[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 06:10:38 -0700
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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