The AP1521 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. The device can drive two, three or four LEDs in series from a Li-Ion cell. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminates the need for ballast resistors. The AP1521 switches at 1MHz that allows the use of tiny external components. A low 300mV feedback voltage minimizes power loss in the current setting resistor for better efficiency.