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 - 11:06:05 PST


Thanks Tom. I'm sure you're right. We also had a rich history of static clamp
tests that were fine. The problems I referenced were pattern and in some cases
temperature sensitive. They produced annoying low failure rate problems hard to
elicit and track. They are not, nor have they been, common or prevalent. It was
interesting though to hear the manufacturer tell us in at least one of the cases
that their diodes were not intended to clamp transmission line transients and
doing so overstressed their part. This was the "flag" I was eluding to in the
original response. Back then we simply added a requirement to our procurement
specs for tolerating short term transients that forward bias the clamps: We
eliminated the vendor.

We don't see a lot of failures. In fact, in our present designs we have been
very carefull to eliminate or carefully understand any ringing that would
forward bias input clamp diodes. On our ASICs, we characterize the clamp
currents and make sure they are well within the vendors design reliability
window.

Brad Henson, Raytheon

"Tom Dagostino" <tom_dagostino@mentorg.com> on 01/10/2000 10:23:11 AM

Please respond to si-list@silab.eng.sun.com
                                                                                
                                                                                
                                                                                

                                                              
                                                              
                                                              
 To: si-list@silab.eng.sun.com
                                                              
 cc: (bcc: Bradley S Henson/RWS/Raytheon/US)
                                                              
                                                              
                                                              
 Subject: RE: [SI-LIST] : Clamp diodes in models (was Input
          switching threshold & CPCI)
                                                              

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
tom_dagostino@mentor.com
503-685-1613

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Bradley S Henson
Sent: Monday, January 10, 2000 8:02 AM
To: si-list@silab.eng.sun.com
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" <Andrew.Ingraham@compaq.com> on 01/08/2000 08:23:33 PM

Please respond to si-list@silab.eng.sun.com

 To: "'si-list@silab.eng.sun.com'"
          <si-list@silab.eng.sun.com>

 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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