> The other is 12-24V and efficiency is not
> top priority so the MC34063 is a possibility, although the user could
> put 1 LED or several in series on any channel, so the design would have
> to work with all combinations of 12V or 24V in, and a single 3V LED to
> (Vin minus say 3 volt headroom) string of LEDs.

MC34063 (and many other ICs) could handle those needs OK.
With CC out it just makes voltage that LEDs need.
Up to ~=3D available Vin if buck mode.

12V/24V is not too demanding. Main impact is probably on inductor
value as it will ramp at double the rate at 24V.

Brain offers that in continuous mode L can be higher than in
discontinuous mode and wide Vin range no issue.

"My" fully discrete  'GSR' buck converter of long ago & much PICList
discussion managed 12V out for Vin of 12V-200V (and an unspecified
amount above that with high side PFET voltage rating setting the upper
limit. This is a hysteretic converter as per Dave's mention. In the
12v out design, below 12V in it passes Vin to Vout with a diode drop
and IR drop of inductor (small) and Vdson of FET (small)  and then
seamlessly drops into buck mode as Vin climbs.
In this application Vout is < 12V for up to 3 x White LEDs so
converter would always run. This uses 3 transistors in basic form
(MOSFET high plus 2 jellybean bipolars (one rated for Vinmax).
In my main application power was moderate (10W?) and efficiency at
200V was only about 50%, but much better  for 12V < Vin < 100V or so -
the usual range of use. (200V is exercise machine at no load with
teenager pedalling utterly flat out trying to blow it up)(Airbus mode
we-know-what-is-best-for-you software loading under such conditions
foiled their game :-) ). I'd not recommend GSR for 200W but at lower
powers where utter low cost is wanted it may give an MC34-63 a good
run for it's lack of money.
..

  Russell
--=20
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