LINEAR dn376f Data Manual

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Monolithic Converter Drives High Power LEDs – Design Note 376 Keith Szolusha

Introduction Both current sense voltages are 100m V and can be adjusted independently using the IADJ1 and IADJ2 pins.

High power LEDs are quickly expanding their reach as the

Efficiencies of up to 91% can be achieved in typical

lightsource of choice for flat panel computer and TV

applications. The LT3477 features a programmable soft-

monitors, TV projection, signage, portable lights and

automotive interior, trim and brake lighting. The input start function to limit inductor current during start-up. Both inputs of the error amplifier are available externally,

voltage and LED voltage combinations across these appli-

allowing positive and negative output voltages. The switch-

cations are as diverse as the applications themselves,

ing frequency is programmable from 200k Hz to 3.5MHz

precluding the ability for a single topology to satisfy the

through an external resistor. It comes in two thermally

needs of them all. Nevertheless, all LED drivers, whether

enhanced packages: a 20-pin (4mm × 4mm) QFN and a

buck, boost, buck-boost or SEPIC must regulate a con-

stant LED current, regardless of input and output volt- 20-pin TSSOP.

ages. Now a single switching regulator with the ability to

Boost Driver

be configured in a large variety of topologies for high

The LT3477’s internal ground-referenced 3A NPN power

power constant LED current is available.

switch is most commonly used for boost applications. The LT®3477 can drive high power LEDs at constant Figure 1 shows a 5V to four 1W LED boost converter with current in any of the topologies stated above. It is a open LED protection and 330m A constant LED current. current mode, 3A DC/DC step-up converter that incorpo- The constant current is regulated using a current sense rates dual rail-to-rail current sense amplifiers and an amplifier and a 0.3Ω sense resistor. The feedback voltage internal 3A, 42V switch. It combines a traditional voltage amplifier is only used for overvoltage protection in case feedback loop and two unique current feedback loops to the LEDs are removed from the circuit.

operate as a constant-current and/or constant-voltage

The forward voltage of the four LEDs ranges from 12V to

source. The floating rail-to-rail current sense amplifiers

16V. The input voltage range is 2.5V (minimum LT3477

allow for both ground-referenced and floating LED solu-

input voltage) up to just below the LEDs’ forward voltage.

tions in different topologies, along with the added benefit

The LT3477 can drive more LEDs as long as the peak

of inrush current or short-circuit protection.

switch current remains below 3A. , LTC, LT and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. L1

VIN 5V

ISN1ISP1 SW 200k

VIN FBN IADJ1 R2 IADJ2 10k

SHDN SHDN LT3477 ISP2

VC ISN2 VREF RT

FBP SSGND

C1: TAIYO-YUDEN LMK316BJ335ML

C2: TAIYO-YUDEN TMK325BJ335MN C3 R3 D1: DIODES INC. DFLS120L 33n FR4 22k

L1: TOKO A915AY-100M

DN376 F01

Figure 1. 330m A Boost LED Driver with Open LED Protection