[SI-LIST] Re: Placement of decoupling capacitors
- From: "Greim, Michael" <mgreim@xxxxxxxxxxxx>
- To: "'khalida@xxxxxxxxxxx'" <khalida@xxxxxxxxxxx>,Larry Miller <lmiller@xxxxxxxxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Wed, 1 Aug 2001 08:06:28 -0400
Hi Khalid,
Well, I think that mfg would take great umbrage with
piggy backing capacitors. For a prototype this might
be possible but not for production. You might want to
make use of plane capacitance to help at high frequencies.
Think of what you are proposing, by piggy backing the
caps, you are minimizing the foot print but not the loop
inductance to the plane. Essentially you are just presenting
a larger cap to the PDS. This will move your resonant
frequency and give you perhaps unexpected behavior.
You may get some additional decoupling through the use of
smaller footprint caps like 0402 and via sharing with the
bga if in fact there is no other way to get the decoupling
in. I can't believe that the board is so packed, that
there isn't a better alternative to piggy backing caps.
If the board is that packed, routing will prove to be your
next problem I suspect.
Do you have a .brd file that I could take a look at and
offer some suggestions on what options you might have.
best regards,
MG
-----Original Message-----
From: Khalid Ansari [mailto:khalida@xxxxxxxxxxx]
Sent: Tuesday, July 31, 2001 5:11 PM
To: Larry Miller; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Placement of decoupling capacitors
Larry,
>I think you have it right on placing the smallest caps (highest frequency)
>closest to the pins, but these are very small (we use 0603's), so the next
I just have enough space to place this one close to the pin, the rest have
to go quite further away due to space constraint. Placing them too far
away I have to worry about the loop inductance.
Are there any issues with piggy backing capacitors?
>bigger (lower frequency) ones are not that much further away. By the time
>you get to the bulk caps, they only have to be in the general neighborhood.
>
>No tricks or piggybacks needed-- the lower the frequency range you need to
>cover the less critical the placement. This is especially true where you
>have power and ground planes that are essentially zero ohms at lower
>frequencies.
>
>My opinion, anyway, and we haven't had any problems following this idea.
>
>Larry Miller
>
>-----Original Message-----
>From: Khalid Ansari [mailto:khalida@xxxxxxxxxxx]
>Sent: Monday, July 30, 2001 2:19 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Placement of decoupling capacitors
>
>
>
>I looked through various emails from Larry Miller, Larry Smith and the rest
>who had contributed to a discussion earlier on decoupling. Those
>emails helped me select the values for decoupling caps but now the
>challenge is, how do I place these? I don't have enough room on each of my
>power supply pins to place these caps around. If I try to place the
>cap that decouples the highest frequency components out of the selected
>ones close to the power pin and the one that decouples the lowest frequency
>components the farthest, the caps are being placed too far away from the
>pin.
>Can I piggy back these caps? What are some of the tricks to overcome this
>problem?
>
>Thanks in advance,
>
>Khalid
>
>
>
>
>
>
>
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