Description

Backlighting in portable device displays has historically utilized difficult-to-power cold cathode-fluorescent (CCFL) tubes. The tubes run on alternating current (AC) and need a large initial voltage of typically greater than 1 kV in order to fire them. Once they have fired: their operating voltage drops to under a kV. Because a notebook computer: for example: typically operates on low DC voltages (12V: 5V: 3.3V: etc.): a Royer oscillator must be used to transform this low voltage to the high-voltage AC required by the CCFL. The high voltages in the system are a potential safety hazard.The CCFL to LED conversion power supply reference design replaces the backlight CCFLs with strings of light-emitting diodes (LEDs). LEDs operate at low voltages of 3V TO 4V each and are extremely durable. They produce a brighter light whose spectral content can be more easily custom tailored to the needs of the backlight. Their brightness can be easily controlled to compensate for changes in ambient light. Also in contrast to the glass-tubed CCFLs: the physically small LEDs can be mounted on flexible strips: allowing the display to be more tolerant of and resistant to sudden shocks.The reference design: with National Semiconductor’s LM3431 three-channel: constant-current LED driver: uses two strings of six series-connected LEDs at a current of 30 mA per string. Each LED has a forward voltage drop of about 3.5V: making the total voltage across each string between 21V and 27V. With a 12V power source: the LM3431 is configured as a boost converter to generate this 21V to 27V output voltage.The design also includes a circuit utilizing a photodiode to make the brightness of the LEDs proportional to ambient light thus improving the readability of the display under all lighting conditions.