I have this simple circuit to drive on and off a 12v rail from a microcontroller. enter image description here

R707 = 10KOhm, R708 = 10KOhm, and Q701 is a P channel mosfet (IRLML6402TRPBF)

I'm driving this with a mircocontroller (3.3V). the EXT_READER_SHDN is configured as open drain output.

When EXT_READER_SHDN is 0 (connected to GND) I have :

Vgs = -10.72V (since the 12V is only 10.73V)--> the mosfet in this case is ON (conducting)and 12v_READER = 10.73V.

When EXT_READER_SHDN is 1 (pin floating) I have :

Vgs is around -2.66V (threshold is 1.2V)--> the mosfet in this case is also ON and 12v_READER = 10.73V.

I have already printed the PCB and need to make a temporary fix until the next version. Any ideas of a simple way to be able to open and close the circuit ?

I thought about reduicing the R707 value to likes 2 or 3 KOhms, but this will apply a hight voltage on the microcontroller pin. but since the pin is on open drain, doesnt this mean that I can go crazy and put 12V and have no issues ?

Thank you guys

  • 1
    \$\begingroup\$ Most MCU data sheets explicitly prohibit applying a voltage to an I/O pin more than a diode drop outside the supply rail. For an MCU it doesn't matter that you have it configured as an open-drain output, the protection diode is still there steering current from your pullup resistor into the MCU supply rail, and if that diode has survived (which with 10K it might, at least initially) probably clamping it a little under 4 volts. There are open drain IC drivers that can tolerate higher voltages, but they aren't usually found in MCUs. \$\endgroup\$ Commented Mar 26, 2018 at 16:55

1 Answer 1


Your MOSFET is backwards.

Also, unless your micro is designed to handle 12V hanging on an open drain output the pin will be limited to whatever the output protects the pin with, usually a diode drop above Vcc.

I suggest you use two transistors.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thank you Trevor, It seems impossible to fix this circuit. I'll do add what you suggested. \$\endgroup\$
    – C.Hicham
    Commented Apr 3, 2018 at 16:30
  • \$\begingroup\$ As a temporary solution, I found the BSS315P where it says i Vgs threshold min -2V, max -1V. putting 100KOhm in R708 should take the voltage down to 0.8V, would this work ? \$\endgroup\$
    – C.Hicham
    Commented Apr 3, 2018 at 16:44
  • \$\begingroup\$ @C.Hicham with that device you need ~5V Vgs to get it on well enough. So 6V will do as per the above schematic. The figures you are quoting is the variance where it is likely to start turning on.. You should be looking at the Rds_on at Vgs numbers. \$\endgroup\$
    – Trevor_G
    Commented Apr 3, 2018 at 17:18

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