Ah, the wonders of Gmail.  Easily putting 2 responses into one
mail...

First, to Spehro-

> > > >Sorry the post is so long, but I'd like to get it right this
> > > >time.  Anyone have any input on these ideas?
> > >
> > > What about a series Schottky on the input side of the LDO?
> > >
> > > What's on the other side of the regulator? A battery & capacitor?
> >
> >Input is battery -> PMOS reverse protection + cap.
> >Output is (currently) bypass capacitors per datasheet.
> >
> >I'm not sure what the series cap for the input of the LDo would do.
> >It gives me input protection, but not protection against Vout<Vin,
> >which is the case when a programmer is powering the circuit and
> >the battery is disconnected.
> >
> >Mike H.
> 
> Won't your reverse protection also prevent current from flowing
> out of the regulator input into the battery?

Yes, and that's the idea.  During normal operation, a 9V battery feeds
a 3.5V regulator, which feeds the rest of the circuit.  During programming
there will be no battery in place, and the programmer will power the
circuit by simply hooking up to PGC, PGD, Vdd, Vss and Vpp of the PIC.

The problem is that the LDO I'm using (TI TPS77001) is a PMOS pass
regulator, so the "inherent" diode which parallels the source and drain
of the transistor becomes forward biased when Vout>Vin.  That means
it conducts, which in turn means that without some form of current
limiting, a rather large current can flow there.

ALTHOUGH now that I think about it, in the case of no battery,
there would be NO conduction, because there's nowhere for the current
to go.  Which makes me an ass, and this thread moot, since the
programming would occur with no battery present.

It would still be nice to be certain that, if someone were to try to program
the device with a battery in place, no smoke escapes.

--------Next, to Wouter-

> note: when your nominal operating voltage is 3V it makes no sense and
> there is no need to verify at 5.5V. IIRC 4.5 is required for some
> programming aspects (chip erase?) but not more.

True.  I'm not sure that I feel like being able to do 4.5V instead of 5V is
a huge improvement, however.

> How about a reverse (schottky?) diode over the regulator, maybe with a
> series resistor at the input of the regulator?

That was the idea I was trying to convey with option 2 from my first
message.  But, see above, this whole thing is now pointless, since I've
realized that with no battery present, the diode has nowhere to conduct
to, which leaves me with only one thing to worry about:  overvoltage on
the supply rail (i.e., someone connects the programmer and tries to
write the chip with a battery in place).

Mike H., feeling sheepish

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