PNP BJT Hi-Side Switch with VPP capability

[Yashu]

I have an application where I need a 5V output signal to have a selectable 510 ohm pullup resistor applied. I have this circuit configured similar to a high side switch using a PNP BJT. The PNP sits between the 5V rail (== PNP emitter) and the 510 ohm resistor (== PNP collector). The PNP base is pulled to ground by a processor output pin (which is set to either an input or grounded output).

Now, an external VPP of 20V needs to be applied to the output signal line. So, this 20V causes 'reverse' current flow thru the 510 ohm resistor into the collector of the PNP and "out" of the base & emitter.

What must I watch out for with this topology? Does Vebo come into play and should I keep the PNP 'on' while VPP is being applied?

 

[felixc]

If your VPP can provide more than 30mA, an easy way to bypass the problem is to put a protection diode between the PNP collector and emitter. Cathode at the emitter. It prevent any reverse bias condition to the transistor, and it would also prevent any high voltage to get back to your processor via the base. I guess that VPP is pulsed. Make sure that the 510 ohm can take the heat.

 

[Yashu]

Are there any long term problems with the circuit as it is now?

 

[felixc]

Well, I remember a failing pnp in a clock radio because of a similar configuration. Keeping the transistor "on" would still reverse-bias it, and the current will unlikely flow in the reverse direction. However, it might work with a PMOS. You may find one with the same pinout as your PNP. Some even come with an integrated reverse diode.