LM3402/LM3404 Fast

Dimming and True Constant

LED Current Evaluation

Board

National Semiconductor

Application Note 1839

Matthew Reynolds

December 10, 2008

Introduction

The LM3402/02HV and LM3404/04HV are buck regulator de-

rived controlled current sources designed to drive a series

string of high power, high brightness LEDs (HBLEDs) at for-

ward currents of up to 0.5A (LM3402/02HV) or 1.0A

(LM3404/04HV). This evaluation board demonstrates the en-

hanced thermal performance, fast dimming, and true constant

LED current capabilities of the LM3402 and LM3404 devices.

Circuit Performance with LM3404

This evaluation board (figure 1) uses the LM3404 to provide

a constant forward current of 750 mA ±10% to a string of up

to five series-connected HBLEDs with a forward voltage of

approximately 3.4V each from an input of 18V to 36V.

Thermal Performance

The PSOP-8 package is pin-for-pin compatible with the SO-8

package with the exception of the thermal pad, or exposed

die attach pad (DAP). The DAP is electrically connected to

system ground. When the DAP is properly soldered to an area

of copper on the top layer, bottom layer, internal planes, or

combinations of various layers, the

θ

can be significantly lower than that of the SO-8 package. The

PSOP-8 evaluation board is two layers of 1oz copper each,

and measures 1.25" x 1.95". The DAP is soldered to approx-

imately 1/2 square inch of top and two square inches of

bottom layer copper. Three thermal vias connect the DAP to

the bottom layer of the PCB. A recommended DAP/via layout

is shown in figure 2.

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FIGURE 1. LM3402 / 04 Schematic

© 2008 National Semiconductor Corporation

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