Hi, I'm trying to use a TP0610 P-channel enhancement mode FET to control the switching on/off of a device's Vcc with an MCU's I/O pin. The problem here is that the device's Vcc is 5.0V while my MCU supplies 3.3V only. I'm wondering if a Vgs of 3.3V will be sufficient to turn on the FET? According to the datasheet, Vt(min) = -1.0V and Vt(max) = -2.4V(condition:Vgs = Vds, Id = -1mA). Does that mean that it takes anywhere from 2.6V to 4.0V at the gate to turn the FET on? If so, 3.3V isn't very safe is it? Also, how do I calculate how much current I should supply from my MCU to my gate(hence calculating that series resistor value)? My device will have a max current of ~30mA and my MCU supplies a max of 2mA. Thanks!
If it's a P channel FET, you want to be pulling the gate LOW to turn it on. Easiest thing to do would be to use an open collector output from the MCU (if it has one) with a pull up resistor to the 5V supply.
With 5V supply I can't manage to simulate switching with any of my spice models, though I don't have the TPO610. Grounding the gate turns them on OK, in fact there's a simple way of making a low-drop "diode" to protect against reverse-battery connection using a p-mosfet on the positive rail with gate to negative rail. But I can't turn them off. Made for this job is Maxim's MAX890. And see this AN36.
I decided to just forgo the 5V operation and run everything at 3.3V. It seems to work now with my test circuit of source->3.3V, drain-> device Vcc with a 200K pulldown, gate-> MCU control with a 50K pullup. I'm simulating my MCU control pin with a jumper to gnd and the switching works fine. Without the pulldown on drain, I'm getting the same problem where I can't turn them off. They're always partially on.
