> 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.


This design is crying out for a buck regulator solution.
Sometimes the cry can be hard to hear :-)
Using a high side discrete P Channel FET or PNP transistor will allow you to
easily address the 80 volt requirement.

I can show you a design which uses 3 transistors (including the high side
switch), an inductor, 5 resistors (possibly 4), a zener and a capacitor.
Voltage withstand is very largely a function of the rating of the high side
transistor although two resistors need to have their dissipation designed
for worst case input voltage. I have a version operating with a 10.5 volt
output and a 0.1 volt output change for input voltages from 12 volts to 200
volts. (It fails gracefully below 12 volts in and output tracks Vin less a
volt or so).

Using surface mount components such as eg SOT-23 transistors (possibly a
SOT223 for the high side transistor) & 0805 resistors and capacitor this
would "probably" be able to be sneaked into a corner of your PCB.


If using a linear solution you could easily meet the 80 volt two minute test
with an input transistor that turned off when Vin rose too high. This
transistor could replace your input dropping resistor during normal
operation.







      Russell McMahon
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