A circuit that can drive three to four high power LEDs is shown on figure below. The ADP 1610 is an adjustable boost regulator. The voltage headroom and current for the LEDs is provided by this device. An average gain of 12 feedback network that servos the Rset voltage and the ADP1610 FB pin 1.2 V band gap reference voltage is formed by the AD5228 and the op amp. The voltage across Rset is regulated around 0.1 V and adjustable by the digital potentiometer as the loop is set.
I led = Vrset/Rset
To conserve power but large enough to limit maximum LED current, Rset should be small enough. To limit the LED current within an achievable range, R3 should also be used in parallel with AD 5228. By lowering the R2 to R1 ratio as well as changing R3 accordingly, a current adjustment range can be expanded. [Source: Analog Devices Application Note]