High power LEDs require large currents to operate. Limiting the drive current can be done in several ways but efficiency needs to be considered. The ideal method for driving these LEDs is with a constant current source specific to their needs. A linear constant current source can waste a lot of power and get very hot doing it. A switching regulator would be an ideal choice.
Here is a circuit that uses a switching regulator to make up a constant current supply with efficiency and stays cool.
The LM2578HV is capable of input voltages up to 60 Volts. It has an internal reference of 1.23 Volts.
The LT1787HV has a fixed gain of 8 and it can also operate up to 60 Volts. The formula to use is:
Vout = Vbias + (8 * Iload * Rsense)
In this design, Vbias is grounded or 0 Volts. Vout is the reference voltage of the LM2578HV or 1.23 Volts. Therefore Rsense = 1.23 / 8 * 0.7 = 1.23 / 5.6 = 0.219 or 0.22 (close enough).
Other output currents can be calculated in a similar manner.
Constant Current Supply for High Power LEDs
A high efficiency constant current supply based on a switching regulator.