RGB LED Colour Mixing
A single LED die can only emit monochromatic light which could be one of the three primary colors - red, green and blue, known as RGB. To realize more colors, three LED dies need to be used together for RGB color mixing. Commonly 7 colors can be produced by controlling the switch of the channel for each primary color. To produce more than 7 colors, each color channel should be able to change in brightness, not just switched on or off. A popular control method is PWM, of which the cycle duty range determines the available brightness levels. The more the levels are available, the more colors can be produced. Apart from the popularity in applications like outdoor decoration lighting in cities, stage lighting designs, home decoration lighting and LED display matrix, RGB color mixing technology can also be found recently in LCD backlighting and projectors.
To implement different patterns of RGB color mixing, each LED die needs a dedicated signal for dimming. One method is using a single driver that has three or more channels and is capable of controlling each channel separately for dimming. The other is using three drivers to provide power and implement dimming function respectively on three channels. Obviously, the former method uses fewer drivers and is suitable for the applications such as backlighting and display matrix that work on lower current and a large number of LEDs. The latter one can be used in high power applications such as decoration lighting.
In addition, a MCU is required to drive LED driver to carry out dimming function and store color configurations of LEDs. For those applications that require more accurate color mixing, color and temperature sensors should be used to monitor the mixed color output and provide feedback.
LEDs used in RGB color mixing are totally depends on application requirement, either High Brightness or High Power LEDs can be used in these applications. The main suppliers include Cree, Broadcom, Lumex and Vishay. LED driver are available from National Semiconductor, ON Semiconductor, TI and Linear Technology.