[SI-LIST] Re: Antwort: Re: Placement of Decoupling Caps
- From: John Barnes <jrbarnes@xxxxxxxxx>
- To: ruston_matt@xxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Wed, 07 Aug 2002 10:09:27 -0400
Matt,
I haven't tested any X5R capacitors, so I can't comment on them at this
time. If you can get me some samples I'd be happy to check them on the
HP 4195A Network/Spectrum Analyzer, and report back here.
Thanks!
John Barnes
dBi Corporation
216 Hillsboro Ave
Lexington, KY 40511-2105
http://www.dbicorporation.com/
ruston, matt wrote:
>
> Istvan, John:
>
> Hi. Any thoughts on using X5R caps instead of X7R. I believe these caps are
> becoming more popular, give almost an order of magnitude more capacitance
> per body size (in 0402 sizes), and have similar dissapation factors and
> aging rates as an X7R cap.
>
> The X5R cap has a better temperature range (-55C to 85C) and tolerance than
> Y5V and Z5U. The cap can be used above 85C with some degradation in
> tolerance, but since they pack more capacitance in the same package and
> decoupling isn't all that sensitive to a few more percent of lower
> capacitance, X5R seem like a win over X7R (assuming you subscribe to the
> decoupling methodology described by John below; higher capacitance is
> better). I could see running X5R up to at least 100C with no major problems.
>
> X5R does have a have a cost penalty. My limited pricing experience shows
> X5R may be 50% -100% more costly. Does anyone have any better information on
> this?
>
> Any other thoughts on X5R (good or bad)?
>
> Regards,
>
> Matt
>
> -----Original Message-----
> From: Istvan Novak [mailto:istvan.novak@xxxxxxxxxxxxxxxx]
> Sent: Wednesday, August 07, 2002 7:25 AM
> To: jrbarnes@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Antwort: Re: Placement of Decoupling Caps
>
> John,
>
> I agree with all three of your statements/conclusions below.
>
> Thanks for posting the Appendix of your design guide. A
> couple of comments:
> - as you mentioned, ESR tends to be a function of several things,
> therefore the quoted ESR value of around 100 milliohms for
> capacitors with X7R and Z5Y dielectrics is true today only for the
> smaller-valued capacitors. In the uF range, some X7R capacitors
> have less than 10 milliohms ESR.
> - I would not limit the useful frequency range of Z5U and Y5U capacitors
> just because their dissipation factor increases with frequency. In fact,
> the
> ideal bypass capacitor would have zero current leakage at DC and arbitrarily
> high conduction (loss tangent) at any AC frequencies: this would in fact
> help
> bypassing. But as you say, the huge variation of capacitance over
> temperature
> and voltage (plus aging) make them an inferior choice anyway.
>
> Regards
>
> Istvan Novak
> SUN Microsystems
>
> ----- Original Message -----
> From: "John Barnes" <jrbarnes@xxxxxxxxx>
> To: <istvan.novak@xxxxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Sent: Tuesday, August 06, 2002 10:50 AM
> Subject: Re: [SI-LIST] Re: Antwort: Re: Placement of Decoupling Caps
>
> > Istvan,
> > I was developing Design Guidelines on Power Distribution for my previous
> > employer back in 2000. I found conflicting advice about choosing
> > bypass/decoupling capacitors in the engineering literature. So I
> > measured a bunch of different types and values of capacitors on an HP
> > 4195A Network/Spectrum Analyzer to try to resolve these questions for
> > myself:
> >
> > 1. You should go for the smallest package you can.
> >
> > Answer: Seems to be true. ESL is usually lower in a smaller
> > package with the same length:width ratio, but ESR showed no obvious
> > pattern of changes. For a given length, a wider package will
> > usually have a lower ESL.
> >
> > 2. You should go for the largest capacitance that you can get in a
> > package.
> >
> > Answer: Seems to be true. ESL showed no obvious relation to
> > capacitance, but ESR often dropped as the capacitance increased.
> >
> > 3. The dielectric does not affect ESR and ESL.
> >
> > Answer: Seems to be true until you reach/exceed the SRF. The
> > impedance of C0G/NP0 capacitors then follows an inductive path,
> > while X7R/Z5U/Y5U/Y5V capacitors wallow around near the ESR for a
> > while then start rising slowly. This is probably good, because the
> > lossy behavior will prevent sharp resonances that could cause
> > unwanted peaks in the power-distribution network's impedance.
> >
> > I personally prefer the X7R dielectric for bypass/decoupling capacitors,
> > as high as it will go. The reasonably tight tolerance over temperature/
> > voltage gives me confidence that all production units will be
> > reasonably close to the units we characterized and qualified during
> > Design Verification Test (DVT). I use some Y5U's as "bulk" ceramic
> > capacitors, usually between 1 and 4 per integrated circuit, to cover the
> > frequency region between the X7R's and the aluminum electrolytic bypass
> > capacitors.
> >
> > To help people choose an appropriate dielectric for a capacitor, here is
> > an appendix from these design guidelines.
> >
> > John Barnes KS4GL
> > dBi Corporation
> > http://www.dbicorporation.com/
> >
> >
> >
> > APPENDIX E: CAPACITOR DIELECTRICS
> >
> > Ceramic capacitors are commonly available in four dielectrics:
> > * C0G or NP0 (titanium oxide, neodymium oxide):
> > - Dielectric constant K of 85-170
> > - Best stability
> > - -55 to 125C operating range
> > - 0 to +/-30ppm/C variation over temperature
> > - 0 to +/-30ppm/C variation over temperature and 0 to rated voltage
> > - Dissipation factor (DF = ESR / Xc) under 0.001 at 25C
> > - Aging rate 0%/decade
> > - Capacitance little affected by frequency
> > - Has the lowest ESR, especially above 30MHz.
