After looking at this, I think I already know the answer, but I just wanted some clarification.

Open collector microcontroller output

I have a part that is going to drive my mosfet at 15V. I have thought about putting in a 10K resistor on the gate and pulling it to ground through the atmega48 microcontroller and sink the current 1.5mA to force the gate to ground. According to the datasheet Port B can sink/source current.

I know from the datasheet that I can sink 10mA per pin at 3.3V with the Atmega48, but based on the datasheet it appears that only ADC4 and ADC5 pins will allow it and I need 4 pins.

So it appears that I have two things going against me. I need 4 pins that can be open collector and the 15V gate driver is much higher than the 3.8V (3.3V (Vcc) + 0.5V) maximum voltage rating per pin.

It appears that I am going to have to use gpio lines and bjt transistors to pull the gate voltage to ground. I was just wondering if I am misinterpreting the data sheet and I can in fact use the micro to pull the 15V gate to ground.


  • 1
    \$\begingroup\$ No you can't use 15V on open collector pins on any micro I have come across. Sometimes you can use 5V on them when running of 3.3V but always check the data sheet. External transistors are the safe option. \$\endgroup\$ Commented Jan 18, 2016 at 20:01
  • \$\begingroup\$ What is the application? For sure no Atmega micro controllers can have 15V sitting on the inputs. Further more, if you want to switch with any speed at all, you will not want a 10k resistor. Your best bet is a gate driver or a transistor buffer as you described... \$\endgroup\$
    – MadHatter
    Commented Jan 18, 2016 at 20:27
  • \$\begingroup\$ Parallel pins won't allow you to pull down a higher voltage. Parallel pins could be used for a higher current though, not that you want that here. \$\endgroup\$
    – Passerby
    Commented Jan 18, 2016 at 21:10

2 Answers 2


If you want to use a PSMNOR9-25, which is an 100A N-Channel MOSFET, then the following is a suitable circuit. You could replace Q1 with a small N-Channel MOSFET such as a 2N7000.


simulate this circuit – Schematic created using CircuitLab


Check out the attached image/schematic. If you use a n-channel FET instead of a p-chan, you can use an active low control circuit with an equal amount of components.

enter image description here


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