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AN2129

APPLICATION NOTE

1 Introduction

Thanks to the high efficiency and reliability, super high brightness LEDs are becoming more

and more important when compared to conventional light sources. Although LEDs can be sup-

plied directly from a simple voltage source (like battery with resistor), for most applications it

is better to use a switching current source to get not even higher efficiency but also to get a

better light output. This paper will focus on a L6902D based DC/DC converter with dimming

interface. For more details about other converters and applications for LEDs available from

STMicroelectronics please refer to other application notes ([1] and [2]).

2 Dimming Concepts

There are two basic principles how the light output of the LED can be controlled. Since the light

brightness is proportional to the current, both methods are dealing with current regulation. The

first and the easiest way is to control the LED current itself, with the principal sketch in Figure

1, where current is changed proportionally with the dimming signal. Disadvantage of this ana-

log control is that there can be a significant change of color (wavelength difference could be

several nanometers) in deep dimming (less that 10%). This potential disadvantage is compen-

sated by a very simple control circuit (usually a simple potentiometer is enough).

Figure 1. Analog current control

Figure 2. Average current control by PWM

I

time

I

time

I

max

I

average

The second method is based on an average current control (digital control) as can be seen in

Figure 2. The current is switched between zero and the nominal current with a frequency high-

er than 100Hz (to avoid flickering). The change of duty cycle and hence the average current

change will be seen as a brightness change, because human eye reaction is slow enough to

"integrate" the light output and it will not be noticed as a blinking.

This method avoids the color change problem, but on the other hand it needs more sophisti-

cated control circuits (usually a microcontroller or another simple PWM generator).

3 L6902D DC/DC Converter

The L6902D is a complete and simple step down switching regulator with adjustable current

and voltage feedback. Thanks to its current control loop with external sense resistor it is able

to work in a constant current mode, providing up to 1A output current with an accuracy of 5%.

Among other features there can be also found general purpose 3.3Volts precise (2%) refer-

ence voltage or 2.5A (typical value) internal current limit for short circuit protection.

DIMMING OF SUPER HIGH BRIGHTNESS LEDS

WITH L6902D

AN2129/0705

Rev. 2