LM3402_1 Datasheet, PDF (1/18 Pages) National Semiconductor (TI) – LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board

LM3402/LM3404 Fast
Dimming and True Constant
LED Current Evaluation
Board
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.
National Semiconductor
Application Note 1839
Matthew Reynolds
December 10, 2008
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_JA of the LM3404/04HV
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.
FIGURE 1. LM3402 / 04 Schematic
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