AN2823 datasheet(1/33 Pages) STMICROELECTRONICS

Part #	AN2823

Description	Current source for LED driving based on the L5973AD

PDF	33 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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1 / 33 page

August 2009

Doc ID 15015 Rev 1

1/33

AN2823

Application note

Current source for LED driving based on the L5973AD

Introduction

LEDs are devices where the light intensity (brightness), measured in millicandelas or in

lumens, is proportional to the forward current flowing through them. Two main classes can

be identified given the typical forward current of the final application:

■ Low current LEDs, from 15 mA to 50 mA, mainly used in the portable market for

backlighting and signaling applications

■ High current LEDs, with forward current between 350 mA to 1000 mA, typically used in

lighting applications

The typical voltage drop across the LEDs when forward-biased depends on the color of the

emitted light, and the spread is high even when considering LEDs from the same production

lot, as shown in Figure 1.

Figure 1.

V-I characteristic of 8 white LEDs of the same type

The spread of the forward voltage and the exponential trend of the I-V characteristic makes

current mode driving more convenient than voltage driving. As a consequence, the current

source regulates the LED current and the output voltage is fixed by the forward voltage

across each LED.

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