This week, an appnote from Freescale on different ways to control high-brightness LEDs:
Bright Approaches to Microcontroller-Based LED Drivers
Driving these LEDs with constant current is really important for consistent results. Wavelength of the light is dependent on the current, so driving a precise value is crucial. However, forward voltage varies with current, and also experiences process variations, so the standard LED in series approach is pretty well useless if you want to control the colour of RGB LEDs properly. Thus, you need a constant current source that doesn’t depend on the forward voltage of the diode. This appnote presents a few solutions, though they head straight for the switching power supply that doesn’t work with RGB LEDs with a common supply terminal.
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