Subject: RE: [IBIS-Users] FW: Extreme currents????
From: Dagostino, Tom (tom_dagostino@mentorg.com)
Date: Thu Jul 11 2002 - 13:14:43 PDT
We've made models from measurements and from SPICE. When doing measurement based models we try, note, try to turn on the protection devices to at least 1.2 volts. Most part can handle this and most will have less than 1 Ampere at this voltage. We have seen some that will turn on at very low voltages, around 0.4 to 0.5 V. These parts have almost a zero resistance and the currents can exceed 1 amp with just 100 mV of forward voltage past the knee. I would expect these parts to easily get to 100 or more Amperes by the time -Vdd was reached given the slope at low voltages.
For SPICE based modeling we have been told by some of our customers that the "excess currents" we see in the simulations are real and please don't "fix" it. In other cases the diode model is just not complete thus causing over 10^9 Amperes to appear in the model.
So I agree with Scott McMorrow that you cannot just say, "Oh, that is wrong" by looking at the currents in the model.
Given that the maximum current you would expect to see in a signal trace on a PCB is 100 mA (5 Volts/50 Ohms), the simulations will probably not see the high current areas of the model. This is not to say that some simulators will not like the model at all.
And yes, I did accidentally forward bias a diode once with 5 Volts, the smell of burning plastic was awful and immediate.
Tom Dagostino
Modeling Manager
Mentor Graphics Corp.
SAS
tom_dagostino@mentor.com
503-685-1613
-----Original Message-----
From: dbcox [mailto:dbcox@micron.com]
Sent: Thursday, July 11, 2002 11:59 AM
To: 'Kim Helliwell'; Lynne Green
Cc: Chris.Dewitt@motorola.com; ibis-users@server.eda.org
Subject: RE: [IBIS-Users] FW: Extreme currents????
Well it looks like everyone has put in their 2 cents worth on this subject.
For what it is worth I loved the model that had 900 AMPS at -3.3V. It was
my favorite. Aside from all the other obvious comments that have been made
How wide would the trace on the PCB have to be to carry this type of load?
Would this device be classified as a fuseable lead frame? It is obvious that
model makers could do a better job in QA'ing their data. There is something
to
be said also about ibis itself here. If the specification allows unrealistic
data to be acceptable then model makers will continue to provide it. Perhaps
extremely high currents should be changed to an error instead of a warning.
This would force correction to this issue at the source. Couldn't resist
commenting on this issue it is one of my favorites........BC
-----Original Message-----
From: Kim Helliwell [mailto:kimgh@apple.com]
Sent: Wednesday, July 10, 2002 4:50 PM
To: Lynne Green
Cc: Chris.Dewitt@motorola.com; ibis-users@eda.org
Subject: Re: [IBIS-Users] FW: Extreme currents????
Frankly, I wish that suppliers of models (whether IBIS
or SPICE) would stop using these "ideal" diodes that
allow such large currents. I don't understand how anyone
can look at such results and think that's OK to ship to
customers!
Maybe they just don't look, but that's hardly an acceptable
excuse, either.
There, I feel better now!
Kim
On Wednesday, July 10, 2002, at 03:19 PM, Lynne Green wrote:
> Hello, Chris,
>
> Different EDA tools have different upper limits on currents that can be
> handled
> by the simulator.
>
> The warning is needed because "Spice" models that use the default series
> resistance (0.0 ohms) for diodes and junction leakage currents result
> in large
> IBIS table currents (often above 1 MegAmp). Most EDA tools don't go
> quite
> _that_ high.
>
> Best regards,
> Lynne
>
>
>
> -----Original Message-----
> From: John Angulo [mailto:jangulo@innoveda.com]
> Sent: Tuesday, July 09, 2002 6:02 PM
> To: ibis-users@eda.org
> Subject: [IBIS-Users] FW: Extreme currents????
>
>
> The following is forwarded on behalf of Chris DeWitt at Motorola TCG.
> Please send all private replies to Chris.Dewitt@motorola.com.
>
> John Angulo
> IBIS Open Forum Postmaster
> -----------------------------------
>
> If I run IBISCHK3 on many models I download I get an "Extreme currents
> present" warning.
>
> What are the limits on currents in the VI tables? I can't find it
> documented anywhere.
>
> Thanks
>
> Chris DeWitt
> Principal Staff Engineer
> Motorola TCG
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Kim Helliwell
Apple Computer
kimgh@apple.com
408 974 9936
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