[SI-LIST] Re: Antwort: Re: Placement of Decoupling Caps
- From: "Istvan Novak" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <jrbarnes@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 7 Aug 2002 07:25:00 -0400
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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