MOSFET pull down in linear region

Thread Starter

matt111111

Joined Nov 26, 2014
6
I have a current sense IC powered directly from a battery. I want the current sense IC to be in the shutdown state when the 3.3V power supply is disabled.


I have added a dual N channel MOSFET circuit (see attachments) to achieve a pull down on the shut down pin (active low and internally pulled high) when the

3.3V supply is not present. The circuit seems to work in LTspice but I am concerned about one of the MOSFETS operating in the linear region. (VDS < VGS - VT)


Is it safe to operate the MOSFET in this configuration or is there anything I'm missing that is considered bad design?
 

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BobTPH

Joined Jun 5, 2013
2,055
If what you are trying to do is shutdown the device when 3.3V is not present, just use a pulldown resistor on the ~SHDN pin, and connect it to the 3.3V supply. No MOSFETs required.

Additionally, linear region for MOSFETs actually means the opposite of what it does for BJTs. The linear region is where you want to be when using the MOSFET as a switch. What is called the linear region on a BJT is referred to as the active region of the MOSFET, this where the current through the MOSFET is sensitive to small changes in Vgs and this is where Vds is high and dissipation is high.

And furthermore, using a MOSFET to pull down in input pin with a > 1Meg pullup is not about to cause enough current to heat up the MOSFET not matter what region it is in.

Fourthly, you are showing 30V on the MOSFET gate, albeit through a 1 Meg register, but the maximum Vgs for most MOSFETs is 20V.

Bob
 
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