User manual

LM3407 Evaluation Board

Reference Design

National Semiconductor

Application Note 1763

SH Wong

January 21, 2009

Introduction

The LM3407 is a Pulse-Width-Modulation (PWM) floating

buck converter with an integrated N-channel power MOSFET

designed to provide precision constant current output for driv-

ing high power LEDs, such as the Lumileds Luxeon® power

LEDs and OSRAM Golden DRAGON® LEDs. The switching

frequency is selectable between 300 kHz and 1 MHz by

changing the value of the frequency setting resistor, allowing

for the use of small external components. The LM3407 fea-

tures a Pulse Level Modulation (PLM) control scheme which

ensures the accuracy of the constant current output well with-

in 10% over input voltage and operating temperature ranges

with an external 1% thick film current setting resistor. The

converter features a DIM pin which accepts standard logic

pulses for controlling the brightness of the LED array, making

the LM3407 ideal for use as a precision power LED driver or

constant current source.

This application note introduces the design of a sample circuit

with the LM3407 providing 350 mA constant current to drive

an LED array of 6 high power LEDs connected in series. The

board can accept an input voltage ranging from 22V to 30V.

The schematic, PCB layout, bill of materials, and circuit de-

sign criteria are shown in detail. Typical performance and

operating waveforms are also provided for reference.

Evaluation Board Schematic

30046701

FIGURE 1. LM3407 Evaluation Board Schematic

LM3407 Evaluation Board Reference Design AN-1763