Voltage regulator with alternate voltage source...

[Vince Mulhollon]

The data sheets all spec stability when the output has a fraction of
one uF on the output, but for a variety of (bad) reasons some
designers insist on putting, sometimes, tens thousands of often very
low ESR uF at the output.  That's extremely hard on the protection
diode and it'll have to be beefy, I'm sure at some size of cap it'll
be bigger than the regulator die, and none the less someone will
attach an even bigger cap (why?) and blow out the reg anyway.  In the
old days regulator ICs were very expensive and caps were cheap, so
you'd try to run entire systems off one expensive reg and one HUGE
cap.  Now, reg ICs are very cheap and small and caps and especially
heatsinks are expensive and relatively large, so its cheaper to
regulate multiply and locally, with small caps, no need for the
diode.. so don't expect changes from the mfgrs any time soon..

Assuming money were no object and people would pay twice as much for a
reg where the die is half protection diode, the next problem is a
reverse biased diode is a RF short, so goodbye 80 dB CMRR or whatever
it is, and uV level output noise performance.  The old 78xx series is
quiet enough for analog or digital use so they'd need to sell quiet
ones for RF and protected ones for digital.  Tens of mV of RF noise at
5 MHz is fine for digital, not so good for a microphone preamp.

I'd guess that "most" IC regs in a circuit with a giant cap are just
driving a giant transistor anyway to get 15 amps out or whatever, and
that junction is the one needing protection anyway.  So the lm317 in
my giant astron 12V power supply will never blow... its "only" driving
some giant pass transistors, which are the junctions actually needing
protection diodes.  So most of the time circuits with giant caps come
with giant transistors making protection diodes on the die a waste.

The 78xx series is short circuit protected and they can sell that with
a straight face without a reverse protection diode.  With a diode, all
you need is the drop from input to output to exceed its PIV (power
spike?  Shorted output?) and it'll zener till the cows come home,
blowing everything on the board with no short circuit protection until
the protection diode melts.  Now the hearsay handwavy starts with "due
to design of the reg, the overall system is more survivable without
the protection diode".

A low PIV might make the reg useless for HV applications.  The 7805
for example doesn't actually output 5V it forces the ground pin to 5
volts lower than the output.  If ground is actually 995 volts, the
output will be a nice regulated 1000 volts for a geiger counter or
whatever.  That means the reverse protection diode "needs" a PIV like
1000+ volts.  On die, thats tricky and expensive.  Its best left off
die for the engineer to optimize.

Finally in my experience its mostly theoretical anyway.  You can't
have reverse current without a complete path, and the reverse path
tends to be pretty low current when the power is off.