If it will be for one LED and low current, you may get away with a MosFet or similar.

But for 21 LEDs, I think you should consider the power loss in heat.

I am currently experimening with devices from ZMDI that are sold by MOUSER.

I test the boost current regulator ZSLS7025 that I need to power 25 watt LED at 720mA and about 36 volt from a 12-24 volt dc input and with Dimming capability.

This company makes such devices either for boost or buck. Have a look at this link:

http://www.zmdi.com/products/led-drivers/

Ioannis