RE: [SI-LIST] : Clamp diodes in models (was Input switching threshold & CPCI)

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From: Tom Dagostino ([email protected])
Date: Mon Jan 10 2000 - 10:23:11 PST


We've measured 10's of thousands of I/O buffers to make models around here.
Our internal spec is to push to part to at least 1 volt past the rail or at
least 100 ma of current. In most cases we exceed these internal specs. It
is rare that this causes any problems in the parts under test and in the
cases where it has happened it seemed to be generic to a particular
manufacturer's process. I cannot say what will happen over time from
continous abuse like this. If you are seeing lots of failures then you are
really putting more current into your devices than you may think.

Tom Dagostino
ICX Modeling Group
[email protected]
503-685-1613

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Bradley S Henson
Sent: Monday, January 10, 2000 8:02 AM
To: [email protected]
Subject: Re: [SI-LIST] : Clamp diodes in models (was Input switching
threshold & CPCI)

Andrew,

Please be carefull about "fixing" a model the vendor warns doesn't tolerate
forward biasing of the "clamp diodes". While I agree that this sounds
ridiculous, I have seen several parts from different vendors that suffered
anomolous operation with very modest amount of over/undershoot. I found this
on
parts the vendors didn't warn about not tolerating overshoot. If a vendor
explicitly warns you about this, you should investigate the robustness of
their
design, redesign to control the overshoot, or consider another part
supplier.

Good luck,

Brad Henson, Raytheon

"Ingraham, Andrew" <[email protected]> on 01/08/2000 08:23:33 PM

Please respond to [email protected]

 To: "'[email protected]'"
          <[email protected]>

 cc: (bcc: Bradley S Henson/RWS/Raytheon/US)

 Subject: [SI-LIST] : Clamp diodes in models (was Input
          switching threshold & CPCI)

Abe Riazi wrote:

> This is interesting and I would like to share related thoughts and
>experience. I have also received from some vendors behavioral models
>which lacked POWER or GND clamp sections, in contradiction to the Bus
>requirements or the actual device.
>
> Of course, this type of model defect is undesirable and requires
>correction, as it can produce simulation results with invalid positive
>or negative overshoot.

I also have been given simulation models that lacked overshoot clamp
characteristics. When queried, the IC vendor said that the signals at his
chip's pins should *never* go even a bit below ground nor above Vdd, so it
shouldn't matter to our simulations. Right!

Many of us have also seen models with incredibly steep clamp diode curves.
I don't know which is worse.

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