The resistor zener supply is pretty robust and foolproof.  You can make
it tougher by using a current limiting (or current regulating) diode
instead of the resistor.  I don't know who makes them but a few years
ago we used them for a bootstrapping supply.  They look like a diode but
function as a current source of about 5 to 20 mA.  You can also use a
JFET with the gate shorted to the source.

Sherpa Doug

> -----Original Message-----
> From: Lyle Killough [mailto:lyle@SUREGRIPCONTROLS.COM]
> Sent: Thursday, August 16, 2001 1:06 PM
> To: PICLIST@MITVMA.MIT.EDU
> 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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