Re: [SI-LIST] : Package Parasitics Modelling

raymondl@qsa.qualitysemi.com
Tue, 30 Mar 1999 11:03:57 +1000

In CMOS, the typical input protection design is somewhat like this:
________
Pin input ------|----------|__ R ___|----------
Internal circuit
|
|
________
| Diode/nfet|
----------------
|
__
GND

The value of R could be a few hundred ohms yet it has no effect at
the current sinking capability of the diode. Moreover, the capacitive
loading of an input pin is contributed majorly by the node before
the R.

Raymond Leung

"Christian S. Rode" <csrode@mediaone.net> on 30/03/99 09:17:38

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

To: si-list@silab.eng.sun.com
cc: (bcc: Raymond Leung/QSA/AU)
Subject: Re: [SI-LIST] : Package Parasitics Modelling

Still, 3 ohms (?) is much better than 0.05 ohms. I guess I wrote that
last message from an obsolete perspective. In days gone by I think
they intentionally used to add a poly serpentine resistor (over thick
field oxide) to limit the discharged current. How they get to 10KV
input protection today is beyond my ken.

A modern model might then be 3 ohms in series with
protection diode capacitance + 100 ohms (several squares of
20 ohm /sq diffusion/poly) in series with the input gate capacitance
(negligible?)

tomda wrote:

> If there was significant resistance between the input pin and the C
> associated with the input protection devices it would show up in the
input
> VI curves. We don't see any significant R in series with the protection
> devices when we make our measurement based models. The current in those
> "diodes" can easily get to over 100 mA before the voltage across the
device
> is 1 volt.
>
> Tom Dagostino
>
> -----Original Message-----
> From: Christian S. Rode [SMTP:csrode@mediaone.net]
> Sent: Monday, March 29, 1999 11:16 AM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Package Parasitics Modelling
>
> A question about IBIS input models. I assume a good portion of the input
> capacitance
> of these models (C_comp) is associated with the input protection
structures
> and that
> there can be significant resistance between that capacitance and the pad.
> That's
> not modeled in (earlier) IBIS models (and the EIA FAQ's "it's included
> in the I-V characteristics" isn't relevant). 50 milliohms (or whatever)
> for R_pkg will
> give an unnaturally high-Q for the input model. What would be a range of
> numbers for
> R between the L and C? (I'm currently guessing 100 ohms)
>
> Chris Rode
>
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