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Most LCD display backlights are designed with LEDs instead of the traditional cold cathode fluorescent lamp. LEDs allow a brighter and sharper image while minimizing power consumption. They can be white on a black & white display or RGB for a chromatic effect. The most common way to control the brightness of an LED is to switch on or off the current flowing through it by applying a Pulse Width Modulation or PWM rather than a variable continuous current. As long as the PWM switching frequency is higher than the human flicker fusion threshold, the backlight will show no flicker. In addition, PWM does not cause color shift, which is the main drawback when using a variable continuous current to dim an LED. The LCD and PWM frequency should both be set higher than the human flicker fusion threshold. The combination of these will generate an intermodulation product or beat frequency. A visible flickering can still occur if the difference between the LCD frame frequency and the LED PWM frame frequency is too small. The problem can be further complicated when the clock source for the LED and LCD are not the same. The separate devices have different responses to temperature and variations over production life, meaning that even if flicker is carefully avoided during the design phase, it may appear later on during production. To solve these issues, NXP has designed the LCD driver family PCx8536 which integrates a PWM controller and up to 6 PWM channels available for LED backlighting and LED button illumination. The LCD frame frequency and the PWM frequency use the same clock source and can be programmed independently; it will be enough to program the two frequencies more than 50Hz apart to prevent any display flickering. To reduce the bill of material costs, as well as testing and evaluation efforts, NXP suggests using the PCx8536 as your LCD driver and LED controller.
For more information, please visit our PCF8536 and PCA8536 product pages.