From: Adrian Shiner ([email protected])
Date: Thu Jan 06 2000 - 11:10:02 PST
The comments about large currents not causing EMI problems lend some support
to my thoughts that voltage based logic is not the necessarily the best way
of designing computers.
----- Original Message -----
From: Larry Smith <[email protected]>
To: <[email protected]>
Sent: 05 January 2000 21:46
Subject: Re: [SI-LIST] : low ESR decoupling capacitors
> DC - the calculations below are just an example for one capacitor.
> To build a robust power system, we need many different valued
> capacitors in parallel. The list probably includes some 100nF, 10nF,
> 4.7nF, 3.3nF, 2.2nF, 1.5nF, 1nF, 820pF, 680pF, 470pF, as well as bulk
> capacitors. Each capacitor value resonantes and presents a minimum
> impedance to the power distribution system at a different frequency.
> To calculate the number of each value to put in parallel, we have to
> know the ESR. Then it is just a simple matter of putting enoungh
> capacitors of each value in parallel to reach down to a target
> impedance. We might end up with 100 capcitors in parallel, some of
> each value. The impdeance over frequency becomes flat and resisitive
> in phase. It is quite practical to have a flat 10 mOhm impedance out
> to 100 MHz or more with 100 capacitors. The phase of the parallel
> impedances does not deviate very far from being resistive.
> You can hit that power system with any clock pulse or any conceivable
> current load waveform and see noise similar to what you would see with
> an ideal voltage source with a 10 mOhm series resistor. It is very
> difficult to predict the load waveform that customer code is going to
> cause the uP and ASICs to draw from the power system. Therefore, we
> want to have a flat impedance across a broad frequency range.
> This makes an almost ideal supply. If your system needs a 5 mOhm
> supply, just double the number of capacitors (...or halve the ESR of
> the capcitors you already have...). There is no voltage ringing in the
> overall system because of the flat impedance profile.
> There is however tremendous current ringing back and forth between
> capacitors. We have not seen an EMI problem from this. In fact, the
> EMI performance of the systems that we have built from this methodolgy
> have always shown an EMI improvement. It seems that power planes with
> bouncing voltages are more harmful than power planes that redistribute
> a lot of current.
> Larry Smith
> Sun Microsystems
> > From: "D. C. Sessions" <[email protected]>
> > Larry Smith wrote:
> > >
> > > Doug - I am changing the thread title to better reflect the subject.
> > > We need to take a look at inductance, resistance and capacitance
> > > to determine the impedance of a capacitor at 200 MHz. If you follow
> > > this through, you will see the great value of low ESR capacitors.
> > >
> > > Inductance is probably the most important parameter of a decoupling
> > > capacitor. You have correctly calculated the inductive reactance of a
> > > typical capacitor at 200 MHz if it is mounted on 5 nH pads. But with
> > > careful pad and via design, we routinely reduce the mounted loop
> > > inductance of 0805 size capacitors to less than 1 nH. One nH gives us
> > > 1.26 Ohms of inductive reactance at 200 MHz.
> > >
> > > But, suppose we mount a 633pF capacitor with 100mOhms ESR on that 1 nH
> > > pad. It forms a nice series RLC circuit that has a minimum impedance
> > > at frequency 1/(2pi*sqrt(LC)) = 200 MHz. The impedance is:
> > >
> > > R + jwL + 1/jwC
> > > = 100m + j 1.26 - j 1.26
> > > = 100mOhm
> > The problem is that most of us aren't trying to draw narrow-bandwidth
> > sinusoidal power from the supply network. For us, the wild resonant
> > swings on the capacitors aren't neatly balanced by equally wild di/dt
> > swings on the inductance, and instead the local supply gets sucked
> > down hard at clock edges and then swings out of safe operating area
> > in between.
> > I'm somewhat familiar with the work being done on resonant clock/power
> > systems, but since our libaries and processes aren't designed with
> > them in mind I really would prefer a supply net that minimized peak
> > excursions from nominal over one that was nominal only at selected
> > > By using a low ESR capacitor, we have presented an impedance to the
> > > power distribution system that is 1/10 of the impedance from the
> > > inductance.
> > >
> > > This is an extremely powerful concept. With low inductance mounting
> > > pads and low ESR capacitors, it is possible to build a high
> > > power distribution system with a fraction of the capacitors you might
> > > have thought you needed. You just have to carefully pick the
> > > capacitance value, carefully design the pads, and have a source of low
> > > ESR caps. THE LOWER, the BETTER!
> > >
> > > We are designing power distribution systems with target impedances
> > > are less than 10 mOhms. Typical NPO and X7R capacitors in the 470 pF
> > > to 10 nF range have ESR greater than 100mOhms. We could use
> > > that have 1/10 the ESR of today's caps. We could reduce the number of
> > > capacitors cluttering up or boards from hundreds to tens. If we only
> > > had lower ESR caps...
> > >
> > > We do not sprinkle in capacitors like salt and pepper but rather have
> > > very deliberate design methodology. It is documented in the IEEE
> > > Transactions on Advance Packaging, Aug 1999, Vol 22, Number 3. The
> > > paper gives much more details on power distribution and decoupling
> > > capacitors than I can give in this space. A soft copy is available
> > >
> > > http://www.qsl.net/wb6tpu/si_documents/docs.html
> > >
> > > Parallel capacitor resonance is definitely an issue. You must have a
> > > methodology that avoids the parallel resonance if you are going to use
> > > low ESR capacitors. You must carefully design the power plane
> > > Placement is critical for capacitors that resonate at a frequency
> > > the dimensions of the board become significant. But fortunately,
> > > software tools will soon become available in the public domain to help
> > > us do all of this.
> > >
> > > Now, all we need is low ESR caps.
> > >
> > > BTW, I really like your time domain method of measuring capacitor
> > > parameters. You won't get any information about mounted inductance
> > > from this measurement, but capacitance, ESR and fixture inductance
> > > determine the shape of the observed waveform. Is there some reason
> > > why you use a 100 Ohm resistor to inject energy into the cap? A 50
> > > Ohm resistor might better terminate the transmission line and avoid
> > > transmission line resonance issues in the measurement.
> > >
> > > regards,
> > > Larry Smith
> > > Sun Microsystems
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