Current generation portable computers and instruments utilize backlit LCDs (liquid crystal displays). These displays have also appeared in applications ranging from medical equipment to automobiles, gas pumps and retail terminals. Cold cathode fluorescent lamps (CCFLs) provide the high- est available efficiency for backlighting the display. These lamps require high voltage AC to operate, mandating an efficient high voltage DC/AC converter. In addition to good efficiency, the converter should deliver the lamp drive in sine wave form. This is desirable to minimize RF emissions. Such emissions can cause interference with other devices, as well as degrading overall operating efficiency. The sine wave excitation also provides optimal current-to-light conversion in the lamp. The circuit should permit lamp control from zero to full brightness with no hysteresis or “pop-on,” and must also regulate lamp intensity vs power supply variations.
This application note reviews the recent technologies and techniques applicable to LCD Backlighting. It presents theoretical considerations, practical suggestions, remedies and circuits.