Re: [SI-LIST] : Best type of models, edge rates & load

D. C. Sessions (dc.sessions@vlsi.com)
Wed, 21 Jul 1999 13:34:47 -0700

Roy Leventhal wrote:
>
> D.C.
>
> It seems to me too that the assumption of behavioral modeling under IBIS is that
> switching behavior is
> quite non-linear and best handled with V-I & V-T curves fed into a Bergeron
> Method Reflection Diagram simulator for the IC elements, a SPICE model simulator
> for terminations, etc., and an RLGC field solver matrix extractor for the
> distributed structures (transmission lines, connectors, cables) seems to be
> working out well. Especially for modeling large signal non-linear behavior. That
> way we don't get into Miller capacitance, die thermal resistance and a whole
> host of other things that are usually only valid under linear, small signal
> assumptions anyway. Or, am I missing something?

I think you missed. Oh well, nobody scores in the 10-ring every time.

The Miller effect that I was referring to is the modulation of edge rate due to
final-stage voltage gain. The IBIS V/T tables tell the simulator what the
predriver acts like, but they necessarily assume that the predriver V/T
waveforms are independent of line loading. This isn't quite true since some
part of the predriver's load is the Miller capacitance of the final drive
devices, which is in turn a function of the final stage voltage gain.
As the loading increases, the predriver will 'see' progressively less load
capacitance and thus go somewhat faster (higher output di/dt).

This is way, WAY down on the list, though, and if anyone has a design that
is that critical they're almost certainly hosed anyway.

> True, we loose something when we increase the level of abstraction as in IBIS.
> We can't for instance model the effects of power supply regulation directly.
> But, are we attempting to blend the device physics world with the abstracted
> IBIS model world here in an entirely productive discussion?
>
> I do agree that we need some realistic characterizations of buffer switching
> speeds into transmission line loads as opposed to 30pf (45pf, 50pf, what?) low
> pass filter. and some work on the standards and definitions of Tco and other
> parameters.

[I had written:}

> The objective is to describe, in as much detail as possible, phenomena that
> aren't easily calculated such as capacitance. The V/T plots are there to
> indicate the predriver behavior, and ANY capacitance hides some of that
> information by filtering out high frequencies.
>
> That said, it's probably not worth worrying about. There are second-order
> effects (thanks to Miller capacitance) that aren't readily accounted for in
> IBIS and which have more effect than small amounts of driver capacitance.

-- 
D. C. Sessions
dc.sessions@vlsi.com

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