http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/XLampXPE.pdf
say you wanted to run 20 xpe reds at 350ma. according to the link, at 350ma a red cree xpe uses 2.1 volts. if u had 20 leds thats about 42 volts total. so you would want a constant...