White LED Charge Pumps Exhibit High Efficiency May 2003 Issue Published Date: May 01, 2003
A new family of high-efficiency, low-noise, constant-frequency charge pumps has been developed for white LED applications. The AAT3123 and AAT3124 devices integrate both a 1.5X fractional charge pump and a 1X load switch, with the dual-mode devices said to push efficiency to new levels by only stepping up the voltage when necessary. The 1X load switch allows battery voltage to pass through to the output when sufficient voltage is available and activates the fractional 1.5X charge pump only when the battery is low. Both devices feature 32-level logarithmic scale LED brightness control through a single-wire digital input using a Simple Serial Control (S2Cwire) interface. The use of constant current outputs is said to ensure uniform LED brightness regardless of differences in manufacturing lots; it also eliminates the need for current-limiting resistors. A 1-MHz switching frequency enables use of very small, 1 µF external capacitors. The devices produce constant current levels up to 20 mA for each output from a 2.7V to 5.5V input. The AAT3123 drives four LEDs, is available in a 12-pin TSOPHW-12 package, and costs $1.80 each/1,000. The AAT3124 drives six LEDs, comes in a 16-pin 4x4 QFN package, and costs $2.04. ADVANCED ANALOGIC TECHNOLOGIES INC., Sunnyvale, CA. (408) 524-9684.
