[SI-LIST] Ground bounce (was VOH datasheet question)
- From: Hal Murray <hmurray@xxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 14 Mar 2006 02:25:45 -0800
> You could clarify this issue from Dr. Johnson article named "Big
> Buffer" (http://www.sigcon.com/Pubs/news/8_07.htm).
As usual, his stuff is very good reading.
This paragraph caught my attention.
> The main factors that affect SSO noise are (1) the aggregate amount of
> current switched by your IC, (2) the rise time of that current, and
> (3) the number and quality of the power and ground connections
> provided in the IC package. Most relevant to this discussion is the
> total amount of current--more capacitive loading enlarges that
> current, increasing the amplitude of SSO noise.
Why does more capacitive loading make SSO worse?
The previous discussion showed that you can compute the output resistance of
the driver from the short circuit current. That doesn't depend upon the
capacitive loading.
I think that capacitive loading will make the duration of the output current
pulse longer. It will slow down the rise time of the signal, but that's the
voltage rise time. The current rise time is what causes ground bounce and
that won't change if you add more load capacitance.
What am I missing?
Is the turn-on time for a driver slow rather than instantaneous? "Slow"
meaning roughly the same as the (normal) rise time so the normal capacitance
gets charged while the driver is turning on and the driver never delivers the
full short circuit current?
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