[SI-LIST] Re: Fw: 5V To 3V

More on this thread from a couple weeks ago ...

Just be aware that there can be some consequences to using the series FET
switch / Quickswitch (TM) approach, even if all the static levels seem OK.

The placement of the Quickswitches, between driver and load, may matter.  In
some cases, having it too far from the 3V part (traces more than a few cm
long between the Quickswitch and 3V part pins) can result in excessive
overshoot at the 3V part's inputs when the 5V part switches, depending on
risetimes.

The Quickswitch effectively open-circuits when the 5V driven waveform goes
about 3V, and now you've got some energy on the other side of the FET that
"rattles around" with little or no damping; both ends of it are hi-Z.

Undoubtably it's not always a problem.  But make sure to simulate it!

I'd suggest SPICE, not IBIS models, because I have a well-earned distrust
for IBIS models, which gets even worse when talking about new enhancements
to IBIS like series FET models which may or may not have been implemented
well enough yet in your simulator.  With SPICE models this should not be an
issue.  Even so, check your SPICE models carefully since even some of them
were crafted by Neanderthals and are bogus; but at least you may be able to
fix them.

Regards,
Andy


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