[SI-LIST] Re: Strange Gnd Clamp V-I curve when generating IBIS
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 15 Dec 2004 10:42:34 -0500
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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