[SI-LIST] Re: Bypass vs Decoupling capacitors
- From: Zhangkun <zhang_kun@xxxxxxxxxx>
- To: weirsp@xxxxxxxxxx, istvan.novak@xxxxxxxxxxxxxxxx
- Date: Sat, 28 Aug 2004 10:20:36 +0800
Steve
Maybe we are different on the defination of the impedance of plane pair. What
is your defination? I use 2D transmission line theory to define the impedance
of plane pair.
Best Regards
Zhangkun
2004.8.28
----- Original Message -----
From: "steve weir" <weirsp@xxxxxxxxxx>
To: "Zhangkun" <zhang_kun@xxxxxxxxxx>; <istvan.novak@xxxxxxxxxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Friday, August 27, 2004 10:34 PM
Subject: [SI-LIST] Re: Bypass vs Decoupling capacitors
> Zhangkun, the issue is what connects the capacitors to the IC. If it is
> traces the inductance will be very high, and that is not what I propose. A
> 4 mil board in FR-4 should come in at about 0.75 ohms.
>
> Regards,
>
>
> Steve.
> At 09:25 PM 8/27/2004 +0800, Zhangkun wrote:
> >Steve
> >
> >If the thickness of dielectric is about 4mil, the impedance of plane pair
> >could be about 5-10 ohm. With good fanout tech, the inductance could be
> >controlled below 0.5nH. By the way, carefully selecting decoupling
> >capacitor is also helpful.
> >
> >Best Regards
> >
> >Zhangkun
> >2004.8.27
> >----- Original Message -----
> >From: "steve weir" <weirsp@xxxxxxxxxx>
> >To: "Zhangkun" <zhang_kun@xxxxxxxxxx>; <istvan.novak@xxxxxxxxxxxxxxxx>
> >Cc: <si-list@xxxxxxxxxxxxx>
> >Sent: Friday, August 27, 2004 9:10 PM
> >Subject: Re: [SI-LIST] Re: Bypass vs Decoupling capacitors
> >
> >
> > > Zhangkun, are you talking about trace inductance as in:
> > >
> > > VRM => trace => decoupling => trace => loads
> > >
> > > OR
> > >
> > > VRM => trace => planar island w/ decoupling?
> > >
> > > I am talking about the second case. The first case has high impedance.
> > >
> > > Power plane SRF is a function of geometry and Er. The issue is what is
> > the
> > > relative cost and headaches to damp the planes making them into
> > > transmission lines, versus chopping them up.
> > >
> > > 100 ohms sounds quite high. We would be thinking in the low ohms or 100's
> > > of mohms. The difficult part is the attachment inductance. If I have a
> > > perfect resistor at 500mohms, but have to attach through 1nH of
> > inductance,
> > > then at 1GHz, the inductive reactance is more than 12 times the resistance
> > > and the phase will be about 85 degrees. Even using an IDC type structure
> > > we are not going to get close to the order of magnitude improvement
> > > needed.
> > >
> > > Now, the one thing that I have not given too much thought to is whether we
> > > can get away with inductive terminations if we remain resistive through
> > the
> > > first few harmonics of the board SRF.
> > >
> > > Regards,
> > >
> > >
> > > Steve.
> > > At 06:25 PM 8/27/2004 +0800, Zhangkun wrote:
> > > >istvan and steve
> > > >
> > > >I want to add some comments on this topic.
> > > >
> > > >1.The power delivery system, which uses traces not plane, will have some
> > > >defect for the leading inductance. We have done a lot of simulation about
> > > >the leading inductance of decoupling capacitor. When the trace is taken
> > > >placed by plane, the ESL gets less of about 6db.
> > > >
> > > >2.The resonance about power plane could be eliminated by means of some
> > > >effort. The frequency could be extended to about 1GHz or higher. However,
> > > >in some product PCB, there is no space for this kind of measures. The PCB
> > > >is too crowded. AVX has provided some high-ESR capacitor, whose ESR could
> > > >be as large as 100 ohm.
> > > >
> > > >3.Using trace for power delivery could be used for some special circuit
> > > >such as PLL. There are some paper in IEEE International Symposium on EMC
> > > >2003 on this topic.
> > > >
> > > >Best Regards
> > > >
> > > >Zhangkun
> > > >2004.8.27
> > >
> > >
>
>
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