macegr, I wondered about that. That schematic is labeled as 'constant current with external switches'. There is another schematic labeled 'constant voltage with external switches' which looks like this:
That one looks more like what I'm used to seeing. RL can be used to set the current through the LEDs. RB = 2K to 5K. Saturation voltage is 0.5V - 0.8V. The formula given for RL is:
RL = (VDD - VLED - VCE) / ILED
I presume VDD = the indicated 24V. VLED ... is that the sum total of the voltage drop of all the LEDs? So if I have 3 of them, each with a VDROP of 3.4, VLED = 3.4 x 3? And VCE = saturation voltage (per datasheet).