MOSFET switch for Microchip uC

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drkidd22

Member

• Oct 8, 2011

• #1

Hello,

I'm trying to build a mosfet switch to be used with a microchip uC. Mainlyto switch power rails from 3.3V to 5V. I have attached an image of what I have so far. I just want to make sure that this will work ok. The output is just for driving logic level signal, just some gates.

 

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• MOSFET Switch.png

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KMoffett

Well-Known Member

• Oct 8, 2011

• #2

https://www.fairchildsemi.com/ds/BS/BSS138.pdf
Looks like it should work. Are you driving TTL ot CMOS gates? You may have to adjust the pullup resistor to suit.

Ken

 

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drkidd22

Member

• Oct 8, 2011

• #3

I'm going to use it to drive A3979 Microstepping DMOS Driver input and output control pins with the uC. So you mean increase the current going into the channel of the mosfet?

 

alec_t

Well-Known Member

Most Helpful Member

• Oct 8, 2011

• #4

When you say "switch power rails from 3.3V to 5V" I take that to mean 'provide a logic-level shift from 3.3V to 5V', since there is no 3.3V rail shown. In that case the circuit should work, provided the FET can be driven fully on with only 3.3V on its gate (I haven't checked the spec for the FET).

 

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drkidd22

Member

• Oct 8, 2011

• #5

The gate treshold is 1.2V. What I meant by switching rails is that the pic has an output voltage of 3.3V from it's pins which will be used to drive the gate of the mosfet. The output of the mosfet will then be 5 volts which is what required by the other IC im trying to drive.

 

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pilko

Member

• Oct 8, 2011

• #6

I don't know if you are aware,but the gate threshold voltage is the gate voltage to just start to turn the FET on,
not when the FET is fully on.

pilko

 

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KMoffett

Well-Known Member

• Oct 8, 2011

• #7

With a Vsg(th) of 0.8 to 1.5V and a supply of 3.3V it should turn on sufficiently to pull a CMOS input low. If it appears not to pull the output low enough, switch to a NPN transistor, like a 2N2222. It only requires a base voltage >0.6V to saturate...plus a little current from the uC output.

Ken

 

Last edited: Oct 8, 2011