RGB LED strings are used in projectors, buildings, displays, stages, and automotive lighting systems because such systems require high efficiency, bright light output. An RGB LED string produces the desired color, with each LED (red, green, and blue) requiring independent and accurate dimming control. High-end systems can use an optical feedback loop to allow the microcontroller to adjust the RGB LED strings to provide accurate color. Adding a white LED to the RGB LED string to form an RGBW LED string increases the color, saturation, and brightness values ​​available in the color illumination system. Four of the LEDs in each RGBW LED string require accurate dimming. Two RGBW LED strings require eight "channels".
There are many ways to drive RGBW LED strings for color and brightness control. One way to drive the RGBWLED string and adjust its brightness is to use four separate LED drivers for four colors (R, G, B, and W), as shown in Figure 1a. In systems employing this approach, the current (or PWM dimming) of each individual LED or LED string is driven by separate drivers and control signals. However, in this type of solution, the number of LED drivers increases rapidly as the number of RGBW LED strings increases. Any lighting system that uses a large number of RGBWLED strings requires a large number of drivers, and the amount of control for synchronizing the control signals of these drivers is also large.
A much simpler (and more convenient) approach is to drive all LEDs at a fixed current with a single driver/converter while PWM dimming each LED with a parallel power MOSFET matrix for brightness control. The matrix dimmer and single LED driver as shown in Figure 1b reduces the circuit size of the solution of Figure 1a. In addition, using a single communication bus to control the matrix LED dimmer makes the RGBWLED color mixing system relatively simple and compact, while driving large current RGBWLED strings, color and brightness control is also very accurate.
Figure 1a
Figure 1b
Figures 1a and 1b: (1a) In a high-power color mixing application, 8 separate LED drivers and PWM signals can be used to drive two RGBW LED strings, or (1b) a single boost-down with serial communication capability Compact LED drivers and matrix LED dimmers enable much smaller and more compact solutions.
The LT3965 matrix LED dimmer can be used to achieve this design, as shown in Figure 5. Each of the LT3965's eight switch matrix dimmers can be paired with a full two RGBW LED strings, allowing each LED to be individually controlled between zero and 100% brightness in 1/256 PWM steps (red, The brightness of green, blue and white light). The two-wire I2C serial interface command provides color and brightness control for all eight channels. The I2C serial interface code provided to the matrix LED dimmer IC determines the brightness status of all eight LEDs and can be used to check for open or shorted LEDs in the event of a fault.
Since each LED in the RGBW LED string is designed as a single point source, red, green, blue, and white light combine to produce a wide variety of colors, and saturation, color, and brightness are controllable. With the high-speed LT3965 matrix dimmer, the brightness of each LED can be set between zero (0/265) and 100% (256/256) brightness in 1/256 dimming steps.
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