Power Electronics / Power Management


White LED driver offers 32 brightness levels

10 March 2004 Power Electronics / Power Management

Advanced Analogic Technologies has developed two advanced Power Management ICs designed specifically for driving and controlling the brightness of up to six white LEDs used to backlight colour LCDs used in mobile phones, smart-phones and PDAs, etc. The AAT3113 and AAT3114 are low noise, constant frequency charge pump ICs, that use fractional (1,5 X) conversion to optimise efficiency in white LED applications.

The devices can be used to produce current levels of up to 20 mA for each output, from a 2,75 to 5,5 V battery input, with a minimum of external components. These devices use only four small (0805), 1 µF ceramic capacitors, and are ideally suited for use in battery-powered applications.

Unlike the common red LEDs, white LEDs require a much higher forward voltage in order for current to flow. In many portable applications, a single cell Lithium Ion battery is the battery of choice. However, during its discharge cycle, the battery voltage drops below the voltage necessary for diode conduction, necessitating the need of a charge pump boost converter. The AAT3113/4 can provide this function as well as regulate the current for uniform brightness. For higher current outputs, each of the output channels can be paralleled to produce up to 120 mA of constant current.

A single wire, serial digital input pin is provided to enable, disable and set the LED drive current. Thirty-two levels of current are obtainable on a logarithmic scale, allowing the brightness to be increased linearly. Unlike other LED driver chips, which use additional components to PWM the output, once the brightness level has been programmed, no further data input is required until a new brightness level is desired. Coupled with the built in soft start, this reduces noise and increases efficiency. When shutdown the device only consumes 1 µA of supply current.



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