Hi Russel, thanks for the email and advice.

I have used the MC34063 in another design.  For the two LED driver=20
designs next up to do, one must be highest efficiency and run from 48V -=20
it is 11 channels driving a total of nearly 200W of LEDs, so MC34063=20
can't do it (40V max input).  The other is 12-24V and efficiency is not=20
top priority so the MC34063 is a possibility, although the user could=20
put 1 LED or several in series on any channel, so the design would have=20
to work with all combinations of 12V or 24V in, and a single 3V LED to=20
(Vin minus say 3 volt headroom) string of LEDs.

The idea of buying any chip for 5c is pretty marvelous.  Nice to know.

Your knowledge and enthusiasm to help is invaluable and appreciated.

J







RussellMc wrote:
> You do not appear to have looked up the Richard Prosser design that I men=
tioned.
>
> Circuit for one version attached.
>>From http://www.piclist.com/techref/postbot.asp?by=3Dtime&id=3Dpiclist\20=
01\08\26\170913a
>
> Circuit operation described here
> http://www.piclist.com/techref/io/relay/HVwLVcoil.htm
>
> ___________________
>
> You do not seem to have priced an MC34063
>
> Example prices here (SOIC-8)
> http://www.digikey.com/product-detail/en/MC34063ECD-TR/497-7852-1-ND/1883=
644
> $US0.17 / 1k, 31c / 100, 41c/10
>
> ONSEMI data sheet here
> http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF
> Internal switch is good for 1.5A.
>
> Lots of app notes and sample PCBs.
>
> Fig 11 does what you want with added 2Rs and  small C, not counting
> any filter caps.
> I'd add a small cap across R2 to improve transient response noticeably.
> See fig 15 for sample PCB layout.
>
> Fig 11 is voltage feedback and you need a tiny bit more circuitry to
> do CC LED drive. ((add a diode and another resistor. )
> NOT the most efficient solution - which may or may not matter to you.
> But also, not totally terrible).
>
> There are Asian sourced ICs that do this cheaper, but if you buy the
> part in Asia it costs about 5 cents in production volumes.
>
> Have this made for you in Asia (or ask Djula and all up cost will be
> under $1 in reasonable volume and maybe usefully so).
>
>
>   Russell
>
>
>
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.