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Putting small color LCDs in handhelds

Posted: 17 Jan 2008 ?? ?Print Version ?Bookmark and Share

Keywords:small color LCD design? battery? backlight display technology?

Over the last several years, small color LCDs have been integrated into an ever-broadening range of products. Color displays, once considered a luxury in cellphones, are now a standard offering even at the entry level. Fortunately, the economies of scale of this industry have lowered the cost of LCD colors and made them attractive for integration in other products

Color LCDs require a white backlight for proper viewing in any lighting environment. This backlight subsystem 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. It is important that the current through the LEDs be closely matched to assure even lighting since the LEDs are typically distributed across one edge of the LCD. Software control is necessary so the user can adjust the brightness and compensate for ambient light conditions. The color 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 color LCD into a design to achieve the right balance of cost, performance and battery lifetime in a handheld product.

? Evaluate the display use time profile. When selecting the white LED driver, take into account how frequently the display is used. If the display is going to be backlit for long periods of time for viewing, having a highly efficient converter will be critical to battery lifetime. Larger displays require many LEDs, and applications with high display use time will benefit from the higher efficiency of a boost topology. Conversely, if the display needs backlighting for only short periods of time, efficiency may not be a critical design parameter.

? Carefully consider the LED selection. LED technology continues to improve at a rapid pace and manufacturers are using new materials, fabrication techniques and LED designs to extract more light output for the same current, so a display that required four LEDs a few years ago may be able to achieve the same backlighting brightness with two LEDs.

? Pay attention to layout. Even though the individual LEDs are driven at low current (10-20mA), the peak input current through the converter is significantly higher. This is especially true of inductive topologies where the peak switch current can be 10x to 20x the average LED current, so proper low loss layout techniques need to be employed. For charge pump topologies, the capacitors should be placed close to the driver to minimize the loop area to avoid radiating switching noise.

? Test your product in real environments. Consider how the display looks in high ambient light conditions and make sure that the software dimming control has enough dynamic range to adequately dim the display in the intended light situations.

? Forget to consider boundary and failure modes. Defects occur, and if an LED is open or shorted to ground, how will the driver handle it? For inductive boost converters, if the LED string is open, the output will fly up since the constant current will charge the output capacitor requiring over voltage protection, which may or may not be integrated into the driver.

? Just look at peak efficiency. Since the backlight intensity is user-adjustable, consider driver efficiency where the display backlight is expected to operate the majority of the time. When looking at driver efficiency, consider the expected operation condition, the battery voltage range and the forward voltage variation of the LEDs. Inductive drivers have better peak efficiency and also are more tolerant to input and output voltage variation.

? Ignore external component selection. In all cases, low ESR X5R or X7R ceramic capacitors should be used to minimize losses. Also, in the inductive case, the forward voltage drop of the Schottky rectifier (if external) and the ESR of the inductor will impact efficiency.

- Bernie Weir
Director of Applications, ON Semiconductor Corp.

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