AN2823 Datasheet, PDF (1/33 Pages) STMicroelectronics – Current source for LED driving based on the L5973AD

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





















)LED;!=
!-V
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.
August 2009
Doc ID 15015 Rev 1
1/33
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