AVR-Chat

I don't like it.

This scheme may work most of the time, but I agree that we can't 
guarantee it will work when the drain is pulled low.  I come from an old 
school that states we should never count on active component 
characteristics that vary widely, that we should use the active 
component as a function block and such things as gain or switching 
thresholds should be determined by external components.  I would also 
advocate a conscious decision be made for transmit and receive 
functions, so I don't see a lot of use for a circuit such as this one.

I got bit once by a circuit in which the original designer counted on 
the base-emitter leakage current of the series pass transistor element 
to start a regulator.  This was back in the days of linear regulators.  
The transistor actually got better as it aged, lower B-E leakage 
current, and the regulator stopped functioning.  That wasn't an easy 
problem to locate.

Fortunately I work in test, instrumentation, and controls where a few 
cents extra for a reliable design is more important than cost saving at 
the expense of reliable performance.  That's how we get trains that 
crash because an oscillator design is marginal or chemical leaks that 
kill hundreds of people.  Designs that work on paper or in theory don't 
always work in the field.


REB



Graham Davies wrote:
> --- 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.
>


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