Re: [SI-LIST] : Output Impedance

D. C. Sessions (
Thu, 02 Apr 1998 08:29:58 -0700

Brian Young wrote:
> I'm convinced IBIS works great if you have no rail collapse so that the IV
> curves are valid. I don't think they are valid under heavy bus switching
> with significant rail collapse, although I hear people claim otherwise. I
> just don't see how IV curves provided at, say 3.3V, work when you have 0.5V
> of collapse. Any comments?

Almost always correct. For normal CMOS outputs, rail collapse steals
drive and thus violates the assumptions used in creating the IBIS model
the first place. The only exception I know of is N-follower pullups,
where the gate drive gets stronger as the rails drop. Combined with a
regular P pullup this can be pretty much Vddq independent. (Doesn't
with the pulldown, though.)

It's been suggested that semiconductor manufactureres (you and me,
produce worst-case models that take into account SSO collapse. This
easy in IBIS as it stands because it involves multiple chip models for
different conditions. If anyone has an inspiration on a way to extend
IBIS to make this easier, by all means forward it to the IBIS forum:

> > ... I have compared the results of IBIS simulation to that of full
> > HSPICE model simulation and hardware measurements for several different
> > transmission line problems. IBIS model simulation compares very well to
> > both.

Certainly within the envelopes of device, dielectric, geometric,
etc. tolerances.

D. C. Sessions