Voltage and Curennt regulator

Hay Guys
I need to put togther a regulator to drive a laser diode,
I would like to have adjustable current and voltage and be able to move between 2.5v and 5v and 20-500ma from what I can see an old lm317 would be good or a L200c,
But im a bit of a noob so can any body point me in the rite direction?

P.S the input would be 6v, two (2) cr123a battery's

Thanks allot,
Reuben

An LM317 needs a minimum input voltage about 2.5V more than the output voltage. You need two LM317 ICs, one for voltage regulation and the second one for current regulation. The current regulating one needs an additional 1.25V to function.

So you need 5V for the max output, plus 5V for the two LM317 ICs plus 1.25V for the current regulator. Therefore the minimum battery voltage is 11.25V.

check the catalogs from National, Maxim and Linear - they have programmable current adjustable voltage switch mode converters that should fit your needs.

One volt of headroom isn't much, even for a switcher - as AG suggests, you'll need to revise your battery voltage if you really want to drive your diode at 5v (seems kinda high). In the case of a switcher, three 3v batteries will give you more than enough room to get 5v, plus you won't be wasting all that extra voltage on heating linear regulators.

well i can live without 5v its just 4.5v would have been nice to run a violet diode

cheers, Reuben