[sdiy] SSM2164 protection

[David G. Dixon]

So, I decided it would be a hassle to try and retrofit negative-rail
protection to all the 2164s in my system.  Also, the idea of putting a
resistor on the +V pin gives me willies, and I don't relish the idea of
having to lay out special power connections for that chip on all my designs.

Hence, I'm thinking that the best solution (for me) is to retrofit the power
distribution boards in my cabinets with p-channel power MOSFETs on the
positive rail (something beefy that won't induce significant voltage drops
-- I'm thinking something like IFR9Z34, which can supply 18A).  I'll wire
the gate to the negative rail through a 20V zener (1N5357B -- thanks for the
hot tip, Richie!), and to the positive rail across a 100k resistor.
Response times should be reasonably fast, and the MOSFET will deactivate the
positive rail whether the negative rail is disconnected or grounded.

Negative rail fails, MOSFET shuts off, entire cabinet powers down safely in
less than a millisecond, 2164s spared, Dixon happy.

OK, where's the flaw?