[SI-LIST] Re: Decoupling
- From: "Bob Weber" <rweber@xxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 10 Jul 2001 13:36:01 -0400
It's not just the cap value that matters, but the whole parasitic circuit
that gets added when a cap is connected to the pwr & gnd planes. The cap has
its nominal capacitance value, plus body inductance, plus the inductance of
the PCB tracks and vias, and the cap ESR. The self-resonant frequency can be
surprisingly low, and the Q is high enough to ring.
The best overall capacitor (still effective over 250MHz) is the power to
ground plane capacitance. However, you need some amount of bulk capacitance
and this is best delivered by larger value caps, e.g. 100nf. Using some 10nf
caps adds a different parasitic resonance, and this can help lower the
overall impedance of the bypass network.
Finally, some damping is useful to kill the Q of the overall pwr / gnd
network. 1nf X7R caps in the same body size as the 10nf and 100nf caps have
a large ESR (check this out on the AVX website
http://www.avxcorp.com/SpiApps/spicap/).
An 0805 X7R 1nf cap has an ESR of about 0.7 ohms, and a 470pf version has an
ESR of about 1.3 ohms. The 100nf cap has an ESR of only .07 ohms, giving it
a much higher Q (and the ability to ring more).
You want high-Q when building an oscillator, but in bypass applications
low-Q is desired (avoid NP0 and C0G caps).
Bob W.
Robert Weber
Principal Engineer
TranSwitch Corporation
3 Enterprise Drive
Shelton, CT 06484
203.929.8810 x2507
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Larry Smith
Sent: Tuesday, July 10, 2001 12:39 PM
To: si-list@xxxxxxxxxxxxx
Cc: ldsmith@xxxxxxxxxxxxxxxxxx
Subject: [SI-LIST] Re: Decoupling
Richard - As another SI-er said, you will probably get a lot of
responses to this one - mostly because there are a lot of opinions out
there on how to do decoupling. I'd like to address this issue because
I think we are on the threshold of doing a lot of good things in the
power distribution and decoupling area. We need some help from the
capacitor vendors.
The short answer to your question assumes that the loop inductance for
the mounted capacitor is 1 nH. The series resonant frequency of a
capacitor is 1/{2pi*sqrt(LC)}. The minimum of the impedance curve
occurs at series resonance and is the value of the capacitor ESR. The
frequency associated with each of your capacitors is:
470 pF 232 MHz
0.01 uF 50 MHz
0.1 uF 16 MHz
Putting low ESR capacitors in parallel is like playing with fire. You
can cook with fire and heat your house, but if you are not careful with
fire, you will get burned. I am a strong advocate of multiple low ESR
capacitors in parallel because I believe we have learned how to make
good, safe use of them. The optimum power distribution system (fewest
components, least cost, least complexity) is obtained from careful
selection and placement of capacitors with a reasonably high Q (low ESR).
The trick is to create a low and flat impedance profile in the
frequency domain by using different valued capacitors in parallel.
Systems behave best when chips look out and see a power distribution
system that is resistive in phase (flat impedance). We like to
establish a target impedance which is defined as
Ztarget = power_supply_voltage * 5% / transient_current.
If your PDS impedance meets the target impedance up to the highest
frequency of interest, your noise will be within acceptable limits.
Much more is written on this topic in
"Power Distribution System Design Methodology and Capacitor
Selection for Modern CMOS Technology"
http://www.qsl.net/wb6tpu/si_documents/docs.html
It is easy to meet a 1 Ohm or 0.1 Ohm target impedance using "rules of
thumb" for decoupling and high ESR capacitors. It becomes a little
more difficult to meet a 10 mOhm target. If you are trying to meet 1
mOhm target impedance up to several hundred MHz, it will be very
difficult unless you have a well defined methodology and some software
tools to help you. On some of our more recent products, at least one
of each of the ceramic capacitors from the following menu are used:
100uF,
47uF, 22uF, 10uF,
4.7uF, 2.2uF, 1uF,
470nF, 220nF, 100nF,
47nF, 22nF, 10nF,
4.7nF, 2.2nF, 1nF,
680pF, 470pF, 330pF, 220pF, 150pF, 100pF
With three capacitors per decade of capacitance, it is possible to make
a flat impedance vs frequency profile from about 200 kHz to 400 MHz
without any problem from parallel antiresonances. The lower the ESL
and ESR (within reason), the fewer components you need. X7R capacitors
tend to have Q's between 2 and 5 and three values per decade are
sufficient. NPO (COG) capacitors may have Q's between 5 and 10 and six
values per decade are useful. Closely spaced power planes may be used
instead of some of the pF capacitors.
We have our own internal software tools to help manage the design.
Cadence is marketing the Power Delivery Tool under SpectraQuest that
does the same thing as our tools. The Cadence tool is even better
because it is hooked up to the design data base for the PCB.
Like all SI tools, these tools are based on models and the analysis is
only as good as the models. These days, I spend half of my life out in
the lab measuring capacitors and reducing the measured data. The
capacitor vendors could help me greatly by measuring the ESR and ESL of
their capacitors and publishing the data. They could also help by
designing capacitors that have the absolute minimum internal
inductance. BTW, traditional measurement techniques do not obtain very
good values for ESR and ESL. (Maybe that should be the topic of
another email.)
regards,
Larry Smith
Sun Microsystems
> Delivered-To: si-list@xxxxxxxxxxxxxx
> Date: Sat, 7 Jul 2001 00:00:30 -0700 (PDT)
> From: richard hill <richard2636@xxxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Decoupling
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>
> I have come across 470 pF capacitors being used
> for decoupling. What frequency range is this
> capacitor effective for decoupling? How about .1 uF
> and .01 uF? What if you put all of them in parallel?
> Is there a good document describing this selection of
> capacitors, I have come across these values quite often.
>
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