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

["S. Weir" <weirsp@xxxxxxxxxx>]

Michael,

The device switching currents are fed by the board power system.  If you 
insert an impedance, you are choking the power supply and the on-chip 
voltage rail drops.  That is a very bad thing.

Regards,


Steve.
At 05:27 PM 7/11/01 -0500, you wrote:

>Can you explain why adding ferrite bead in series with a power lead is a
>bad thing? Thanks.
>
>Regards,
>Michael Chan
>
>
>-----Original Message-----
>From: Ritchey Lee [mailto:leeritchey@xxxxxxxxxxxxx]
>Sent: Wednesday, July 11, 2001 4:48 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: decoupling
>
>
>
>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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