[SI-LIST] Re: Beyond Rail Operation in IBIS
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 21 Oct 2002 11:58:59 -0400
> "How important is it to have accurate beyond rail curves?
Personally, I would suggest that accuracy is desirable out to around a
half to one volt past the point where the on-die clamp diodes start to
conduct. Beyond that, accuracy is less important than just being in the
ballpark.
> Does the IBIS
> spec. allow models to not have IV curves beyond rail voltages?"
No.
Some people differ with me, but my opinion (again) is that the I-V
curves ought to make some attempt to represent some semblance of reality
all the way from -Vcc to +2*Vcc. While the simulator may never
*converge* more than a volt beyond the rails, it is likely to *try*
points beyond this, and the I-V data out there needs to push it back
towards reasonable voltages and currents. The slope of the I-V data is
probably most important. Truncating the data means zero slope.
Extrapolation is better. You want to be in the ballpark.
Certainly you wouldn't want the simulator to converge at -5V because
someone "doctored" the I-V data in the model to keep the currents small.
If the current at -5V is enough to vaporize silicon, make sure that's
what you put into the IBIS model! You don't want the simulator to
converge there.
Andy
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