mosfetpower supply
I'm using an N-channel mosfet to supply power to a servo. The problem is that even with the gate set at low, I'm still getting some amount of voltage (~0.9V in a 5V system) coming through. Do I need a pull-down in case the pin is floating?
Here's the current schematic:
There is an error in the O.P. circuit. It will not be able to turn off the LED, so the LED will be on all the time. To turn off the P-channel MOSFET, you need to pull the Gate to Source. The Source is always at +9V in your circuit, but the pullup R1 goes only to +5V (VCC).
A high side P-channel MOSFET switch often looks like this.
Q2 can be a small N-channel MOSFET or a small NPN transistor.
On the other hand, is there a reason for using a high side P-channel switch (as opposed to a low side N-channel switch)? Are you making this setup just to understand P-channel MOSFETs better?
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
Yes, if you just tristate after pulling it high, then the gate will stay floating high. You either need a resistor to pull it down to ground or you need the input signal to drive it low.
The resistor can/should be relatively high valued compared to your input resistor to prevent excessive voltage drop when you have it set as a high input. You only have to drain the inherent capacitance on the MOSFET gate when you're pulling it low so even at a high resistance to ground the RC time constant is usually relatively short.