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'[EE]: 5V Logic Bypass caps - SMD value/phy size'
2000\06\02@115640 by James Paul

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Jim P,

In my experience, I have found that a 20 uF or so electrolytic,
usually Tantalum, and a .1 or .01 uF to be sufficient.  I haven't
tried to find an optimum value because these values always worked
for me in the past.  As far as package size goes, the 20 uF is
usually in the 1210 package and the .1 or .01 is either the 0805 or
0603 package.  Which package size I use depends on the size of the
IC I'm bypassing, the size of the board I have to work with, and the
number of components I have to mount.   Regarding the hand built
status, most of the boards I work with are hand built (stuffed)
because the volume is so small.   But I have no difficulty in
soldering packages as small as the 0603.  With some difficulty, I
can solder smaller packages, but them my soldering tip size becomes
a major hurdle.  I've also tried solder paste with some success.
Bottom line, the 1210, 0805 and 0603 packages work for me.  They
probably will for you too.  And that brings up my last point.  If
these boards are to be manufactured after the prototypes have been
proven and/or perfected, check with the manufacturing facility and
ask them their preferences and/or limitations on component size.
It would be a shame to design a circuit, get it perfected with 0603
components, and then find out that the manufacturing facility can
only handle down to 0805 size components.   Also, make sure you
leave enough space between components so that the Pick and Place
machines can get into posotion to place the component where it
belongs.  I'm sure you know most of these points, but I figured I'd
mention them just in case you didn't.

                                            Regards,

                                              Jim







On Fri, 02 June 2000, Jim P wrote:

{Quote hidden}

spam_OUTjimTakeThisOuTspamjpes.com

2000\06\02@115650 by James Paul

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Jim P,

In my experience, I have found that a 20 uF or so electrolytic,
usually Tantalum, and a .1 or .01 uF to be sufficient.  I haven't
tried to find an optimum value because these values always worked
for me in the past.  As far as package size goes, the 20 uF is
usually in the 1210 package and the .1 or .01 is either the 0805 or
0603 package.  Which package size I use depends on the size of the
IC I'm bypassing, the size of the board I have to work with, and the
number of components I have to mount.   Regarding the hand built
status, most of the boards I work with are hand built (stuffed)
because the volume is so small.   But I have no difficulty in
soldering packages as small as the 0603.  With some difficulty, I
can solder smaller packages, but them my soldering tip size becomes
a major hurdle.  I've also tried solder paste with some success.
Bottom line, the 1210, 0805 and 0603 packages work for me.  They
probably will for you too.  And that brings up my last point.  If
these boards are to be manufactured after the prototypes have been
proven and/or perfected, check with the manufacturing facility and
ask them their preferences and/or limitations on component size.
It would be a shame to design a circuit, get it perfected with 0603
components, and then find out that the manufacturing facility can
only handle down to 0805 size components.   Also, make sure you
leave enough space between components so that the Pick and Place
machines can get into posotion to place the component where it
belongs.  I'm sure you know most of these points, but I figured I'd
mention them just in case you didn't.

                                            Regards,

                                              Jim







On Fri, 02 June 2000, Jim P wrote:

{Quote hidden}

.....jimKILLspamspam@spam@jpes.com

2000\06\02@120508 by David VanHorn

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At 10:35 AM 6/2/00 -0500, Jim P wrote:
{Quote hidden}

You want the caps to absorb well at the third harmonic of the clock.
0.1uF does well at 3 MHz, 0.01 at 30. 0.001 at 300.
It's a broad response, so dont think that there's one specific value.
However, if you use 0.1uF on a 20 MHz part, you won't get the supression
that you could if you used 0.047uF.


>I've got access to the under side of the board - and that's where
>I had intended on placing the SMD bypass caps.

Place the cap at the ground pin, and route power to the cap first, and from
there to the chip. Otherwise, you compromise the bypass.
Murata and panasonic make through-hole 3 leaded caps which help eliminate
this by having ground, in and out leads (I and O are reversible)

>The first few boards will be hand-built prototypes (of course!) - so
>am I better off with the larger size (1210 etc) SMD caps?

I do tons of work with 1206 and smaller with no special tools, though 0402
is pretty difficult.

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2000\06\05@164743 by Robert A. LaBudde

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part 0 1140 bytes
This suggests that lower capacitance a priori works better at higher speeds.

My understanding of the specifications suggests the opposite: for any make
of capacitor, higher capacitance always works better at any frequency.

What might be true is that certain types of capacitors are only available
in small denominations, and these types have lower ESR and ESI so they work
better at higher frequencies.

What's important is that the impedance of the capacitor be only 1 ohm or so
at the frequency of interest to reduce ripple at the current draw required.

</x-flowed>
Attachment converted: creation:capacitor impedance vs freq.gif (GIFf/JVWR) (00016199)
<x-flowed>================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ralspamKILLspamlcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================
</x-flowed>

2000\06\05@181949 by David VanHorn

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>This suggests that lower capacitance a priori works better at higher speeds.
>
>My understanding of the specifications suggests the opposite: for any make
>of capacitor, higher capacitance always works better at any frequency.

That would be true if they didn't have internal inductance that is
proportional to the capacitance.


>What might be true is that certain types of capacitors are only available
>in small denominations, and these types have lower ESR and ESI so they work
>better at higher frequencies.

Smaller values of C have smaller internal inductance, in any given package.

>What's important is that the impedance of the capacitor be only 1 ohm or so
>at the frequency of interest to reduce ripple at the current draw required.

More properly, as low as possible.
A cap with a Z of 1 ohm won't do a lot in a circuit with comparable impedance.

My measurements were made in a 50 ohm system, (stripline on G-10 PCB)
Thinner tracks will be higher impedance, and a given cap will perform
better with higher track impedances. (provided of course proper layout!)




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