PNP for soft switch and reverse battery protection

put a reverse diode on the inside of the protection to short any small leakage so if the battery is connected backwards, 100uA will flow through the diode to ground Mark

Batteries usually discharge at nearly constant voltage, and drop off rapidly at end of life. So a series diode won't affect lifetime much.

You might give up 300 mv to a schottky diode.

Do Schottky diodes leak that much? That seems like a lot. (un-measurable on my DMM uA scale... leakage less than 50 nA.) (Well that's a small diode.. 1N5711)

George h.

it's not diode leakage I'm taking about

the 100 ua leakage is from the PNP circuit when the battery is connected backwards.

The OP is concerned this 100 uA leakage will damage his load.

A SHUNT diode will forward conduct the 100uA leakage to ground and almost no reverse voltage will appear across the load.

Oh sorry my mistake.

GH.

I would be shocked if any logic device is unable to survive at least as much reverse current as the ESD protection diodes (i.e., the input current limit on a CMOS input logic pin), if not the Vdd/Vss pin current limit (which is carried by transistors not diodes, but the diodes are at least as big as the transistors, see--). So, 10 to 100mA, not ~uA.

I can't conceive of a reason how or why a CMOS IC should fail from that. It's just substrate forward bias!

And, for that matter, I would be very concerned if I discovered a chip that does die so easily! Assuming I have any choice in its selection, of course...

In any case, you can prevent such exposure by shunting Vss to Vdd with another small schottky, which will keep it well below a PN junction Vf.

Tim