You might look into something like a LM259xHV from national (auto approved &
<=60v inputs)  --a bit overkill for 20ma though.  I've used their simple
switchers before with good results --not very picky component wise.  Anyway,
you should be able to get it into your size requirements with careful layout
& small inductor. You also might look at sipex.   I'm not sure what your
noise req's are though...

Also TI has some 3 leg complete switchers you might try, but they are fairly
tall and might not fit in your potting box (can you stand a protrusion?).
I'd have to look around to find a part #, I've got a few samples around here
somewhere...
-Dal
----- Original Message -----
From: Lyle Killough <lyle@SUREGRIPCONTROLS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, August 16, 2001 11:06 AM
Subject: [EE]: tiny power supply


> I have a power supply problem which I am hoping to get some good
suggestions
> on..... (I have lots of my own bad ideas)
>
> I am designing a small PIC-based device for use in mobile hydraulic
> machinery.  There is VERY little room on the board - the
> board will be less than .85x.75 inches / 22x19 mm, and there will be a 3A
> high-side (solenoid) driver along with a PIC12CE673 and a linear Hall
effect
> sensor in a TO-92ish package.  The board will be potted into a .25" / 6mm
> thick package.
>
> The device will be connected to anything from 10 to 26 VDC with a power
> supply requirement of 20 mA @ 5 VDC.  All of the hideous automotive-type
> abuse will be present; e.g. reversed battery, load dump, battery
disconnect
> and voltage peaks.  In addition, one of our customers has specified a 2
> minute, 80 volt survival test.
>
> The obvious solution of a LM2931-5.0 with a 32V over-voltage shut-down
will
> not work.  Way too much heat for a little linear regulator.
>
> I have a working option which will allow 10 to 80+ volts in, 5 volts out
> with about .25 volts of ripple.  This is really just a voltage comparator
> maintaining a voltage on a capacitor with a Schottky diode in the
hysteresis
> loop.  This is built with discrete transistors to allow the large supply
> voltage range.
>
> I know very close to nothing about switching power supplies, but I suspect
> that there is a buck regulator which will do this in a neat package.  I
have
> looked in the obvious places, but I cannot find a device which will take
the
> high input voltages in a small (low pin count) package.  I prefer to not
> have an inductor in this supply, current demand is low, I wish to minimize
> stray magnetic fields due to the Hall effect sensor, and there is so
little
> space.
>
> If any of you PICsters have any suggestions or cautions about how to
achieve
> the above, I'd love to hear about them.
>
> Thanks!
>
>
> ____________________________________________
>
> Lyle Killough      lyle@suregripcontrols.com
> ____________________________________________
>
>            Sure Grip Controls Inc.
>              4 - 1413 McGill Road
>             Kamloops, BC, V2C 6K7
>                    Canada
>
>               phone 250-374-2278
>               fax   250-374-1099
>
> ____________________________________________
>
>           www.suregripcontrols.com
> ____________________________________________
>
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