Subject: Questions on the relationship between IBIS model & datasheet

From: Su Ming TAI <[email-masked]>
Date: Tue, 05 Dec 2000 11:40:50 +0900

Dear SI & IBIS gurus,

Hi! I'm a newbie in the world of SI and I have question that some of you 
may be able to explain it to me.
I have a datasheet and an IBIS model of a same device. On the datasheet 
(see pg 4), the Ioh and Iol values are -24mA and 24mA respectively when Vcc 
is 3V.
Now, on the IBIS model, at the pullup and pulldown section, when the 
voltage is at 3V, the current are not anything near to 24mA or -24mA at all.
My question is:
1) Why is that the datasheet values do not seem to correlate with the IBIS 
model?
2) How do we know that the IBIS model is correctly generated based on the 
values from the datasheet?

I welcome any comments or explanations on this issue. Thanks.

Regards,
Su Ming TAI

---

From: "Ingraham, Andrew" <[email-masked]>
Date: Tue, 5 Dec 2000 10:54:16 -0500

> (see pg 4), the Ioh and Iol values are -24mA and 24mA respectively when
 
Those particular values mean that they do not want you to sink or source
more than 24mA at Vcc=3V.  The device may be capable of sinking or sourcing
more than that, but those are the recommended maximum output currents.

> voltage is at 3V, the current are not anything near to 24mA or -24mA at
 
Careful here.  The "voltage" that is 3V, is Vcc, the supply voltage.  It is
NOT the axis or independent variable of the [Pullup] and [Pulldown] curves!

> 1) Why is that the datasheet values do not seem to correlate with the IBIS
 
The IBIS datasheet (IBIS model) hopefully represents actual performance of
the device.  The Spec sheet (what you called datasheet but I need to call it
something else to differentiate it from the IBIS datasheet) describes how to
use the part.  Some of the information, for example page 4, puts constraints
on how you may use it.  Other sections, for example page 5, give the
absolute worst-case performance you can (hopefully) ever expect, even when
they migrate the part to a new IC fab which could result in a different IBIS
datasheet being generated.

There is almost always some "padding" included in the Spec sheet data.  For
example, the minimum tpd (according to all simulations and projections made
by the vendor) might be 1.7ns, hypothetically, but the vendor publishes 1ns
to be safe.  It's a marketing decision.  They might not add the same amount
of "padding" to their IBIS models, which are intended to better represent
actual performance.

In other words, when comparing "apples to apples" (identical
characteristics), I would not be surprised if the Spec sheet was more
conservative than the IBIS datasheet.

But let's take a closer look at your example.  The spec sheet on page 5 has:

  Voh(min) = 2.2V  @ Vcc = 3V, Ioh = -24mA
  Vol(max) = 0.55V @ Vcc = 3V, Iol =  24mA

Voh = 2.2V means Vcc-Voutput = 0.8V.  Checking the IBIS [Pullup] curve, when
Voltage = 0.8V, I(min) = -21.8948mA, which is close to -24mA.  It doesn't
quite meet the Spec.  At Ioh = -24mA, Vcc-Vout = slightly more than 0.9V, so
Vout = slightly less than 2.1V.  Not quite 2.2V minimum.

For Vol = 0.55V, the IBIS [Pulldown] curve says I(min) = between 32.8243mA
and 38.6213mA.  For Iol = 24mA, Vout = about 0.36V.  This meets the Spec
sheet (max. 0.55V).

> values from the datasheet?
 
I would hope that both the IBIS datasheet and the Spec sheet are generated
from simulations and/or measurements, not one from the other.  Neither sheet
alone has sufficient information to generate the other.

The IBIS datasheet has considerably more data about I vs. V, so it could
never be generated from a Spec sheet alone, but the IBIS datasheet usually
lacks the Absolute Maximum and Recommended Maximum conditions that are
included on the Spec sheet.

Regards,
Andy

---

From: "Dagostino, Tom" <[email-masked]>
Date: Tue, 5 Dec 2000 12:00:40 -0800

When Vol is specified at 0.55 volts and 24 mA it means the part will put out
less than 0.55 volts when 24 ma of current is being sunk into the output.
This is what the manufacturer "guarantees".  Since the manufacturer wants to
ship all possible die from a wafer the output spec is conservative, all
parts should meet this spec.  It is not unusual to see 2 to 5 time the spec
current at the specified output voltage. If the model shows less than the
specified current at the specified voltage I would be suspect of the model.
I have rarely seen a part that would not meet its Vol or Voh spec.  I have
seen many parts that would product more current at a given voltage than the
model said it should (measured Iol at Vol was greater than the Max in the
model).  

Tom Dagostino
IBIS and Tau Modeling Manager
SDD
Mentor Graphics Corp.
503-685-1613
[email-masked]

-----Original Message-----
From: Ingraham, Andrew [mailto:[email-masked]]
Sent: Tuesday, December 05, 2000 7:54 AM
To: 'Su Ming TAI'
Cc: [email-masked]; [email-masked]
Subject: RE: Questions on the relationship between IBIS model &
datasheet

 

 

Regards,
Andy

---

From: "Haque, Moshiul" <[email-masked]>
Date: Thu, 14 Dec 2000 11:25:08 -0600

Dear Su Ming TAI

When comparing the IBIS data to the datasheet spec 

Check the section that says electrical characteristics over recommended
operating  free air temperature range.

PARAMETER    TEST CONDITIONS    VCC  	MIN 
Vol		Iol=24 ma    	  3v    	0.55
Voh		Ioh=-24ma	 	  3v    	1.2

Now in the IBIS file in the pulldown data
[Voltage range]        3.3         3.0         3.6
500.00000mV            44.1271m    32.8243m    54.8473m
600.00000mV            52.2894m    38.6213m    65.2873m  

VCC  range is 
[Voltage range]        3.3         3.0         3.6

and the pullup current.
1.20000V           -41.5252m           -29.1685m           -52.8087m

[Voltage range]        3.3         		3.0	         	3.6

The Iol in datasheet is should be the addition of groundclamp current and
pulldown current in the IBIS file.Groundclamp current is negelegible in this
device.As you can see that at VCC=3.0 volt with 0.55v volt applied minimum
current is greater than 32 ma which agrees with the datasheet that says with
3v vcc and 0.55v applied at output in low state you should get atleast 24ma.

At the pullup
1.20000V           -41.5252m           -29.1685m           -52.8087m
[Voltage range]        3.3         		3.0	         	3.6

pullup data is VCC relative meaning that if its min 2.2v in datasheet that
actually is max 1.2v (3-1.2v) in the pullup data in IBIS file.The Ioh in
datasheet  should be the addition of pwrclamp current and pullup current in
the IBIS file.Pwrclamp current is negelegible in this device.As you can see
from the pullup data with 1.2v at output high state the current is at -29ma
which agrees with the datasheet.  

Best Regards.
Moshiul Haque
Application Engineer
Standard Linear and Logic Marketing
Texas Instruments 
Sherman , TX
Tel -(903) 868-7153
Fax-(903) 868-5980
e-mail- [email-masked]

-----Original Message-----
From: Su Ming TAI [mailto:[email-masked]]
Sent: Monday, December 04, 2000 8:41 PM
To: [email-masked]; [email-masked]
Subject: Questions on the relationship between IBIS model & datasheet

Dear SI & IBIS gurus,

Regards,
Su Ming TAI