AVR-Chat

On Wed, Apr 8, 2009 at 1:22 PM, Graham Davies <Yahoo37849@ecrostech.com>wrote:

>
> My question for the non-"digital only" folk is this ... will just about any
> MOSFET (with an appropriate threshold) perform well in this circuit?  I
> understand that the beginner-level textbook structure of a MOSFET is
> symmetrical with respect to source and drain, but there must be some
> difference in practice with devices designed for specific purposes.  How
> should I choose a MOSFET for this application?  For example, the
> manufacturer tells me the gate-to-source threshold voltage and (as mentioned
> above) this is an important parameter here.  I have no idea what the
> threshold will be when the drain and source terminals are swapped, but this
> is just as important.
>

The threshold voltage for the MOSFET needs to be safely below the lower of
the two supplies.  For the 3V supply shown, the logic level FETs with a
threshold below 2V should work fine.

MOSFETs are typically either built laterally (very common in IC's, not
common in discrete devices), or vertically.  The lateral devices are
symmetric, in that the drain and source can be swapped with no effect in
operation.   Vertical MOS's are not symmetrical.  The trick here is that the
body diode in the vertical MOSFETs used pull down the source voltage to
where the FET channel conducts and pulls it even further down.  In this case
it is still the gate to source voltage that is turning on the MOSFET.  The

One subtle point of the circuit is that the gate needs to be tied to the
lower of the two supply voltages for the circuit to work.



Jon


[Non-text portions of this message have been removed]