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I'm working on a low power application. With a PIC24F (3.3V) I want to control the power to an SD card, turning it on and off. Typical current draw is 50mA, but can go as high as 200mA. I already have a 3.3V supply.

I was thinking of something using a 2N7000 MOSFET. What gate-source voltage would saturate these? Is it possible to do this on the high side or will it have to be on the low side?

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Low-side configuration are fine for simple loads like relays or lights, but you don't want to use a low-side configuration in this case, since it would place the ground of the SD card at a different level than the ground for the PIC24, which would interfere with the logical levels used to drive the data interface.

I like to use P-channel MOSFET's in a high-side configuration for this purpose, with the gate tied to the source lead through a 499K resistor to keep the device off when starting up. You can then turn the MOSFET on by configuring the pin on the PIC24 in an open-drain configuration and driving it low.

You want a P-channel MOSFET with a very low Rds (below 1 ohm) and low Vgs, such as this one. (I couldn't find any that met your criteria in a thru-hole package.) The MOSFET you chose (2N7000) has an Rds of 5 ohms, which means there would be a 1 volt voltage drop at 200 ma.

Here's a document that explains why it is best to use P-channel MOSFET's in a high-side configuration.

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  • \$\begingroup\$ Thanks! I will use a P-ch fet then - sounds great. Through hole/surface mount is not an issue. \$\endgroup\$ – Thomas O Oct 2 '10 at 13:30
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You may also use a TPS2041B device from TI. This is a high side switch with low on resitance and overcurrent protection.

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