>
>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.

Your dissipation, at 26V with a linear reg will be 0.42W, no matter how you
do it.
At 80V, 1.5W

A zener supply might work, if the rest of the components can handle the
3W/min of heat during the test, and live with the .42W continuous.   This
will depend on the total thermal mass, since you can dump heat into the
potting compound in the short term. Make sure that the resistor dosen't get
hot enough to decompose the potting compound though! :)

Are you using electrolytics? If so, mount them as far from the resistor as
possible.
Use a ceramic or monolythic cap to bypass the zener.

Linear and national both have 8 pin SMPS chips that can do buck, and you
may get away with the on-chip transistor at this power level.

The R-Zener shunt reg will get rid of most of the junk. A series diode will
protect against reversals, and you certainly have the input headroom to use
it.
--
Dave's Engineering Page: http://www.dvanhorn.org

I would have a link to http://www.findu.com/cgi-bin/find.cgi?KC6ETE-9 here
in my signature line, but due to the inability of sysadmins at TELOCITY to
differentiate a signature line from the text of an email, I am forbidden to
have it.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics