On the application of strengthening backlight brightness control technology

The brightness control of backlight LED can be achieved through pulse width modulation (PWM) or constant current control. PWM brightness control requires a constant current driver to drive the LED, but the on/off time needs to be adjusted to achieve the desired brightness. Therefore, PWM is more complicated than direct constant current control. So a new solution is presented. For this purpose, RGB LED backlighting is used as an example.

The pixels in an LCD display are divided into three primary color grids: red, green and blue. The color of the pixel is defined by the mixture of these three primary colors.

When using RGB backlight, the driver must correct the brightness balance between the three primary colors of red, green and blue to prevent white point displacement when the LED temperature changes. In addition, the driver must ensure that the correct intensity of light is maintained at any operating temperature. In terms of compensation, closed-loop or open-loop forms can be used. If closed-loop compensation is used, a photosensitive sensor is required to measure the white point and its intensity. On the contrary, if open-loop compensation is used, the temperature must be measured in advance and the brightness balance is adjusted through a pre-defined compensation curve. For example, LP5520 is an example of an RGB backlight driver, which is an open-loop compensated LED driver. Figure 7 shows the principle of open-loop color compensation.

Figure 7

The temperature compensation curves are measured using RGB LEDs in real applications, and these curves are programmed into the EEPROM memory inside the chip. The chip is integrated into an LCD display module, and the module manufacturer programs the compensation curves during production. In addition, the RGB LED backlight can also be used as an optimized color filter.