Re: [SI-LIST] : Decoupling strategy in tight spaces

Roland F. Portman (portman@bitmicro.com)
Wed, 15 Sep 1999 11:44:57 -0700

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Chris,

Why the BIG package sizes? BIG package means BIG inductance which negates
the capacitance.

Consider the following. (You said 'mil' so I used X7R dielectric
with very stable temperature response.)

0.1uF, 0603,16wvdc GRM39X7R104K016A?
0.1uf, 0805, 50wvdc GRM40X7R104K050A?
4700pf, 0603, 50wvdc GRM39X7R472K050A

Murata Ceramic Capacitor CATALOG

If this isn't enough, then get exotic for a little more money and longer leadtimes.
Try the AVX 0508 LowInd Ceramic Caps. Yes I did type 05-08. The terminals
are along the long side of the cap and thus reduces the effective inductance.

For low ESR, take a look at Murata's 10uF ceramic replacements for tantalum
Also look at Kemet, but the word on the street is tantalum caps now have very
long lead times (20 weeks???)

Roland

(No I don't work for or gain benefit from suggesting Murata. I just like their product
line and the completeness of their data book. It has nice graphs, charts and notes.)

Chris Bobek wrote:

> Hi,
>
> I am trying to come up with a decoupling strategy for a board I'm
> working on. Unfortunately, there are some constraints that make it more
> difficult than other boards I've done.
>
> For various reasons (i.e., mil project), we have the following
> capacitors to work with:
>
> 4700pF -- 0805 (largest value for smallest case size, for our
> application)
> 0.1uF -- 1812 ("")
> 15uF -- 0.6ohm ESR, 1412
>
> For parts with a lot of switching and I/O, my thoughts were to:
> - use a 4700pF at each power pin because it has a smaller case size than
> the 0.1uF which will yield lower inductance, thus better response for
> higher freq's.
> - use a 0.1uF at each power pin for most of the other freq's.
> - use the low ESR 15uF where possible to provide large switching
> currents
>
> For other, smaller parts, I would just use a mix of 4700's and 0.1's.
>
> Are there any major problems with my reasoning? Does anybody have
> "better" recommendations... and can back them up ;-)
>
> I will follow this question up with another later, concerning capacitor
> placement and vias.
>
> Thank you,
>
> Chris
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Roland F. Portman
BiT Microsystems, Inc.
48499 Milmont Drive
Fremont, CA  94538

(510) 623-2341 Ext 129 (510) 623-2342 Fax (925) 328-7128 Pager

portman@bitmicro.com http://www.bitmicro.com

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Chris,

Why the BIG package sizes?   BIG package means BIG inductance which negates
the capacitance.

Consider the following.  (You said 'mil' so I used X7R dielectric
with very stable temperature response.)

0.1uF, 0603,16wvdc   GRM39X7R104K016A?
0.1uf, 0805, 50wvdc   GRM40X7R104K050A?
4700pf, 0603, 50wvdc  GRM39X7R472K050A

 Murata Ceramic Capacitor CATALOG
 

If this isn't enough, then get exotic for a little more money and longer leadtimes.
Try the AVX 0508 LowInd Ceramic Caps.  Yes I did type 05-08.  The terminals
are along the long side of the cap and thus reduces the effective inductance.

For low ESR, take a look at Murata's 10uF ceramic replacements for tantalum
Also look at Kemet, but the word on the street is tantalum caps now have very
long lead times (20 weeks???)
 

Roland

(No I don't work for or gain benefit from suggesting Murata.  I just like their product
line and the completeness of their data book.  It has nice graphs, charts and notes.)

Chris Bobek wrote:

Hi,

I am trying to come up with a decoupling strategy for a board I'm
working on.  Unfortunately, there are some constraints that make it more
difficult than other boards I've done.

For various reasons (i.e., mil project), we have the following
capacitors to work with:

4700pF -- 0805  (largest value for smallest case size, for our
application)
0.1uF -- 1812  ("")
15uF -- 0.6ohm ESR, 1412

For parts with a lot of switching and I/O, my thoughts were to:
- use a 4700pF at each power pin because it has a smaller case size than
the 0.1uF which will yield lower inductance, thus better response for
higher freq's.
- use a 0.1uF at each power pin for most of the other freq's.
- use the low ESR 15uF where possible to provide large switching
currents

For other, smaller parts, I would just use a mix of 4700's and 0.1's.

Are there any major problems with my reasoning?  Does anybody have
"better" recommendations... and can back them up ;-)

I will follow this question up with another later, concerning capacitor
placement and vias.

Thank you,

Chris

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Roland F. Portman
BiT Microsystems, Inc.
48499 Milmont Drive
Fremont, CA  94538

(510) 623-2341 Ext 129
(510) 623-2342 Fax
(925) 328-7128 Pager

portman@bitmicro.com
http://www.bitmicro.com
  --------------D351BCF5AD7F2398A86DB3E5-- **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****