[SI-LIST] Re: Antwort: Re: Placement of Decoupling Caps

  • From: "Ibrahim Khan" <ikhan@xxxxxxxxxxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 7 Aug 2002 09:24:55 -0500

Way to go John.  Appreciate your contribution.

Regards
Ibrahim Khan

-----Original Message-----
From: John Barnes [mailto:jrbarnes@xxxxxxxxx]=20
Sent: Wednesday, August 07, 2002 9:09 AM
To: ruston_matt@xxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Antwort: Re: Placement of Decoupling Caps



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:
>=20
> Istvan, John:
>=20
>  Hi. Any thoughts on using X5R caps instead of X7R. I believe these=20
> caps are becoming more popular, give almost an order of magnitude more

> capacitance per body size (in 0402 sizes), and have similar=20
> dissapation factors and aging rates as an X7R cap.
>=20
>  The X5R cap has a better temperature range (-55C to 85C) and=20
> tolerance than Y5V and Z5U. The cap can be used above 85C with some=20
> degradation in tolerance, but since they pack more capacitance in the=20
> same package and decoupling isn't all that sensitive to a few more=20
> percent of lower capacitance, X5R seem like a win over X7R (assuming=20
> you subscribe to the decoupling methodology described by John below;=20
> higher capacitance is better). I could see running X5R up to at least=20
> 100C with no major problems.
>=20
>  X5R does have a have a cost penalty. My limited pricing experience=20
> shows X5R may be 50% -100% more costly. Does anyone have any better=20
> information on this?
>=20
>  Any other thoughts on X5R (good or bad)?
>=20
> Regards,
>=20
> Matt
>=20
> -----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
>=20
> John,
>=20
> I agree with all three of your statements/conclusions below.
>=20
> Thanks for posting the Appendix of your design guide.  A couple of=20
> comments:
> - as you mentioned, ESR tends to be a function of several things,=20
> therefore the quoted ESR value of around 100 milliohms for capacitors=20
> 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=20
> milliohms ESR.
> - I would not limit the useful frequency range of Z5U and Y5U=20
> capacitors just because their dissipation factor increases with=20
> frequency.  In fact, the ideal bypass capacitor would have zero=20
> current leakage at DC and arbitrarily high conduction (loss tangent)=20
> 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.
>=20
> Regards
>=20
> Istvan Novak
> SUN Microsystems
>=20
> ----- 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
>=20
> > Istvan,
> > I was developing Design Guidelines on Power Distribution for my=20
> > previous employer back in 2000.  I found conflicting advice about=20
> > 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=20
> > 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=20
> > capacitors, as high as it will go.  The reasonably tight tolerance=20
> > over temperature/ voltage gives me confidence that all production=20
> > units will be reasonably  close to the units we characterized and=20
> > qualified during Design Verification Test (DVT).  I use some Y5U's=20
> > as "bulk" ceramic capacitors, usually between 1 and 4 per integrated

> > circuit, to cover the frequency region between the X7R's and the=20
> > aluminum electrolytic bypass capacitors.
> >
> > To help people choose an appropriate dielectric for a capacitor,=20
> > 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 =3D 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 <=3D 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 <=3D 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 <=3D 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 =3D 0.0
> >    -  M =3D 1.0
> >    -  P =3D 1.5
> >    -  R =3D 2.2
> >    -  S =3D 3.3
> >    -  T =3D 4.7
> >    -  U =3D 7.5
> > *  Second (number), multiplier of temperature coefficient:
> >    -  0 =3D -1 part per million / degree C (ppm/C)
> >    -  1 =3D -10 ppm/C
> >    -  2 =3D -100 ppm/C
> >    -  3 =3D -1000 ppm/C
> >    -  4 =3D -10,000 ppm/C
> >    -  5 =3D +1 ppm/C
> >    -  6 =3D +10 ppm/C
> >    -  7 =3D +100 ppm/C
> >    -  8 =3D +1000 ppm/C
> >    -  9 =3D +10,000 ppm/C
> > *  Third (letter), tolerance of temperature coefficient:
> >    -  G =3D +/-30 ppm/C
> >    -  H =3D +/-60 ppm/C
> >    -  J =3D +/-120 ppm/C
> >    -  K =3D +/-250 ppm/C
> >    -  L =3D +/-500 ppm/C
> >    -  M =3D +/-1000 ppm/C
> >    -  N =3D +/-2500 ppm/C
> >
> > Examples:
> > *  R2G =3D -220 ppm/C +/-30 ppm/C =3D -250 to -190 ppm/C (N220)
> > *  S2H =3D -330 ppm/C +/-60 ppm/C =3D -390 to -270 ppm/C (N330)
> > *  U2H =3D -750 ppm/C +/-60 ppm/C =3D -810 to -690 ppm/C (N750)
> > *  M7G =3D +100 ppm/C +/-30 ppm/C =3D +70 to +130 ppm/C (P100)
> >
> > *  C0G =3D NP0 =3D MIL-C-20D CG
> > *  S1G =3D N030 =3D MIL-C-20D HG
> > *  U1G =3D N080 =3D MIL-C-20D LG
> > *  P2G =3D N150 =3D MIL-C-20D PG
> > *  R2G =3D N220 =3D MIL-C-20D RG
> > *  S2H =3D N330 =3D MIL-C-20D SH
> > *  T2H =3D N470 =3D MIL-C-20D TH
> > *  U2J =3D N750 =3D MIL-C-20D UJ
> > *  P3K =3D N1500
> > *  R3L =3D N2200
> >
> >
> > EIA RS-198 designations for general-purpose Class 2 dielectrics:
> > *  First (letter), lowest rated temperature:
> >    -  X =3D -55C minimum
> >    -  Y =3D -30C minimum
> >    -  Z =3D +10C minimum
> > *  Second (number), highest rated temperature:
> >    -  2 =3D +45C maximum
> >    -  4 =3D +65C maximum
> >    -  5 =3D +85C maximum
> >    -  6 =3D +105C maximum
> >    -  7 =3D +125C maximum
> > *  Third (letter) tolerance:
> >    -  A =3D +/-1.0% tolerance
> >    -  B =3D +/-1.5% tolerance
> >    -  C =3D +/-2.2% tolerance
> >    -  D =3D +/-3.3% tolerance
> >    -  E =3D +/-4.7% tolerance
> >    -  F =3D +/-7.5% tolerance
> >    -  P =3D +/-10% tolerance
> >    -  R =3D +/-15% tolerance
> >    -  S =3D +/-22% tolerance
> >    -  T =3D +22 to -33% tolerance
> >    -  U =3D +22 to -56% tolerance
> >    -  V =3D +22 to -82% tolerance
>=20
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