[SI-LIST] Re: Power integrity / Transient current in Target impedance
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: <saeedbakhshi@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 11 Dec 2003 08:53:19 -0700
Saeed,
For the transient current I use I = C dV/dt = C (Vcc/trise). C is the
receiver input capacitance on chip B x the number of drivers used for that
receiver type on chip A. Then Ztarget = (Vcc * 0.05)/I (for a 5%
regulation). You can do this for slow/typical/fast if required. Keeping
the target impedance to "several harmonics of the clock frequency" is, in my
opinion, impractical from a cost perspective (you would have a gazillion 100
pF caps on the board), and unnecessary from an SI perspective (interplane
capacitance). If the noise regulation for a particular chip is unspecified,
I default to 5% on an original design, and then get some lab data to see if
I can go to 10%. In general, just make Zo vs Frequency hit between 5% - 10%
regulation up to the frequency of operation and it will work fine. I usually
go a few percent over fops, but not multiples of fops.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Saeed Bakhshi
Sent: Thursday, December 11, 2003 2:13 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Power integrity / Transient current in Target
impedance
Hi
I have reviewed all si-list threads regarding to power integrity and
decoupling cap selection. all have said that there must be a target
impedance and this target impedance should be met up to several harmonics of
existing clocks frequencies ; and the target impedance is calculated using
the below equation:
Target impedance= VDD x (Voltage tolerance) / "I_transient"
OK, But nobody explain how "I_transient" value could be calculated.
In all datasheets there is a ICC(current drawn from power supply) in
absolute maximum rating table. Is this ICC the same "I_transient"? or a
fraction of ICC should be used as "I_transient"?
What is the relationship between ICC mentioned in the "Absolute maximum
rating table of Chip datasheets" and the "I_transient" in target impedance
formula?
any comment is appreciated.
Regards,
Saeed Bakhshi
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