> > - Tends to be most expensive for a given capacitance and voltage
> > (CV).
> > * X7R and BX (barium titanate):
> > - K of 600-4000
> > - Poorer stability than C0G
> > - -55 to 125C operating range
> > - +/-15% variation over temperature versus capacitance at 25C
> > - BX has +15 to -25% variation over temperature and 0 to rated
> > voltage
> > - X7R may drop 20-45% from 0 to rated voltage
> > - DF <= 0.025 over temperature, drops as temperature and DC voltage
> > increase, increases as AC voltage and frequency increase.
> > - Aging rate maximum -2.5% per decade, typically -0.8 to -2% per
> > decade time
> > - Capacitance may drop 10-18% from DC to 10MHz
> > - ESR is about 100 milliohms from 10-30MHz.
> > * Z5U (barium titanate):
> > - K of 4000-18,000
> > - Poorer stability than X7R
> > - 10 to 85C operating range
> > - +22 to -56% variation over temperature versus capacitance at 25C
> > - May drop 60% from 0 to rated voltage
> > - DF <= 0.030 over temperature, drops as temperature and DC voltage
> > increase, increases as AC voltage and frequency increase,
> > increases greatly above 1 to 20MHz, so maximum usable frequency is
> > about 50MHz.
> > - Aging rate -3% to -5% per decade time
> > - Capacitance may drop 20% from DC to 10MHz
> > - ESR is about 100 milliohms at 5MHz.
> > - Is piezoelectric-- can generate voltage spikes if jolted or
> > vibrated.
> > * Y5U and Y5V (lead perovskite):
> > - Highest K
> > - Poorest stability.
> > - -30 to 85C operating range
> > - Y5U has +22 to -56% variation over temperature versus capacitance
> > at 25C
> > - Y5V has +22 to -82% variation over temperature versus capacitance
> > at 25C
> > - May drop 60 to 80% from 0 to rated voltage
> > - DF <= 0.050 over temperature, drops as temperature and DC voltage
> > increase, increases as AC voltage and frequency increase,
> > increases greatly above ??MHz, so maximum usable frequency is
> > about ??MHz.
> > - Aging rate about -5% per decade time
> > - Capacitance may double or treble from DC to 2MHz
> > - ESR is about 10-60 milliohms
> >
> > EIA RS-198 designations for temperature-stable Class 1 dielectrics:
> > * First (letter) significant digits of temperature coefficient:
> > - C = 0.0
> > - M = 1.0
> > - P = 1.5
> > - R = 2.2
> > - S = 3.3
> > - T = 4.7
> > - U = 7.5
> > * Second (number), multiplier of temperature coefficient:
> > - 0 = -1 part per million / degree C (ppm/C)
> > - 1 = -10 ppm/C
> > - 2 = -100 ppm/C
> > - 3 = -1000 ppm/C
> > - 4 = -10,000 ppm/C
> > - 5 = +1 ppm/C
> > - 6 = +10 ppm/C
> > - 7 = +100 ppm/C
> > - 8 = +1000 ppm/C
> > - 9 = +10,000 ppm/C
> > * Third (letter), tolerance of temperature coefficient:
> > - G = +/-30 ppm/C
> > - H = +/-60 ppm/C
> > - J = +/-120 ppm/C
> > - K = +/-250 ppm/C
> > - L = +/-500 ppm/C
> > - M = +/-1000 ppm/C
> > - N = +/-2500 ppm/C
> >
> > Examples:
> > * R2G = -220 ppm/C +/-30 ppm/C = -250 to -190 ppm/C (N220)
> > * S2H = -330 ppm/C +/-60 ppm/C = -390 to -270 ppm/C (N330)
> > * U2H = -750 ppm/C +/-60 ppm/C = -810 to -690 ppm/C (N750)
> > * M7G = +100 ppm/C +/-30 ppm/C = +70 to +130 ppm/C (P100)
> >
> > * C0G = NP0 = MIL-C-20D CG
> > * S1G = N030 = MIL-C-20D HG
> > * U1G = N080 = MIL-C-20D LG
> > * P2G = N150 = MIL-C-20D PG
> > * R2G = N220 = MIL-C-20D RG
> > * S2H = N330 = MIL-C-20D SH
> > * T2H = N470 = MIL-C-20D TH
> > * U2J = N750 = MIL-C-20D UJ
> > * P3K = N1500
> > * R3L = N2200
> >
> >
> > EIA RS-198 designations for general-purpose Class 2 dielectrics:
> > * First (letter), lowest rated temperature:
> > - X = -55C minimum
> > - Y = -30C minimum
> > - Z = +10C minimum
> > * Second (number), highest rated temperature:
> > - 2 = +45C maximum
> > - 4 = +65C maximum
> > - 5 = +85C maximum
> > - 6 = +105C maximum
> > - 7 = +125C maximum
> > * Third (letter) tolerance:
> > - A = +/-1.0% tolerance
> > - B = +/-1.5% tolerance
> > - C = +/-2.2% tolerance
> > - D = +/-3.3% tolerance
> > - E = +/-4.7% tolerance
> > - F = +/-7.5% tolerance
> > - P = +/-10% tolerance
> > - R = +/-15% tolerance
> > - S = +/-22% tolerance
> > - T = +22 to -33% tolerance
> > - U = +22 to -56% tolerance
> > - V = +22 to -82% tolerance
>
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