FreeLists / si-list / [SI-LIST] Re: decoupling

[Ritchey Lee <leeritchey@xxxxxxxxxxxxx>]

Yes, at 250 MHz and up, the plane capacitor is the work horse.  Imagine what
happens when someone tells you to insert a  ferrite bead in series with a power
lead, isolating it from the plane capacitor.  Really bad things!  Sadly, many
applications notes tell you to do this, as do some of the less well trained EMI
people.  I spend a lot of time fixing designs that have followed this advice.

Lee

Khalid Ansari wrote:

> Larry,
>
> What happens much above 250 MHz, do we need any more
> capacitors or is the power to ground plane capacitance
> sufficient?
>
>  >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
>
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