How does a bidirectional level shifter work when shifting 5V to 3.3V?

[likephysics]

I stumbled across this ckt - http://husstechlabs.com/support/tutorials/bi-directional-level-shifter/

It says it's a bidirectional level shifter (3.3 to 5v or 5 to 3.3v)

I can't figure out how it would work when level shifting 5v to 3.3v
When the drain is 5v, the source should be 3.3V
When the drain is 0v, the source should be 0v (how can the drain be 0v)?

 

[mfb]

If the source is at 3.3V, the MOSFET is not conducting, and the drain is at 5V.
If the source is at 0V, the MOSFET is conducting, and the drain is (close to) 0V as well.

Signal transmission goes from left to right.

 

[likephysics]

If the source is at 3.3V, the MOSFET is not conducting, and the drain is at 5V.
If the source is at 0V, the MOSFET is conducting, and the drain is (close to) 0V as well.

Signal transmission goes from left to right.

mfb, that part I get. But it says bidirectional level shifter. So it has to work from right to left also.

 

[mfb]

Hmm... I would expect that the other direction needs a different setup.
A reverse diode (some MOSFETS have that included) can bring the left side to "low" if the right side is at 0V.

 

[likephysics]

Hmm... I would expect that the other direction needs a different setup.
A reverse diode (some MOSFETS have that included) can bring the left side to "low" if the right side is at 0V.

yup. That's it, its the rev diode.

 

[Jony130]

All discrete MOSFET has a built-in body diode. And try read this:
http://ics.nxp.com/support/documents/interface/pdf/an97055.pdf