[SI-LIST] Re: Ground bounce (was VOH datasheet question)
- From: steve weir <weirsi@xxxxxxxxxx>
- To: billw@xxxxxxxxxxx, Edi.Fraiman@xxxxxxxxxxxxx
- Date: Tue, 14 Mar 2006 09:06:07 -0800
I have to agree with Bill here. Draw out the circuit including the
driver and the power distribution. The output current depends on the
R-L path through the driver to the load capacitance. Increasing the
capacitance increases the time constant of the network. It reduces
the characteristic impedance on the capacitor side, but not the input
side. Where I will disagree slightly is that as that time constant
goes up, for a fixed frequency the total amount of energy
transferred through the package interconnect increases, while the
impedance stays the same, leading to droop.
Steve.
At 05:57 AM 3/14/2006, Bill Wurst wrote:
>Edi,
>
>This is a common misconception. If you perform a cursory analysis, you
>will find that loading an output with more capacitance, while it
>increases the transition time (decreases the slew rate), does not change
>the amplitude of the rail bounce but it does have the undesirable effect
>of extending its duration.
>
> -Bill
>
>
> /************************************
> / billw@xxxxxxxxxxx /
> / /
> / Advanced Electronic Concepts, LLC /
> / www.aec-lab.com /
> ************************************
>
>Edi Fraiman wrote:
> > Hi,
> >
> > I concur with you.
> > Increased capacitive load reduce the slew rate on the driver outputs.
> > The slower the output slew rate, the lower the ground bounce will be.
> >
> > Edi
> > =20
> >
> >
> >
> >>-----Original Message-----
> >>From: si-list-bounce@xxxxxxxxxxxxx
> >
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >
> >>On Behalf Of Hal Murray
> >>Sent: Tuesday, March 14, 2006 12:26 PM
> >>To: si-list@xxxxxxxxxxxxx
> >>Cc: hmurray@xxxxxxxxxxxxxxx
> >>Subject: [SI-LIST] Ground bounce (was VOH datasheet question)
> >>=20
> >>=20
> >>
> >>>You could clarify this issue from Dr. Johnson article named "Big
> >>>Buffer" (http://www.sigcon.com/Pubs/news/8_07.htm).
> >>
> >>=20
> >>As usual, his stuff is very good reading.
> >>=20
> >>This paragraph caught my attention.
> >>=20
> >>
> >>>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.
> >>
> >>=20
> >>Why does more capacitive loading make SSO worse?
> >>=20
> >>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.
> >>=20
> >>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.
> >>=20
> >>What am I missing?
> >>=20
> >>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?
> >>=20
> >>=20
> >>=20
> >>--
> >>The suespammers.org mail server is located in California. So are all
> >
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> >
> >>other mailboxes. Please do not send unsolicited bulk e-mail or
> >>unsolicited
> >>commercial e-mail to my suespammers.org address or any of my other
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> >>=20
> >>=20
> >>=20
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