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

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From: Bradley S Henson (bhenson@notes.west.raytheon.com)
Date: Mon Jan 10 2000 - 08:02:29 PST


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" <Andrew.Ingraham@compaq.com> on 01/08/2000 08:23:33 PM

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 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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