The AP1521 is a step-up DC/DC converterspecifically designed to drive white LEDs with aconstant current. The device can drive two, three orfour LEDs in series from a Li-Ion cell. Seriesconnection of the LEDs provides identical LEDcurrents resulting in uniform brightness andeliminates the need for ballast resistors. TheAP1521 switches at 1MHz that allows the use oftiny external components. A low 300mV feedbackvoltage minimizes power loss in the current settingresistor for better efficiency.