Super basic N MOSFET logic level shifter - done correctly?

[FlashEF]

Hi. Could anyone tell me if the fallowing circuit is going to work?



It is supposed to do a non-inverted conversion from 1.8 V logic to 3,3V. It is surprisingly hard to find information regarding those sort of circuits as they are super basic and simple. I ended up scratching my head and wondering am I right or not.

[AndyC_772]

I'm afraid not. If you apply 1.8V to the gate, and the Vgs threshold is 0.5V, then the output voltage will be about 1.3V.

There's no completely trivial solution that I'm aware of. An inverting level shifter is easier - it's more or less the circuit you've drawn, just turned upside down. Two inverters in series makes for a non-inverting circuit.

[FlashEF]

Doesn't the Vgs treshold mean the point where the MOSFET starts to turn on? The ON resistance for this model is roughly in 200 mOhm so shouldn't it be a voltage divider with one resistor of 10k and the other 200mOhm? What am I missing here?

[SeanB]

0.5V with respect to the substrate/source. In this case the source will be at the top end of the 10k resistor. You have made a very nice voltage follower. To make a level shifter you will need to make this an inverter by swapping the mosfet and resistor and add another after to invert again. If you need to do this more than once on a board use a proper level translator.

[fmaimon]

Check out this application note. Page 10 is what you are looking for. http://ics.nxp.com/support/documents/interface/pdf/an97055.pdf

[FlashEF]

Thanks for the datasheet and all other help. The method mentioned on the 10-th page seems easy to implement but I'll probably test it first cause it seems somewhat dependent on the MOSFET model.

[AndyC_772]

Doesn't the Vgs treshold mean the point where the MOSFET starts to turn on? The ON resistance for this model is roughly in 200 mOhm so shouldn't it be a voltage divider with one resistor of 10k and the other 200mOhm? What am I missing here?

I'm sure you've got it by now anyway, but: Vgs is the voltage between the gate of the transistor and the source of the transistor. Think carefully about what these two terminals are connected to; if the source were connected to ground, then it would indeed turn fully on with a 1.8V input. But it isn't!

[FlashEF]

Thanks AndyC. I taught of the first split second when the bottom of the transistor is pulled to ground, but yes - after the voltage raises the MOS will start to close and settles on it's treshold voltage. Something to keep in mind for the future.