Since the conception of cellular phones, PDAs, and handheldcomputers, there has been a continuing push for moreuseful and dynamic displays. One of the more drasticchanges in miniature display technology has emerged due tothe availability of Internet content, pictures, and videos onever-shrinking personal devices. The promise of more contentand functionality has caused a migration toward higherresolution color displays. This presents some added designissues, however, as a color LCD display requires whitebacklighting as opposed to the more standard green. Thecurrent options are using a cold cathode fluorescent lamp(CCFL), an electroluminescent backlight, or newer whiteLEDs. White LEDs are quickly becoming the light of choicebecause of their falling costs, longer life, and smaller size.The problem this presents is that the white LED has a highvoltage drop (3.1V to 4.0V depending on manufacturer) ascompared to the green LED with a voltage drop of 1.8V to2.7V. Whereas the green LED can be powered directly fromthe commonly used Li-Ion battery, with a linear regulator,and a ballast resistor, the white LED used for backlight orfrontlight purposes will requires the battery voltage beboosted.National has many solutions for driving LEDs includingswitched capacitor converters and inductive based switchingregulators. This application note will describe some differentswitched capacitor methods of driving white LEDs and thebenefits of each. The main areas of concern for most designersof portable equipment are efficiency, size, cost, functionality,and LED current matching. Balancing these competingdemands will help designers make the right choice forhis or her application.
