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

In this circuit, the anode (+) of the diode is connected to positive supply=
?=20
What is the typical resistor value?


--=20
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