I just tried to use LM317 input 24vdc output 21.5~22V and have a good result , Thank you for all advises.
(240R+trimmer 5K) direct to 6leds, at this time just try without additional resistor stable at 490mA. I would like to drive at maximum 750mA by change the resistor value.
Prevent overheat, I need to add temperature controller please advise for simple scheme precision.
Assumed critical temperature 50deg C and ideal 35-38C max.

 

The original thread got a bit tangled -http://forum.allaboutcircuits.com/showthread.php?t=31738 , so I have placed the new question in a new thread.

We need more input from you - what are some parameters? How much power has to be controlled? How precisely must the temperature be maintained? What factors might cause problems?

 

The LM317 really needs about 3v "headroom" when used as a current regulator.

This is based on the typical dropout of 1.7v, plus the nominal Vref (measured from the OUT terminal in respect to the ADJ terminal) of 1.25v; 1.7v+1.25v = 2.95v. However, the dropout varies as a function of current through the regulator and the temperature of the regulator.

See the "dropout voltage" plot in National Semiconductor's datasheet for the LM117/LM317, page 6, 2nd down on the right.

 

In regards driving 6leds @3W in a string, what do you think to LM317
is this good choice or any other convenience ICs?
I choose this thought easy and very simple, but not sure is this common or not pls advise because I have a plan to make 6 parallel in a board.

 

In this case, "3W LEDs" is rather ambigous.

We also need their Vf @ current rating.

However, I suspect that you're going to need something like a BuckPuck to drive them.