This circuit will drive 1watt white LED from a 12v supply and is capable of delivering 300mA. The driver transistor is BD 327 and the inductor is 70 turns of 0.25mm wire wound on the core of a 10mH inductor.
The voltage across the LED is approx 3.3v - 3.5v
The 1R is used to measure the mV across it. 300mV equals 300mA LED current.
The diode MUST be high speed. Non- highspeed diode increases current 50mA! This circuit is the best design as it does not put peaks of current though the LED. Reduce 390R slightly to increase max current.