[SI-LIST] Re: IBIS Driver Models, Simulators and CComp

  • From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 10 Jan 2002 08:34:14 -0800

Greg,

I have to fess up and admit that I can't give you an answer
because I am not a circuit designer or a process engineer.
I can only give you some hand waving explanations which you
may even be able to do yourself.

Consider that the channel and its parasitic capacitance is
a parallel RC circuit.  If that channel is an ideal switch,
it is a short when it is on, and an open when it is off.
How much capacitance do you see when you look into this
"RC" circuit when it is "driving", or "receiving"?  It
will be 0 and C, respectively.  Of course we don't live in
an ideal world, so in reality we would see some non zero
values, but I believe this could be part of it in addition
to some lower level (quantum?) internal device behaviors
inside the transistor itself.  Take this a step further,
add the predriver and Miller capacitance (and gain) to
the circuit.  Got the picture?...

Anyone else have more info in this?  I would be interested
in hearing about it too.

Regarding the rest of the comments on how to make C_comp more
realistic, I would like to mention that I am beginning to be
more and more convinced that we should model the buffer's
upper and lower halves as two (or more) complex impedances.
(I already gave a presentation on this a couple of summits ago,
but didn't have time to continue the work yet).  In this approach
we would have a real part, which is the same as the IV curve
today, and is frequency independent (it is only a voltage,
and temperature dependent resistance), and an imaginary
part, which is frequency (and voltage, and temperature) dependent
and describes the parasitic capacitances (series and/or parallel)
and of course those resistances which are in series with
capacitances.  This approach would put the C_comp in order.
Any comments?

Thanks,

Arpad
=============================================================

-----Original Message-----
From: Gregory R Edlund [mailto:gedlund@xxxxxxxxxx]
Sent: Thursday, January 10, 2002 6:09 AM
To: si-list@xxxxxxxxxxxxx; arpad.muranyi@xxxxxxxxx
Subject: [SI-LIST] Re: IBIS Driver Models, Simulators and CComp


Arpad,

What are the physical origins of the difference in C_comp between
the driving case and the receiving case?  Suppose C_comp = 2 pF.
How much change could one expect to see?

> Regarding the variability of the value of C_comp when the buffer
> is driving or receiving, you are also correct in your observation.
> It does depend on that as well as voltage, so a single value C_comp
> really doesn't cut it.  For more precise modeling we would need
> multiple curves (C vs. V).

Greg Edlund
Electronic Packaging & Integration
IBM Server Technology Development
3605 Hwy. 52 N, Dept. HDC
Rochester, MN 55901
gedlund@xxxxxxxxxx

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