AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Mike Perks" <mikep@...> wrote:
>
> To answer your question I googled the following paper:
> http://www.vartists.com/engineering/N-MOSFET_bidirectional_level_shifter\
> _analysis.pdf

That's an interesting paper for anyone not familiar with the circuit, but I don't think the analysis is correct.  Maybe Roy will offer an opinion.

In the problematic case where the MOSFET is used "backwards", the author points out that the diode, present in many MOSFETs from drain to source, will turn on and pull down the source when the drain is low.  But, he then argues that lowering the source voltage in this way turns the MOSFET on, connecting source and drain and passing the full low voltage across the device.  I'm uncomfortable with the argument that a MOSFET turns on is this "normal" manner when the drain voltage is below the source voltage.  I think it turns on because of the gate-drain voltage, but we still have the problem that we don't know the threshold voltage in this mode of operation so we can't be sure the circuit will work as we intend.

Someone might argue that the diode itself passes the low from "right" to "left".  This could be true if it is a Schottky diode with very low forward voltage drop.  So, maybe the answer to my question is to use a MOSFET with a built-in Schottky diode from drain to source and don't worry about the gate-to-drain threshold voltage.

Graham.