I have this P channel MOSFET switching power to another device for me, by a digital pin on my 3.3V feather sense board. my feather sense and digital pins are 3.3V. but I just realize I need to actually switch the lipo battery voltage. 3.7-4.3 volts. So in this schematic, I will be replacing the +3.3V with the slightly higher battery voltage, but this will not harm the digital pin that doing the switching will it? The higher battery voltage would appear to be connected to the A5 pin thru the 10K resistor, so I'm not sure.
I already have the custom board made, and am hoping I can just green wire the correct voltage to the pin, and cut a trace...
Check the datasheet for the processor and see what it allows.
the voltage I need will be a lipo, at 3.7-4.2
It does sound like this is too much for that pin. I'm hoping to find a solution that doesn't involve remaing the whole board. I can cut the trace and wire the LiPo voltage to the source of the MOSFET. I can remove the 10K pull-up resistor on the A5 pin on my feather. I power of the feather by the EN pin on it. I wonder if the MOSFET will be floating enough to be active, and let power go thru to my itsybitsy board? If so I could add a pull-up to A5, that is connected to the 3.3v.
See my updated photo. I can manage this without a new PCB. would this work?
sorry if it's nt drawn correctly, I'm self taught
Maybe
connected to the A5 pin
The A5 pin of what?
Read the datasheet of your mosfet. Schottky diode on A5 with your original circuit could do the trick. Or not.
If the battery voltage never goes over 4.0V.
If source voltage were 4.2V and gate voltage were 3.3 the Vgs would be -0.9V, datasheet says typical Vgs threshold is -1V so MOSFET could be conducting.
https://assets.nexperia.com/documents/data-sheet/PMV48XP.pdf
Thanks. I'll have to do some testing.
Typically the 10K resistor would be connected to the mosfet source. Then the mosfet would be off unless A5 draws the gate down by at least the minimum threshold voltage, which is 0.75V for this mosfet. So 4.2V - 0.75V = 3.45V, which is still too high for A5 at 3.3V. So you could add a schottky diode in series into the A5 line, which is another 0.3V drop, which should be enough. A5 would have to be higher than 3.15V to turn off the mosfet, which, at 3.3V, it is.
However, if you’re just building one of these, your particular copy of the mosfet may have a threshold as high as 1.25V, in which case the schottky wouldn’t be needed.
Edit: In fact, you shouldn’t use the schottky unless it’s needed to turn off the mosfet. If you have too much drop, the mosfet may not turn fully on when A5 goes low.
You are welcome, but you don't need to do testing. If the battery voltage is over 4.0V there is no guarantee that the MOSFET will be off