[SI-LIST] Re: Beyond Rail Operation in IBIS

  • From: "Abhijit Mahajan" <amahajan@xxxxxxxxxxxx>
  • To: rhaller@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Thu, 17 Oct 2002 12:58:07 -0700

Bob,

Thanks for your reply.  I am glad you understand my situation.  However 
I did not quite understand the last part of your email.  

Are you suggesting that one should interpolate the curve and 
approximate it as a linear ramp beyond the inital diode "Knee" ?

If yes, are there any established methods for doing so?

Thanks again!

Abhijit.





Robert Haller wrote:
> 
> Abhijit,
>         The IBIS spec requires you to model a curve from -Vcc to +2Vcc.
> This is well beyond the rail voltages (and can often Melt silicon :-) .
>    Many people do not correctly model clamp curves. One of the reasons
> is often CMOS I/O cells contain a lateral PNP structure that has
> spreading resistance. This manifests itself by modifying the actual
> behavior of the ESD clamp structures. I spent a significant amount of
> time working with a semiconductor vendor to get this right. Even if they
> extract their ESD clamp diode structures accurately, they may miss the
> vertical PNP's.
>   Practically The important parts of the clamp model is the turn on
> behavior and resulting Slope. Theoretically you need to get enough
> points on the "turn on" and to establish a slope.  If you don't want the
> a tool to interpolate do the sweep from -VCC to +2 Vcc.
> Regards,
> Bob
>    --
> Robert J. Haller (rhaller@xxxxxxxxxx)
> Principal Consultant
> Signal Integrity Software Inc.
> 6 Clock Tower Place, Suite 250
> Maynard, MA 01754
> Phone: (978) 461-0449, ext 15
> 
> Volk, Andrew M wrote:
> > It is important if your signal goes any significant amount beyond the rail.
> > Series terminated buses like PCI depend on the clamping diodes for signal
> > integrity, to stop excessive reflections that can also impact timing.  Some
> > interfaces depend on them to limit the voltage swing at the receiver to safe
> > limits on newer processes.  If you need to check for these kind of effects,
> > the clamps are essential.
> >
> > Andrew Volk
> > Intel Corp.
> >
> > -----Original Message-----
> > From: Abhijit Mahajan [mailto:amahajan@xxxxxxxxxxxx]
> > Sent: Thursday, October 17, 2002 11:15 AM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Beyond Rail Operation in IBIS
> >
> >
> >
> > It is often difficult to get accurate curves for beyond rail
> > operation of IO Buffers.  The problem is mainly in diode models that
> > are not accurate and results in 100s of Amps of current.
> >
> > I am sure this question has been asked before,
> >
> > "How important is it to have accurate beyond rail curves? Does the IBIS
> > spec. allow models to not have IV curves beyond rail voltages?"
> >
> > Thanks for your help.
> >
> > Abhijit.
> >
> >
> 
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