RE: [IBIS-Users] Problem in developin NMOS Open Drain type IBIS

Open drain models are not tricky at all.  The output hi voltage is dependent
on three things. Vdd, Rload and the off current of the open drain driver.
Vdd is normally the power supply voltage. The resistor used in IBIS modeling
is usually 50 Ohms and the current in an off FET is usually 0.00mA.  So the
off voltage is usually Vdd.  

Vout = Vdd - Ioff*50Ohms = Vdd - 0 = Vdd

If you are getting a different value you are mostly likely not having the
proper Vdd value, the off current is not zero or you have other currents in
your model that are being summed with the FET's current in such a way that
there is current flowing in the load causing an IR drop.  I would check the
power and ground clamp tables to see if they are contributing any current to
the output.

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@teraspeed.com 
www.teraspeed.com 

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827

-----Original Message-----
From: owner-ibis-users@server.eda.org
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Amit KUMAR
Sent: Friday, September 21, 2007 6:19 AM
To: ibis-users@server.eda.org
Cc: amit kumar
Subject: [IBIS-Users] Problem in developin NMOS Open Drain type IBIS


Hello all,


I am generating IBIS model(ver 4.2) for a circuit which is working in 
Open drain mode.
I have included in the IBIS file one pulldown curve and 2 V-T waveforms 
(both with high fixture, one falling and one rising). Simulations show that
the operating point for high input is different in 
case of original circuit and IBIS file.
For low input it is the same.
So when i give some Falling input at the input of model both the IBIS 
waveform and original waveform fall to the same voltage but
at the start they are at different voltage.
Infact i get operating point for high input half of the voltage which i 
should actually get..


Is their something tricky in modelling of Open Drain model ??
I have modelled a few IBIS models of output type and I/O type but never 
did i get such a problem.

Thanks for your support
Amit Kumar
Design Engineer
ST Microelectronics India


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