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Optoelectronics/Displays  

How to implement small LCD backlighting--LED driver considerations

Posted: 04 Oct 2007     Print Version  Bookmark and Share

Keywords:liquid crystal display  LED  LCD-backlighting 

Over the last several years, small colour LCD displays have been integrated into an ever-broadening range of products. Colour displays, once considered a luxury in cell phones, are now a standard offering even at the entry level. Fortunately, the economies of scale of this industry (approaching 1B handsets/year) have lowered the cost of LCD colour displays and made them attractive for integration in other products, be it a handheld medical instrument, a universal entertainment remote control, a digital picture frame/viewer, educational toy, or the latest WiFi enabled VoIP cordless phone. Colour LCD displays require a white backlight for proper viewing in any lighting environment. This backlight sub-system consists of an array of high brightness white LEDs, a diffuser to distribute the light and a backlight driver to convert the available power into regulated constant current to drive the LEDs. A 1-1.5" display may have 2-4 LEDs while a 3.5" display could easily have 6-10 LEDs. For LEDs, the light output is proportional to current and since LEDs have a very steep I-V curve it is important that the current through the LEDs be closely matched to assure even lighting since LEDs are typically distributed across one edge of the LCD display. In addition software control is necessary so the user can adjust the brightness and compensate for ambient light conditions. The colour point of the LED can shift depending on the current through the LED so it is common to set the LED current to a fixed value and pulse width modulate the LEDs to reduce the average light output. There are a number of factors that need to be considered when incorporating a small colour LCD display into a design to achieve the right balance of cost, performance and battery lifetime in a handheld product.

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