Okay, per a thread from a few weeks ago, I'm redesigning
to make robust ICSP a reality.  My system will normally
operate at 3.5V, so I need to isolate the PIC power
supply from the remainder of the system.

I was thinking about just sticking a diode in there, such
that it would be reverse biased during programming and
protect the rest of the supply rail from the voltage on the
PIC.  I was also thinking about using a Schottky.

The more I think about that, the less I like it.  Although
the current draw on the PIC is minimal (very low power
app), there's always that chance that something I do
will have an unexpected side effect and cause the drop
over the diode to exceed .3V, at which point the signal
from the SPI peripherals exceeds Vdd+.3V, which is
the limit specified by the datasheet.

The only device that I really feel the need to protect is
the regulator.  It's a low power PMOS pass LDO, so
if the output drops below the input, the parallel diode
in the PMOS pass element will back-conduct, and
there's no current limiter there.

All the other devices are fine with a brief (or even not
so brief) excursion to 5.5V for programming and
verification.  So what is the best way to protect my
regulator?  Three options spring to mind:

1.  Series diode, forward biased during normal
operation.  Of course, this means my system voltage
is not necessarily constant.  I dislike it.
2.  Reverse-biased Schottky from Vout to Vin.  That way,
in a situation such as programming where Vin = 0V,
the diode conducts "around" the regulator, protecting it.
Requires another reverse biased diode from Vin to gnd.
Again, not sure I like it.
3.  The "backwards PMOS" method, where a PMOS
with its drain tied to gnd through a large resistor is
hooked up "backwards" to the output of the regulator.
In normal operation, it would appear as a small resistance,
but since the current draw of the system is ~2mA at worst,
that's not such a big deal.

Sorry the post is so long, but I'd like to get it right this
time.  Anyone have any input on these ideas?

Mike H.

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