[SI-LIST] Re: Bypass vs Decoupling capacitors
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Zhangkun <zhang_kun@xxxxxxxxxx>, istvan.novak@xxxxxxxxxxxxxxxx
- Date: Fri, 27 Aug 2004 07:34:19 -0700
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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