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SC4505

High Efficiency Boost Converter for Backlight/Flash LED Driver

Semtech Corporation 

SC4505

POWER MANAGEMENT

Overview

The SC4505 contains a 1MHz fixed-frequency, current-

mode boost converter, and two independent LED current

regulators. The LED current setpoints are chosen using

external resistors, and the PWM controller operates

independently to keep the two currents in regulation.

Since the SC4505 receives feedback from both of the

LED current regulators, either or both LED strands can

be on at any given time. Additionally, different numbers

of LED can be used in the two strands with no resistor

ballasting, or preset output voltage setpoint.

A typical application would use 3-6 backlight LED, driven

with 20mA, and 2-6 flash LED, driven with 20mA during

torch mode (video recording), and driven with 100mA

during flash mode (for photographs). Usually only the

backlight LED are used, but during some cases both

strands must be on at the same time. As the output

voltage is different for each case, a designer is often

forced to use lossy ballast resistors to balance the LED

currents, or to use two separate converter ICs, greatly

increasing component count and BOM cost.

The states of the two LED current regulators are chosen

by a three-bit digital input. Either or both of the current

regulators can be on at any given time. The converter

automatically shuts down to zero-current shutdown mode

if all pins are low.

EN_BL – Enable Backlight regulator (IO1).

EN_FL – Enable Flash/Torch regulator (IO2)

FL_TRB – Select flash or torch mode for IO2. The current

in torch mode is approximately 1/5 of the current in flash

mode. Flash mode when FL_TRB = 1.

The SC4505 solves these issues by controlling the boost

converter set point based on instantaneous requirements

of the two current regulators. Therefore, only a single

inductor and power switch is needed to provide power

to the entire lighting subsystem, increasing efficiency and

reducing part count. A digital interface allows independent

control of all LED currents with no converter

“babysitting”. Interface to output control is high-

bandwidth, supporting digital PWM dimming on any

channel at 50Hz to 50kHz dimming frequency, while

aggressively shutting the entire IC down to less than

1µA shutdown mode when all LED strands are turned

off. In shutdown mode, leakage through the current

regulator outputs is also < 1µA, keeping the output

capacitor charged and ready for instant activation of the

LED strands.

1 MHz switching speed provides high output power using

a tiny 1.0mm high inductor, maximizing efficiency for

space-constrained and cost-sensitive applications. In

addition, converter and output capacitor are protected

from open-LED conditions by overvoltage protection, and

flash LED are protected from burnout by a user-settable

time-out feature.

 2007 Semtech Corp.

6

www.semtech.com

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