On Tue, 18 Jan 2011 15:11:46 -0700, "Forrest Christian" said:

> But I would love to find a switchmode regulator IC with some sort of=20
> overvoltage protection built in.
>=20
> As I've posted on here a couple of times recently, I'm dealing with=20
> various input protection circuits which generally clamp to the rails -=20
> sometimes raising the power supply rail to a too-high level due to the=20
> power being clamped to the rail being in excess of what is being used on=
=20
> that rail.

I've always had to deal with that problem on the board, preferably close
to the part of the circuit that dumps the current into the rails. For
protecting against positive swings, a PNP transistor with collector to
ground, emitter to your input pin. A small diode from positive supply to
base and a resistor from ground to base.=20

The advantage to this circuit is that your I/O excesses don't do
anything to your supply voltage, rather than a supply clamp which allows
the supply voltage to raise to the clamp voltage when clamping occurs.

You can use one circuit per protected pin. Or share the diode/resistor
combination among several transistors. Or share the whole thing using
one resistor, two series diodes, one transistor and then just one diode
per circuit.

Basically it is an emitter follower with the base tied to a reference
voltage one diode drop below positive supply. If you use a PN2907 it can
take a lot of current.

Friendly regards, Bob

--=20
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