LINEAR LT3743 User Guide

L DESIGN FEATURES

High Current/High Speed LED Driver Revolutionizes PWM Dimming

by Josh Caldwell Introduction

Power drivers that can produce regu- lated high current pulses are used VIN VIN

in a number of lighting applications, LT3743 HG

ranging from high current LEDs in

DLP projectors to high power laser CBOOT IREG

LED LOAD

diodes. For instance, in high end SW

video projectors, high power LEDs are

VCC_INT

used to produce color illumination.

The RGB LEDs in these projectors

LG VOUT FOR IREG VOUT FOR IREG require precise dimming control for DETERMINED DETERMINED

CTRL_SEL GND

BY CTRL_H BY CTRL_L

accurate color mixing—in this case,

SENSE+

more control than simple PWM dim- SENSE ming can offer. Typically, to achieve PWMGH

CTRL_L

the wide dynamic range required in color mixing, LED drivers must be CTRL_H PWMGL able to rapidly switch between the two disparate regulated peak current

Figure 1. Basic switched-capacitor topology

states, and overlay PWM dimming without disruption. The LT3743 has

the ability to meet these demanding Switched Output the LT3743 acts as a regulated cur- accuracy and speed requirements. Capacitor Topology rent source driving into the load. The

The LT3743 is a synchronous In traditional current regulators, the voltage drop across the load for a given buck DC/DC controller that utilizes voltage across the load is stored in the current is stored in the first switched fixed-frequency, average current mode output capacitor. If the load current output capacitor. When a different control to accurately regulate the is suddenly changed, the voltage in regulated current state is desired, the inductor current through a sense the output capacitor must charge or first output capacitor is switched off resistor in series with the inductor. discharge to match the new regulated and a second capacitor is switched in. The LT3743 regulates the current in current. During the transition, cur- This allows each capacitor to store the any load with an output voltage range rent in the load is poorly controlled, voltage drop for the load corresponding from 0V to 2V below the input rail with resulting in slow load current response to the desired regulated current. ±6% accuracy. time. Figure 1 shows the basic topology Precision, broad-range LED current The LT3743 solves this problem with the various control pins. The PWM control is achieved by combining ac- with a unique switched output capaci- and CTRL_SEL pins are digital control curate analog dimming (high and low tor topology, which enables ultrafast pins that determine the state of the states) with PWM dimming. Analog load current rise and fall times. The regulated current. The CTRL_H and dimming is controlled via the CTRL_L, basic idea behind the topology is that CTRL_L pins are analog inputs with a CTRL_H, and CTRL_T pins; PWM

dimming via the PWM and CTRL_SEL CTRL_SEL

pins. A rapid transition between the

high and low analog states is made

possible with the LT3743’s unique use SW

of externally switched load capacitors, ICTRL_H

which allows the LT3743 to change INDUCTOR

CURRENT ICTRL_L

regulated LED current levels within

PWMGH

several microseconds. The switching

frequency may be programmed from PWMGL

200k Hz to 1MHz using an external re- ICTRL_H

sistor and synchronized to an external LED CURRENT

ICTRL_L

clock from 300k Hz to 1MHz.

Figure 2. LED current PWM and CTRL_SEL dimming

Linear Technology Magazine • December 2009