Charge pumps boost white LED drive efficiency

A new generation of high efficiency charge pumps has been developed for white LED applications.

A new generation of high efficiency charge pumps has been developed for white LED applications.

Designed to maximise power efficiency in battery-powered portable designs, the AAT3140 and AAT3141 charge pumps combine a load switch (1x) with fractional (1.5x) and doubling (2x) conversion to support LEDs with a wide range of forward voltages (VF).

"Matching LED brightness without wasting power has always been a formidable challenge for cellphone and other portable device manufacturers", says Jan Gripsborn, Manager of Field Applications Engineering at AnalogicTech.

"When battery voltage drops, competitive trimode parts typically require that all LEDs in a display run off the less efficient charge pump, even if only one LED requires it".

"By combining our constant current source architecture with built-in intelligence, we can provide power from the charge pump only to those LEDs which need it and, in the process, enhance overall system power efficiency without the additional cost of white LED VF binning".

The AAT3140 produces current levels up to 30mA on each of four current source outputs.

The outputs may be operated individually or in parallel.

The device can be used to drive a wide variety of LED configurations from a 2.7 to 5.5V output.

Unlike boost-convertor-based solutions which demand physically large, external inductors, these low-noise, constant frequency charge pumps operate at a high 1MHz switching frequency and require only four small (1uF) external capacitors.

Moreover, AnalogicTech's unique charge pump design also offers lower EMI and noise than an inductor-based boost convertor.

The AAT3140 also provides 32-position logarithmic scale LED brightness control via AnalogicTech's Simple Serial Control (S2Cwire) single wire digital input.

This interface provides a much simpler design than competing offerings that use a PWM signal to control LED brightness.

The use of constant current outputs ensures uniform brightness control regardless of differences in manufacturing lots.

The AAT3140 also features a thermal management system to protect the device from short circuit conditions and integrates soft-start circuitry to prevent excessive inrush current during startup.

The AAT3141 offers all the same features as the AAT3140, but adds addressability to its digital input control.

This Advanced Simple Serial Control (AS2Cwire) interface allows designers of portable devices that run multiple displays, such as clamshell-style cellphones, to independently control three LEDs to backlight a main display and one LED (or two in parallel) to backlight a sub-display.

Both the AAT3140 and AAT3141 come in compact 12-pin TSOPJW packages specified over the -40 to +85C range.

Pricing in 1000 unit-quantities starts at $1.95 and $2.09, for AAT3140 and AAT3141, respectively.

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