Create colourful and bright LED light with a matrix dimmer
September 25 , 2016

Red, green, and blue (RGB) LEDs are used in projector, architectural, display, stage, and automotive lighting systems that require efficient, bright output. To produce predictable colors from an RGB LED, each of its component LEDs requires individual, accurate dimming control. High- end systems can use an optical feedback loop to allow a microcontroller to adjust the LEDs for color accuracy. Adding a white LED to an RGB LED to produce an RGBW LED extends the hue, saturation, and brightness values available in the color system (Fig. 1). Each RGBW LED requires accurate dimming of four component LEDs. You can create vibrant and dynamic light from such LEDs, but the driver circuitry can be complex - two RGBW LEDs can require eight driver channels. Let's discuss how a matrix approach can simplify the drive and control of such systems.

 

There are various ways to drive RGBW LEDs for color and brightness control. One way to drive and dim RGBW LEDs is to use four separate LED drivers, one for each color as shown in a two-LED system in Fig. 2. In such a system, the LED current, or PWM (pulsewidth modulation) dimming, of each individual emitter or string is driven by separate drivers and control signals. In this solution, though, the number of LED drivers increases quickly with the number of RGBW LEDs. Any lighting system with a significant number of RGBW LEDs requires a substantial number of drivers and synchronization of the control signals to those drivers.

 

The matrix approach


A much simpler and more elegant approach is to drive all of the LEDs with a single driver/power converter at a fixed current, while using a matrix of shunting power MOSFETs to PWM dim the individual LEDs for brightness control. The matrix dimmer and single LED driver shown in Fig. 3 reduce the circuit size relative to the approach in Fig. 2. Furthermore, a single communications bus to control the matrix LED dimmer makes RGBW color-mixing LED systems relatively simple and compact, while driving high-current RGBW LEDs with accurate color and brightness control.

As seen in Fig. 3, the LT3965 matrix LED dimmer enables such a design. Each LT3965 8-switch matrix dimmer can pair with exactly two RGBW LEDs, allowing control of the individual brightness of each individual emitter (red, green, blue, and white) in PWM steps of 1/256 between zero and 100% brightness. Two-wire I2C serial commands provide both color and brightness control to all eight channels. I2C serial code to the matrix LED dimmer IC determines the brightness state of all eight LEDs and can check for open and short LEDs in case of a fault.

Since each RGBW LED is designed as a single point source, the red, green, blue, and white light combine to produce color variety, with saturation, hue, and brightness control. Each LED can be set in 1/256 steps between zero (0/256) and 100% (256/256) with the high-speed LT3965 matrix dimmer.