[SI-LIST] Re: Strange Gnd Clamp V-I curve when generating IBIS
- From: "Rohan Hubli" <Rohan.Hubli@xxxxxxxxxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 16 Dec 2004 15:47:08 -0800
Andrew, =20
The leakage current is pretty significant which makes me uncomfortable.=20
If the current was of the order of uA, nA.. it can be neglected I agree. =
But what I am seeing currents of the order of 0.1A, and going upwards to =
about 4A @ ~4.5V.=20
The leakage current is pretty significant, which leads me to question =
the
quality of the spice models from the foundry vendor as Tom so rightly=20
pointed out.
Rohan=20
-----Original Message-----
From: Andrew Ingraham [mailto:a.ingraham@xxxxxxxx]
Sent: Wednesday, December 15, 2004 7:43 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Strange Gnd Clamp V-I curve when generating IBIS=20
How do you know the current is due to the GND clamp? Are you measuring =
it
at the clamp diode, or at the I/O pin?
Normally we measure clamp currents (for IBIS models) at the I/O pad or =
pin,
and from there you can't really tell whether the current goes into the
Ground clamp or the Power clamp. You'll see both GND and Power clamp
currents, added together. It's up to you to separate it into the GND =
and
Power clamp curves.
GND clamp currents can happen anywhere from -Vcc to +2Vcc. If your =
device
has positive overshoot clamps that connect to Vss (zener diodes or =
active
clamps, for example) and are not Vcc-relative, then I believe it would =
be
correct to make them part of your IBIS model's GND clamp curves, not the
Power clamp curves which represent leakage and clamp currents that do =
track
with Vcc.
Are the currents above +2.5V significant? If they are in the microamp =
(or
less) range, they may be "normal" for your particular device, and =
hopefully
this much leakage current is listed on the spec sheet. Check with the
device's designers to see if this is normal. (It has nothing to do with
IBIS.)
Personally, I take the phrase "5v tolerant" with a gallon of salt, and =
want
to see further proof. Some manufacturers use that phrase despite the =
fact
that their devices sink large currents at, or even below, 5 volts. I
suppose they believe "5v tolerant" just means the chip won't blow up if =
you
apply 5 volts to the pins.
Regards,
Andy
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=20
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