.

A maximum output voltage of 20V allows for up to five LEDs

to be connected in series, providing even illumination by

passing the same output current through each LED.

Designed for a low 199mV, the feedback voltage has a

direct effect on the converter efficiency. Because the volt-

age drop across the feedback resistor does not contribute

to the output power (LED brightness), the lower the feed-

back voltage, the higher the efficiency. A 1.2MHz oscillator

frequency saves board space by reducing external compo-

nent sizes, making the SC4509 ideal for battery-operated

products.

The SC4509 can be configured as a voltage regulated

boost converter or as a current-regulating boost converter

for driving white LEDs. The device is designed to be stable

without additional external components in both cases.

Over-Voltage Protection (OVP)

To prevent the output voltage from exceeding the maxi-

mum switch voltage rating (30V), an over-voltage protec-

tion circuit is integrated. As soon as the output voltage

exceeds the OVP threshold, the converter stops switching

and the output voltage starts to drop. When the output

voltage drops back below the OVP threshold, the convert-

er resumes operation. This sequence will repeat for as

long as is necessary.

Enable and PWM Dimming

The SC4509 provides the EN/SS pin to serve multiple

functions. Its main function is as an enable pin (active

high). It also can be used to control LED brightness

when a PWM signal (up to 10kHz) of varying duty cycle is

applied. A third function acts as a soft-start control. Soft

start can be implemented by ramping the voltage applied

to the pin at startup or by slowly increasing the PMW duty

cycle.

© 2007 Semtech Corp.

6

www.semtech.com

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