Electronic Design

Power Management For Portable Equipment Lighting

Sponsored by: NATIONAL SEMICONDUCTOR

What is behind the move to white LEDs in portable equipment?

The trend for color displays in handhelds has produced a parallel move to white LEDs for LCD backlighting. White LEDs provide the desired appearance for backlighting color LCDs. They're also more rugged and efficient, simpler to drive, and smaller, thinner, and far less noisy than the cold-cathode florescent tubes previously used for backlighting.

Why use driver ICs for white LEDs?

The simplest drive configuration for an LED is a voltage source and current-limiting resistor. However, a driver IC provides better overall performance. It can control white LEDs with a constant-current source that regulates their current regardless of power-supply voltage variations or variations in LED forward voltage drops. Also, the driver allows you to boost the voltage applied to the LEDs because the battery voltage can fall below the value required to turn on the LED.

What are the requirements for a white LED driver IC?

Many portable systems use a single lithium-ion (Li-ion) or multiple nickel-metal-hydride (NiMH) batteries to drive the white LEDs used for backlighting LCDs. The driver must provide the proper forward voltage for the white LEDs, which typically range from 3.3 to 4.0 V.

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