INTRODUCTION

　　In August of 1992 LTC published Application Note 49， “Illumination Circuitry for Liquid Crystal Displays.” One notable aspect of this event is that it generated more response than all previous LTC application notes combined. This level of interest， along with significant performance advances since AN-49’s appearance， justifies further discussion of LCD backlighting circuitry.

　　This publication includes pertinent information from the previous effort in addition to updated sections and a large body of new material. The partial repetition is a small penalty compared to the benefits of text flow， completeness and time efficient communication. The most noteworthy performance advance is achievement of 92% efficiency for the backlight power supply. Additional new benefits include low voltage operation， synchronizing capability， higher output power for color displays， and extended dimming range.

　　A practical 92% efficient LCD backlight design is a classic study of compromise in a transduced electronic system. Every aspect of the design is interrelated， and the physical embodiment is an integral part of the electrical circuit. The choice and location of the lamp， wires， display housing and other items has a major effect on electrical characteristics. The greatest care in every detail is required to achieve a practical high efficiency LCD backlight. Getting the lamp to light is just the beginning！

　　Current generation portable computers and instruments utilize back-lit liquid crystal displays （LCDs）。 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 highest 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 the form of a sine wave. 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 also permit lamp intensity control from zero to full brightness with no hysteresis or “pop-on.”

　　The LCD also requires a bias supply for contrast control. The supply’s output should be regulated， and variable over a considerable range. The small size and battery powered operation associated with LCD equipped apparatus mandate low component count and high efficiency for these circuits. Size constraints place severe limitations on circuit architecture and long battery life is usually a priority. Laptop and hand held portable computers offer an excellent example. The CCFL and its power supply are responsible for almost 50% of the battery drain. Additionally， these components， including PC board and all hardware， usually must fit within the LCD enclosure with a height restriction of 0.25“.