mosfetswitches
I am looking for a high side switch to turn a Bluetooth module off when not in use.
The module uses 100mA at most and runs on 3.3V.
I am using a ATMega328 to talk to the module and it runs at 5V.
How can I use a P-MOSFET in a high-side switch circuit to switch these low current levels using 5V logic?
Lacking the ability to add an answer:
Would this work, perhaps?
simulate this circuit – Schematic created using CircuitLab
Simplest way is to put something like a zener diode structure from the drain to the gate. That way when the zener diode voltage is exceeded by a bit the MOSFET turns on and clamps the voltage. Something like this:
simulate this circuit – Schematic created using CircuitLab
If I read the datasheet correctly, the part you mention clamps at about -22 to -16V with a supply voltage 6~20V, so there's probably something a bit different going on- the zener is changing a logic level relative to ground (before a level shifter).
Holding the P-channel MOSFET closed (gate at 5V) will use hardly any power. It is basically the same as holding an N-channel MOSFET gate at 0V.
(See http://www.vishay.com/docs/91076/91076.pdf for a p-channel version of the IRF530 - same gate-source leakage current of +-100nA max)
To prevent powering the LED array via CLK or DAT you will need to either drive them low or set them to high-impedence when the MOSFET is off. Since CLK and DAT are both outputs, simply driving low is fine.
Note: This does not apply to your circuit here, but for inputs you can use a tristate buffer like the 74LVC1T45 or similar to prevent powering via signal pins... See FTDI (FT232RL) powers itself through RX/TX lines for the solution I used.
Edit:
Alternative solution with existing parts
Actually, you could just move the N-channel MOSFET to the low side and drive CLK and DAT to 5V when you want to turn the LEDs off and go to sleep. That would address the problem in your first idea.
Best Answer
Your schematic is largely OK, with these issues:
Added:
Russell pointed out that the digital signal to control the gate is 0-5 V. I guess that you think the diode is there to protect the gate from reverse voltage. That is very likely unnecessary. As always, read the datasheet for the parts you are using. 1.7 V reverse on the gate is probably fine.
If you're worried about the processor dumping current onto the 3.3 V line thru R1, then make R1 bigger. It only functions when the processor pin is not driven, which should only be for maybe a few 10s of ms while the processor starts up and before the firmware gets around to driving the pin one way or the other. Does it matter if the bluetooth module powers up and draws 100 mA for a few 10s of ms at power up? If not, then you can leave off R1 altogether. Since both the FET gate and the processor output will be high impedance when the processor pin is configured as input, a rather high value resistor will do. 100 kΩ should be fine. Probably even 1 MΩ would be fine, but check the input pin leakage current in the processor datasheet.