Reference Design

CCFL to LED Conversion Power Supply

Replace backlight cold cathode fluorescent (CCFLs) tubes with strings of LEDs for lower power consumption, better durability, smaller size, and brighter light that is more easily custom tailored to compensate for changes in ambient light.

Source: National Semiconductor
11/10/2008

Backlighting in portable device displays has historically used cold cathode-fluorescent (CCFL) tubes which are difficult to power. The tubes run on alternating current (AC) and need a large initial voltage of typically greater than 1 kV to start. Once they have fired, their operating voltage drops to under a kV. Because a notebook computer, for example, typically operates on low DC values (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, 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 3.5V to 4.5V 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 a display to be more tolerant of and resistant to sudden shocks.

The reference design, with National Semiconductor’s LM3431 3-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 3.5V to 4.5V, 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, improving the readability of the display under all lighting conditions.