[Fwd: [SI-LIST] : Inductance and Decoupling]

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From: Itzhak Hirshtal (hirshtal@is.elta.co.il)
Date: Mon Mar 19 2001 - 06:07:13 PST


--
Itzhak Hirshtal
Elta Electronics
POB 330 Ashdod
Israel 77102
Tel: 972-8-8572841
Fax: 972-8-8572978
email: hirshtal@is.elta.co.il

attached mail follows:


Mar-19-01

James,

You're right, under the condition that the power plane is close enough to the components in question. However, there might be 2 cases where this is not always true:

(1) If you have a double-sided PCB (components on both sides), and/or (2) You have more than 1 supply voltage (very regular in these days), which makes it a little difficult to put them all close enough to the top and/or bottom layer.

The only possible "solution" I can see for the seeming problem is the notion (raised by some participants of this forum) that the peak currents I've calculated are supposed to be supplied by a good decoupling power-ground pair, which presumably has a very low inductance. The only problem is that this "capacitor" itself must be connected with a low-enough inductive path to the supply pins of the device. Since each device is connected through (relatively) many supply vias, then the effective inductance of this path is actually reduced by a factor equal to the number of supply pins.

I'm not sure if I'm right, or if this is indeed the "complete" picture. Will be glad to get more responses for this.

Thanks anybody

James Antonellis - Sun BOS Hardware wrote:

> Itzhak, John, > > Please excuse the informal greeting. > > I was wondering if one source of a slight correction could be > the following... > > In regards to Itzhak's comment: > > "let's take via length to be 60mil and its radius to be 6mils." > > The stackup is arranged such that the planes providing the worst > di/dt (io or core depending on your application) are closest to > the top and/or bottom of the pcb. Therefore, we see ~9.3 mils > of depth before we reach the first of the two planes. > > Thusly, is a 60mil length still valid given an effective current > distance of 10mils vs the total length of 60mils?? > > Just a thought, > jim > > ------------- Begin Forwarded Message ------------- > > From: Itzhak Hirshtal <hirshtal@is.elta.co.il> > MIME-Version: 1.0 > To: John Lipsius <jlipsius@JasmineNetworks.com> > CC: si-list <si-list@silab.eng.sun.com> > Subject: Re: [SI-LIST] : Inductance and Decoupling > > ------------- End Forwarded Message ------------- > > - > Jim Antonellis Sun Microsystems, Inc. > jim.antonellis@sun.com MS: UBUR03-204 > Phn: (781) 442-0392 1 Network Drive > Fax: (781) 442-1575 Burlington, MA. 01803 > > ------------------------------------------------------------------------ > > Name: NoName > NoName Type: Plain Text (TEXT/plain) > Description: NoName > > Name: NoName > NoName Type: Hypertext Markup Language (TEXT/html) > Description: NoName > > Name: vcard.vcf > vcard.vcf Type: VCard (TEXT/x-vCard) > Description: vcard.vcf

--
Itzhak Hirshtal
Elta Electronics
POB 330 Ashdod
Israel 77102
Tel: 972-8-8572841
Fax: 972-8-8572978
email: hirshtal@is.elta.co.il



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