LINEAR High Voltage Buck Converters for Driving High Power LEDs - Design Note 392 by Keith Szolusha Owner's Manual & User Guide

High Voltage Buck Converters Drive High Power LEDs Design Note 392 Keith Szolusha Introduction gate signal to extend the dimming ratio of the converter

High power LEDs continue to replace traditional bulbs by maintaining constant output capacitor voltage and in new automotive, industrial, backlight display and control loop state during PWM dimming off-time. Shut- architectural detail lighting systems. LEDs excel in a wide down and external analog current adjust pins provide range of performance and cost parameters, including simple interface for further LED light and current control excellent spectral performance, long life, robustness, fl exibility in any system.

falling manufacturing cost and relatively safe materials.

Single Buck 1A LED Driver

Linear Technology offers a large and growing family of

The LT3474 buck converter 1A LED driver shown in Fig-

high voltage DC/DC converters tailored specifi cally to

ure 1 has features that suit it to automotive applications

drive high-powered LEDs.

(and other battery-powered applications) or to industrial

The LT®3474 and LT3475, for example, are high voltage, EF FI CI EN CY

applications with limited board space, high voltage and

high current, single- and dual-channel buck LED convert- high ambient temperature. This scheme uses a high side ers with wide PWM dimming ratios that can drive one integrated 100mΩ sense resistor for true LED current or more LEDs up to 1A and 1.5A for 80 lumens to 120

sensing and regulation, superior to the common and less

lumens per LED (or more as higher output LEDs become

effi cient method of biasing LEDs with a constant voltage

available). These dedicated LED drivers have onboard

and a power wasting bias resistor.

high voltage NPN power switches and internal sense

The 4V to 36V input voltage range makes it ideal to use with

resistors to minimize board space, reduce component

little-to-no input transient protection circuitry in automo-

count and simplify design.

tive, industrial and avionic applications where long cables

With their high side sense resistors, the LT3474 and

from the battery result in very high input spikes.

LT3475 can drive LEDs tied to ground, an important

advantage in many systems. Current-mode control and The boosted NPN power switch results in high effi ciency a precise reference voltage optimize loop dynamics for for both 1- and 2-LED applications (Figure 2). The boost a well regulated, low ripple constant LED current. Ther- diode is integrated to further reduce component count. mally enhanced exposed pad packages keep the junction Driving the shutdown pin to ground turns off the LEDs temperature low during high power operation in stressful and reduces the input current to less than 2µA for bat- environments. A PWM pin uses the dimming MOSFET tery longevity.

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation. All other

VIN = 12V

trademarks are the property of their respective owners. TWO SERIES CONNECTED

WHITE LUXEON III STARS

VIN 5V TO 36V

VIN BOOST 0.22µF 2.2µF 10µH

SHDN SW ONE WHITE LUXEON III STAR

LT3474 RT BIAS

REF OUT

DIMMING

VADJ PWM

80.6k 10k CONTROL

VC LED 0.1µF 3.3n F LED1 GND

DN392 F01

LED CURRENT (m A)

DN392 F02

Figure 1. LT3474 High Voltage Buck Figure 2. LT3474 Buck Drives Single or LED Driver Regulates 1A Multiple LEDs with High Effi ciency