Subject: Re: R_Pad?

From: [email-masked] (Scott Schlachter)
Date: Tue, 29 Oct 96 18:38:47 PPE

Dileep and Arpad:

HSPICE will model the inherent CMOS diodes (D to Bulk) within the transistor 
model.  From what I can tell, there seems to be a fallback to this specification
of the model:  the diode saturation current equations do not appear to have
any way to include any parasitic resistance whatsoever.  I believe that you both 
have valid points here.  HSPICE can model the diode correctly, but it seems 
that the only way is to extract it from the said transistors, and model it 
seperately - a definate unexpected extra step.  When modeled this way, there 
is no problem assigning the appropriate parasitic impedances associated with 
the diodes.  However, in trying to keep the power and ground clamp diodes as 
being inherently modeled within the pull-up and pull-down transistors, there
seems to be a flaw with the HSPICE model.  Perhaps there is something that I'm 
overlooking with this, and if anyone knows how to deal with this in the
HSPICE transistor models, please respond.  Thanks,
-Scott Schlachter
 Actel Corporation 

> Arpad Muranyi, Intel Corporation wrote:
>  
> > I have seen many SPICE models that ignore the resistive properties of the ESD 
> > diodes, yielding such large currents at 1 Volt of over or undershoot.  So much 
> > for the accuracy of the SPICE models... 
> > 
> 
> Please do not get confused between the model EQUATIONS and model PARAMETERS.
> SPICE diode model equations have the provision for specifying the parasitic
> resistance. Of course, if it is not specified in the model PARAMETERS,
> it can produce large currents. IBIS models will exhibit similar unrealistic
> behavior if the parasitic resistance is not taken into account while
> making the ibis model from simulations. It is always possible to use a very
> good model improperly to produce unrealistic results. SPICE is not to be
> blamed for that.
> 
> ------------------------------------------------------------
> Dileep Divekar
> Applied Simulation Technology, Inc.
> 2188 Bering Drive
> San Jose, CA 95131
> 
> Phone - (408)-434-0967 x 100
> Fax   - (408)-434-1003
> Email - [email-masked]
> ------------------------------------------------------------
>