[SI-LIST] Re: Decoupling capacitors
- From: Larry SMITH <Larry.Smith@xxxxxxx>
- To: weirsi@xxxxxxxxxx
- Date: Mon, 16 May 2005 09:14:02 -0700
Steve - The "big V" approach makes a very low impedance at the series
resonant frequency of the capacitor chosen. With this method, problems
can arise about a decade above and below the SRF where the decoupling
capacitors have to interface with other components in the system.
On the low frequency side, the output of the VRM is inductive. It's
impedance crosses the capacitive impedance of all the big V caps in
parallel and usually makes a fairly high Q resonant peak in the kHz or
low MHz region.
On the other end of the spectrum, the inductance of all the caps in
parallel usually resonates with the capacitance of the power planes and
make a high Q resonant peak in the 100MHz region. These are the two
"gotcha's" associated with the big V method. "Antiresonant" peaks at
any frequecy can (and often does) cause trouble in power distribution
systems.
You can add additional capacitor values to address the newly created
peaks, but then you end up with more peaks.. If you add enough
capacitor values to eliminate all the peaks, you end up with a flat
frequency profile. This is the one I like.
regards,
Larry Smith
Sun Microsystems
steve weir wrote:
> Joe Paul, the ESL may be the same but the ESR of the 1uF in the same
> chemistry and voltage will definitely be about 35-40% that of the
> 0.1uF. It is very unlikely that 0.1uF will provide any cost or performance
> advantage over 1uF in the same 0603 case from the same mfg at a low voltage
> rating. Due to cover layer considerations, depending on the voltage and
> chemistry, the 1uF may actually exhibit lower mounted inductance than the
> 0.1uF
>
> At 2ns/ 160MHz, both capacitors are fully inductive and cover layer issues
> aside will have very similar performance. The 1.0uF capacitor has the
> benefit of more capacitance which generally makes it easier to stabilize
> the transition from the bulk capacitors / VRM. Fans of the big "V" like
> Dr. Johnson, Istvan Novak, and myself will usually advise that at the same:
> cost, package size and chemistry, take the bigger capacitor. If you want
> to find out why some people do things differently, take a look at Larry
> Smith and company's papers on multipole capacitor networks. If nothing
> else, those papers should help you better understand what you are doing
> whether or not you elect to follow the methods they describe.
>
> If you really want to see how your capacitors perform and have access to a
> VNA, I suggest building a test board. You can get details on such a board
> from Istvan's papers on his web site, or from mine on the X2Y web
> site. You can put together a decent set of CPW test fixtures for under
> $200. cash and some time. Your biggest expense will be a pair of SMA
> connectors per fixture.
>
> If you want more information on bypass network design, Istvan's web site,
> the Teraspeed web site, and the X2Y web site all have papers on the subject.
>
>
> Steve.
> At 04:58 PM 5/16/2005 +0530, Joe Paul M wrote:
>
>
>>I have a doubt regarding decoupling capacitors.
>>
>>I have the option for using 1 uf9AVX 06036D105KAT2A) or 0.1uF (AVX
>>0603ZC104KAT2A) at same cost.
>>
>>Concerned rise time is about 2nS.
>>
>>Is there any issue in using 1uF caps, if it has same ESR and ESL and
>>package (0603) as 0.1uF.
>>
>>Thanks all
>>Joe Paul
>>
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