Appliance Design - November 2007 - (Page 24)

DISPLAYS & INDICATORS Driving LED Backlights Fig. 1. Yield regions on a typical LED wafer. When converting from CCFL to LED backlights in LCD panels, new driving methods are needed. V by achilles wang Achilles Wang is applications engineer at Maxim Integrated Products, Ltd., Wan Xin, China. 24 applianceDESIGN November 2007 endors of large-area and mediumarea displays are switching from cold-cathode fluorescent lamps (CCFLs) to the more environmentally friendly LED backlights, as “green” technology permeates more and more systems. LED backlights are also thinner, consume less power, and are more reliable than CCFLs. As designers face tighter environmental regulations, they focus on reducing the number of environmentally harmful substances in their systems. And, as consumers become more environmentally conscious, they demand replacement of the lead in integrated circuits and the mercury in CCFLs. Some of these changes are mandated by organizations such as the EEE, which developed the Restrictions on Hazardous Substances (RoHS) directive for electrical devices in the European Union. Thus, manufacturers of LCD panels and modules all over the world are actively looking for substitutes for CCFL backlights. When compared with traditional CCFL backlighting, LED backlights meet many of the new environmental requirements simply by being hydrargyrum-free (free of mercury). They have many other advantages, such as good color reproduction, fast response speed, lower-power operation, improved safety (no glass to break or vapor to disperse), long life, and small size. Until now, the main drawback has been cost: matched, high-brightness LEDs have been much more expensive than CCFLs. But, that situation is changing, now that the sales volume of LED backlights has begun to increase. The effect of superior features in LED backlights is now driving their rapid adoption by display manufacturers. LED sources have replaced CCFLs in many applications requiring small-area backlights, and are now the mainstream choice for those applications. Similarly, the large increase in production of medium-size LCD screen modules for control panels, flatscreen monitors and televisions will lower the cost of LED backlights for those applications, again making them the mainstream choice for the larger LCD panels. The driving scheme for medium-tolarge LED backlights differs from that used for the small LCD screens found in mobile telephones and other portable products. To accommodate larger screens, the backlight must include several dozens, or even hundreds of high-brightness LEDs illuminated simultaneously. That configuration usually means higher driving voltage and higher current, more rigorous cooling requirements, more flexible controlling methods, and most important — adequate current matching and equal currents among the LEDs. Thus, LED backlighting for medium-LCD and large-size LCD screens imposes more stringent requirements on the driving circuitry. www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - November 2007 Contents Editorial Shipments/Forecasts News Watch Today’s LEDs are Brighter and More Colorful than Previous Versions, Allowing Designers More Opportunities to Improve and Differentiate Their Products. When Converting from CCFL to LED Backlights in LCD Panels, New Driving Methods are Needed. Electrowetting Technology Allows Displays to be Designed in an Extensive Range of Colors. Noise Problems are Not Always Easy to Find or Fix, but Acoustic Labs Can Help Designers Ferret Out the “Good” Noise from the “Bad.” A New Sanitization Protocol for Clothes Washers Combines the Best of Three Well-Established Provisions to Ensure Efficiency, Quality and Safety. Stricter EMC Requirements Raise the Bar for Appliances with Electronic Controls and Require Testing to New Levels. Despite Added Cost and Complexity, Gas-Modulating Systems have Migrated from Large-Scale Boiler Applications to Smaller Residential and Commercial Furnaces and Appliances. A New Treatment Technology Hardens the Surface of Light Alloys such as Aluminum and Magnesium and Expands Their Potential Applications. DesignMart Advertiser’s Index Association Report: AHAM

Appliance Design - November 2007

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