Re: [IBIS-Users] Problem with POWER_CLAMP data

Vivek,

There is no problem, except maybe that your SPICE models are unrealistic
when stressed this far. The real device probably can't sink 49 amps, but
your SPICE model says that it does. This is not an uncommon problem with
SPICE models, many of which forget to include diode series resistance. Thus
you get extremely steep curves with huge currents in forward conduction, in
the SPICE model.

There is nothing wrong with the fact that the POWER CLAMP goes from 0V
to -1.8V. Remember, the POWER CLAMP table is VDD-relative, and if VDD is
3.3V, then 3.3V at the device pin corresponds to 0V in the IBIS table, and
5.1V at the device pin corresponds to -1.8V in the IBIS table.

Is the 49A value a problem if left alone? Maybe, or maybe not. If the
SPICE model was OK, then so is the IBIS model; that is, the IBIS model is no
worse than the SPICE model. It is at the extreme far end of the clamp
curve, well away from normal operation, and it is unlikely that the
simulator will converge on a voltage near that point. That large a current
value helps push the simulator back into the more normal operating region
where the currents are less. This will happen with both the SPICE model and
the IBIS model.

If you don't like the 49A, you should fix the SPICE model.

Regards,
Andy

> In the POWERCLAMP curves, for extreme voltage conditions, I get a
very
> high current of about 49A( which is wrong). Now if i closely observe the
> pct1.spi, i find that the sweep has been set in the following manner :
>
> .DC VOUTS2I 3.3 5.1 0.1
>
> But in the IBIS file, the POWER CLAMP data has been generated
from -1.8V
> to 0.0V. And the current at -1.8V is 49A.
>
> What can be the problem ? Is it the behaviour of the buffer or is it
a bug
> with s2ibis2 ? How can i get out from this problem ?

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Received on Fri Jan 14 07:31:08 2005