- File#1 AVAGO_AV02_0257.pdf (409.1 KB) , download : 33 - By Kwong Yin Leong, Avago Technologies -

This paper describes RGB LED color mixing control using linear dimming with Avago Technologies' color management controllers, ADJD-J823 and HDJD-J822. The evaluation circuit consists of the LED backlight driver board, the HDJD-JD04 kit, and a small LED backlight screen .

The LED mixing board has a field sequencing mode to sequence the RGB colors. The color mixing performance, ie du'v' measurements, is taken during linear dimming with and without field sequencing. The control measurement is taken with PWM dimming and the du' v' is computed.

From the results, linear dimming with or without field sequencing is a viable method to achieve precise RGB color control. The maximum du'v' result obtained is 0.006. One possible application in linear dimming is the DLP projector. This paper also explains why the Pixel memory synchronization can be easily resolved with linear dimming.

VLINR linear dimming voltage for red LEDs
VLING linear dimming voltage for green LEDs
VLINB linear dimming voltage for blue LEDs
ENR Enable Red LED
ENG Enable Green LED
ENB Enable Blue LED
PWM Pulse width modulation
DLP - Digital light processor
PWMR Pulse width modulation for red LEDs
PWMG Pulse width modulation for green LEDs
PWMB Pulse width modulation for blue LEDs
SYNC Pulse to trigger the loading of image pixel data to DLP mirror

PWM dimming is typically the preferred dimming method, as the wavelength (ie the color) does not shift during dimming. The RGB mixing application is easier to control with PWM.

Linear dimming is limited to RGB LED applications due to the wavelength shift caused by changes to the LED current during linear dimming. The shift in wavelength makes precise color control difficult to achieve. However, the use of Avago's color controller, ADJD-J823 or HDJD -J822, can provide precise color control.

The usage of LED dimming is in the DLP projector. The usage of LED lamps in the DLP projector allows for savings in the motor and color filter disc, with a potential reduction in lamp and maintenance costs.

Block diagram description
The ADJD-J823 from the HDJD-JD04 kit has three PWM outputs: PWMR, PWMG and PWMB, which enable Red, Green and Blue LEDs respectively. The ADJD-J823 controls the proportion of Red, Green and Blue amounts by modifying the PWMR, PWMG and PWMB duty cycles to produce the desired color on the backlight screen.

The Red channel is discussed and shown in Figure 1. The Green channel is the same, except the PWMG and ENG is used in placed of PWMR and ENR respectively. similar, the Blue channel uses PWMB And ENB signals instead of PWMR or ENR.

The key components involved with the linear dimming are the low pass filter and the AS1104 LED driver.

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