Power Electronics

Boost Converter Powers White LEDs

Semtech’s SC4505 is an integrated boost dc-dc converter for driving white LEDs in LCD backlight and camera flash applications in cell phones and PDAs. Using a single inductor, the part drives 12 white LEDs with greater than 82% efficiency. The SC4505, which features a 2-A power switch, generates adjustable output up to 28 V for driving two LED strings with up to six LEDs per string.

For each LED string, the chip provides an independent user-selectable current source. The backlight string supports up to 75 mA and the flash string supports up to 125 mA. Backlight dimming can be achieved through digital PWM control signals, which maintain peak LED current for maximum brightness. The camera flash string features a user-controllable flash timeout that reduces LED burn out. In addition, the flash can be set to 20% of maximum flash current for continuous LED lighting.

The converter operates at a nominal switching frequency of 1 MHz with an input range of 2.6 V to 12 V. Driving the LEDs in series results in excellent current matching for uniform lighting. The SC4505’s high operating frequency and higher integration reduces board space and BOM costs of the display/flash subsystem.

The SC4505 uses a regulation technique that optimizes its operation to seek the highest efficiency irrespective of load current in each string. This innovative power regulation technology adapts to the instantaneous power requirements of the system, reducing power dissipation while maintaining high efficiency. The device comes in a 3-mm x 3-mm, 16-pin MLPQ package that is lead free and RoHS/WEEE compliant. Pricing is $1.56 each in 1000-piece lots.

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