I was implementing a circuit to only turn on a voltage to a sensor ("Measure Pin") when there was VCC (3.3V) on my board. (Vin may be there even when VCC is not). I have attached my circuit here. Basically, Vin should go through to "A" and further down the voltage divider only when there is VCC (DVCC3P3 in my schematic).

However, I would like to have Vin be in the range of 6V-36V. From the NTR0202 spec, it seems that the gate-source voltage should only be a maximum of 20V. So I think if Vin is greater than 20V I'll have an issue, correct? Are there any recommendations for the replacement of this P-channel MOSFET?

I found the ZVP4525GTA, with Gate-Source of +/-40V, but with a Drain-source of -285V is this overkill?

Any other comments would be appreciated. Thank you very much.

 

You can add some resistance between the drain of the 7002 and the gate of the 4525. Then add a zener - say 12 volt from gate to source.
On a side note you might want a logic level fet for the 7002. It may work but not for sure all of them will.

 

Ron - thanks for the reply.
1. So you would recommend going with the 4525 instead of the NTR0202pl?
2. Will a logic-level FET do much different than the 2N7002?
3. If I go with the 4525, then I wouldn't need the resistor / Zener, correct? Would this only be if I kept the NTR0202PL? Which is better? Switching to the 4525? Or should I do both (switch to the 4525 and also add the resistor/zener)?

Thanks again.

Any other comments appreciated.

 

Sorry I wrote down the wrong one.

The 7002 will probably be ok. The gate threshold voltage is 2.5 volts.

 

Another option would perhaps be to clamp Measure Pin to ground using a BJT whose base is driven by another BJT. Less critical than using FETs.

 

Alec, if we clamp the measure pin to ground whenever Vcc isn't there then we would constantly have some current through the 100k resistor when the board is off. (Correct?) If not, could you explain a bit further?

Ron, do you recommend keeping the 7002? Just wondering why?

Doesn't moving to the 4525 solve the issue?

And even if I do change to the 4525, should I still put in the resistor on the gate of it? (And/or the Zener?)

Any replies welcome. Thanks!

 

if we clamp the measure pin to ground whenever Vcc isn't there then we would constantly have some current through the 100k resistor when the board is off. (Correct?)

Correct. So assuming the maximum 36V at Vin the current would be 360uA.
The first attached circuit uses two NPN transistors. If the 360uA is undesirable then the second attached circuit, using an NPN and a PNP would do the job. General purpose small-signal transistors with a >36V rating should be easy to source.