Subject: RE: removing non-monotonic points?!
From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Thu Feb 07 2002 - 10:28:14 PST
As I've spoken my mind about this recently, I figured I should throw my
two cents in. I disagree that I-V points more than 800 mV beyond the
rails are meaningless.
Even though a simulator may not CONVERGE on a point that is 2V beyond
the rails (because of stiff clamping), that doesn't mean that the
simulator didn't TRY that point. If the I-V data at that point in the
model is totally out to lunch, say zero, then you wouldn't expect the
simulator to figure out it needs to get back into a more normal
operating region.
That's one reason why I-V tables should never be truncated, even if you
can't measure the current that far out without frying the part. The
data out there needs to be reasonable enough ... by extrapolation if
need be ... to guide the simulator back toward reality. Otherwise it
might actually converge way out there, or may give up and abort the
simulation.
Also, there are plenty of duff SPICE models that have unusually stiff
clamps because they forgot to include any diode resistance. With these
models you might never find a simulated result that is more than 800 mV
beyond the rails, no matter how hard you try; whereas the real devices
are not that ideal. I have seen negative overshoot on a few devices go
as far as about -2.5V ... although this is at the package pin, not on
the die. Suffice it to say that some parts clamp less well than most.
Regards,
Andy
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