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I am controlling the voltage of a 3.6V lithium battery with a MCP112-250 (I know it is 3V but I have a diode in series with my battery).
And the output is connected to the gate of a N-channel MOSFET, IRFB7430PbF according to this schematic : enter image description here The MCP112 is doing his job by toggling his ouptup from high to low.
When Vin>2.5V I have my MCP112 output high and I have 2.5V at Vout of the LDO, and when Vin<2.5V I have my MCP112 low but I don't have 0V as expected but aproximately 1.2V when Vin=2.2V.
My circuit is not shut down as expected... Theoretically when I have a 0V on the gate of a N-channel it has to be "open" so my system is off.

But why is this working like there were no MOSFET ?

EDIT:

It seems I have some current leaking from somewhere... Because when the MOSFET is removed I have Vout=~2V ...But from where ? I used a GND power plane on my board. Is it possible that current is leaking between the pads and the power plane ?

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  • \$\begingroup\$ First of all your FET has a max threshold voltage of 3.9V, so you may never get it fully enhanced. What's the load on your output? You will have some leakage through the FET so it's conceivable that you would see some voltage on the output if there was no load. Or you may have damaged the FET with ESD or in some other way and the gate oxide has been punched through. Try putting a 1K load on the output and see what happens. \$\endgroup\$ – John D Mar 9 '17 at 1:58
  • \$\begingroup\$ It was with no load, but with a 12kOhm load (I juste have this one on my hand) I have the same problem. Vin=2.4V and Vout=1.5V instead of 0V.. \$\endgroup\$ – Tagadac Mar 9 '17 at 2:06
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    \$\begingroup\$ You have the FET in backwards (drain and source swapped) or you have a bad FET, or other wiring error. \$\endgroup\$ – John D Mar 9 '17 at 2:09
  • \$\begingroup\$ @JohnD It's soldered on a board with silkscreen so it can't be backward and my package is good... I am going to change it to see what happens. I let you know ! \$\endgroup\$ – Tagadac Mar 9 '17 at 2:16
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    \$\begingroup\$ Does the MCP112's output go down to 0V with the FET disconnected? The MCP1700 is rated for 0.25A max. Why are you using a FET rated for 195A? An IRLML2502 would be more than adequate - and is specified for 2.5V Gate drive. \$\endgroup\$ – Bruce Abbott Mar 9 '17 at 2:49
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SOLUTION:

If you are "cutting" the ground make sure you are measuring your output from where you realy want to measure it !
In my case I was measuring Vout from Vout of the LDO and the negative pole of the battery, so when Vin>2.5V Vout=2.5V, but when Vin<2.5V the MOSFET disconnect the ground of the battery from the rest of the circuit so between Vout and the negative pole of the battery there is a remaining voltage. But if you are measuring between the 2 pins of J1 or J2 you should see 0V, but if you are measuring from Vout on J1 or J2 and the negative pole of the battery you are not measuring what you are expecting... So this is why I got strange voltage on my multi-meter...

In short:
When you are measuring a voltage with your multi-meter make sure to measure with the good ground reference !

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