Re: about offseting the pu and pc

Text item:

Syed,

The equation Vtable = Vcc - Vmeas assumes that you are measuring the output
voltage with respect to ground while you are sweeping the device. When you
select the sweeping range for a 5 volt device, you will have to sweep 5 volts
above Vcc and 10 volts below Vcc, which is -5 to +10 volts in ground relative
terms for the typical case.

Now, if you move Vcc +- 5% for min and max, you will use 4.75 and 5.25 volts.
Using these Vcc numbers, you will still have to sweep 5 volts above and 10 volts
below Vcc, which translates to -5.25 to 9.75 for the minimum and -4.75 to 10.25
volts for the maximum curves (in ground relative mode).

Imagine a circuit that has a variable voltage source connected between the Vcc
and output pins to do this sweep. Notice that the sweeper does the same thing
regardless of what Vcc is. It forces the output pin 5 volts above and 10 volts
below the Vcc voltage for all three cases (typ, min, max). This is the voltage
axis of the I-V tables in IBIS.

Also note that if your sweeper is at 0 volts in the above configuration, the
output to Vcc difference is equal to 0 volts. If you have a CMOS device driving
high, it will give you 0 amps regardless of what Vcc is. Therefore you will get
the zeroes all lined up at 0,0 for all three cases (typ, min, max). (Your
stement below is incorrect regarding this).

You can achieve the same results if you seep the pullup in a ground relative
mode. In that case the variable voltage source is between GND and the output.
If your device is driving high, you will get 0 mA when your sweep voltage equals
Vcc. For the three cases (typ, min, max) this will be at 5.0, 4.75, and 5.25
volts, respectively. So you will get 5.0V,0mA 4.75V,0mA and 5.25V,0mA where
the voltages are your Vmeas. Aplying the equation

Vtable = Vcc - Vmeas

you get the following three voltages for Vtable typ, min and max:

Vtable_typ = Vcc_typ - Vmeas_typ ---> 5.00 - 5.00 = 0
Vtable_min = Vcc_min - Vmeas_min ---> 4.75 - 4.75 = 0
Vtable_max = Vcc_max - Vmeas_max ---> 5.25 - 5.25 = 0

That is how they line up in the Vcc relative mode. I know this can be
cunfusing, but I tried my best to explain it. I hope it helped.

Sorry, but I do not know how s2ibis handles this.

Arpad
===============================================================================

Hi,

I have a ques on the Vcc offseting of the pu(Pullup) and
pc(Power Clamp) V/I data.

I am creating a CMOS model with 3.3V supply. This means
TYP=3.3V, MIN=3.0V and MAX=3.6V

There is 'one' voltage table for the three current table.
When I use the Vtable = Vcc - Vmeas to offset the voltage
table, 'Which' Vcc should be used ??

If I use 3.3V, only the TYP curve goes thru the 0,0 point
and the MIN and MAX will not ! The 0,0 point is when
Vcc=Voh (pu curve)

I am not sure how s2ibis2 handles this issue.

Pls comment.

Regards,
Syed.
National Semiconductor Corp.

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Subject: about offseting the pu and pc
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From: huq@rockie.nsc.com (Syed Huq)
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