Richard:
Look at the picture in the last posting I made. If you put an N-FET into the spot that Koosha illustrated for the switch you will end up with the N-FET providing current to the load (target MCU) in a source follower mode. The voltage delivered to the load will be at least the VGS of the FET less than the +VDD supply unless the gate is driven above the VDD level.

It is wonderful that there are many "logic level" FETs available these days and that you enjoy the benefits of using them. However the fact of the matter remains that, unless you can live with a VDS drop across the N-FET, it is unsuitable for switching the VDD of a target load if the target needs a full VDD voltage and the gate drive voltage of the N-FET is limited to the VDD level. This limitation will persist even if logic level N-FETs are used.

Consider the circuit shown here that I quickly cobbled up in LT-Spice:



In the above circuit the VDD is simulated at 5 volts and the gate of the N-FET is driven by a 0->5V drive signal such as may come from an inverter driving the gate. (For this simulation I have neglected consideration of the inverter characteristics at powerup/powerdown and assumed full rail to rail swing which is quite possible with a CMOS type part). The N-FET is shown in a source follower mode with the load being in the source circuit. In this example I am simulating Koosha's target MCU via a 15 ohm resistor. Note that the FDS5680 N-FET model I chose is for a device that has a minimum VGS of about 2.0 volts which probably doesn't qualify the device as a "logic FET" but at the simulation current level the data sheet shows a VGS of about 2.0 volts. The Fairchild data sheet for the FDS5680 can be seem here.

Now consider the simulation waveforms at the 15 ohm load. This load represents just under 200 mA of current through the N-FET.



It is possible to clearly see the point that I am making. When the gate of the FET is driven to 5V the source voltage follows by a lower value equal 5V - VDS@200mA - VGS@200mA) ~ 2.9V.

I did not mark your posting as "answer is wrong" as I'll leave it to you to qualify your comments and point out that to use an N-FET as a high side switch requires a drive voltage higher than the VDD level.

Michael Karas