Nov 21, 2016: In order to deliver efficient and bright output, RGB (red, green, and blue) LEDs are used across varied applications such as automotive, architecture, display, stage, etc.
But to get the right colors from an RGB LED, its component LEDs need accurate dimming control. Many designers add a white LED to an RGB LED to get the right RGBW LED with the correct hue, brightness and saturation.
Each RGBW LED requires precise dimming of four component LEDs. Even though it is possible to create bright and dynamic light from such LEDs, the driver circuitry may be complex as two RGBW LEDs can involve eight driver channels. This is where matrix approach comes to the forefront as it can simplify the drive and the control of such systems.
The matrix approach
The matrix approach is an easy and elegant approach that helps to drive LEDs with a single driver/power converter at a fixed current. It uses a matrix of shunting power MOSFETs to PWM dim individual LEDs for brightness control.
The matrix dimmer and single LED driver reduce the circuit size relative to the approach.In addition, a single communication bus to control the matrix LED dimmer paves the way for RGBW color-mixing LED systems which are comparatively simple and compact, while driving high-current RGBW LEDs with perfect color and brightness control.
A matrix LED dimmer can be paired with two RGBW LEDs to enable control of the brightness of each emitter (red, green, blue, and white) in PWM steps of 1/256 between zero and 100% brightness. As an individual RGBW LED is designed as a single point source, the lights merge to create color variety, with saturation, hue, and brightness control.
RGBW LEDs can create precise color
RGBW LEDs can create precise color and brightness with PWM dimming of all the red, green, blue, and white emitters. Matrix dimming approach using PWM dimming surpasses drive-current schemes in correctness of color and brightness.
The transient response and bandwidth of the LED driver affects the color accuracy. With more than 10-kHz crossover frequency, the compact boost-buck converter reacts quickly to changes in the number of driven LEDs since the matrix dimmer turns its switches on and off.
Accuracy can be enhanced down to 1/256 PWM dimming with a high-bandwidth buck converter LED driver. However, this will involve the cost of adding another step-up converter to generate a regulated, greater-than-30V output voltage.
Matrix LED color mixer
The matrix dimmer requires an appropriate LED driver to power the sequence of eight LEDs from a number of inputs—standard 12V ±10%, 9V-16V (auto) or 6V-8.4V (Li-ion). The LT3952 boost-buck LED driver steps up and steps down input-to-LED voltage, and also provide slow ripple input and output current.
Matrix dimmer can create colorful and bright LEDs
The matrix dimmer controls LED brightness by shunting the LEDs with parallel power MOSFETs. The LEDs are not required to be connected to floor with the floating output boost-buck LED driver.
Matrix LED dimmer can be paired with LED driver
A matrix LED dimmer like LT3965 can be paired with a boost-buck LED driver to create a precise color RGBW LED color mixing system. With the LT3952, it can be used to drive two RGBW LEDs at 500 mA with 350-kHz switching frequency from a 6V to 20V input. This adaptable system can be powered with automotive batteries, 12V power, or Li-ion batteries.
High color accuracy results from the fast transitory response of the boost-buck LED driver topology and expected dimming control via the 256:1, I2C-controlled matrix system.