Driving MOSFETS with open drain output of Comparators.

[Arsh ahmad]

Is it possible to drive a MOSFET using a comparator having an Open-drain output?

[Zero999]

Yes, very slowly. Add a pull-up resistor. Again note, it'll be slow so no good for PWM.

[Arsh ahmad]

I don't want to use it for PWN I only want to turn it on or off with an input signal.

[Siwastaja]

With a small MOSFET (small Qg) and no concern about power dissipation and hence suitably low value pullup, such as 1kOhm, it isn't even that slow.

[Arsh ahmad]

Yes, I think this might work.

[Peabody]

Yes you can.  I've used the INT/SQW alarm output of a DS3231 RTC, which is open drain, to turn on a P-channel mosfet.  But you do need the pullup resistor so the gate never floats.  I used 100K, but this was just a low-current power switch, so there were no real thermal considerations.  I guess in theory it could also work with an N-channel mosfet.  I feel less comfortable about that, but haven't really thought it through.

They do make comparators with push/pull outputs, which would eliminate the need for the resistor, but I don't think they are common.

[Arsh ahmad]

Thanks for the replies, I made a hardware prototype and it is able to drive both n channel as well as p channel MOSFETs.

[Peabody]

With an N-channel mosfet, the mosfet will be turned on by the pullup resistor when the comparator output is off, so the mosfet is on by default.  That also means the mosfet will be turned on when the comparator is powered down, which might not be what you want.  With a P-channel mosfet, the mosfet is off by default, and only turns on when the comparator output is low.  That would be considered the safer option in most cases.

[Zero999]

With an N-channel mosfet, the mosfet will be turned on by the pullup resistor when the comparator output is off, so the mosfet is on by default.  That also means the mosfet will be turned on when the comparator is powered down, which might not be what you want.  With a P-channel mosfet, the mosfet is off by default, and only turns on when the comparator output is low.  That would be considered the safer option in most cases.

That depends on whether the pull-up is connected to the same power supply as the comparator or not. The comparator might also behave in an unpredictable manner, during power cycles, especially if the ramp up/down is slow.