[SI-LIST] : pcb plane cap and mixed bypass cap value....

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From: Chris Cheng (chris.cheng@3pardata.com)
Date: Mon Jun 12 2000 - 19:06:50 PDT


there seems to be a lot of discussions about using pcb capacitance
or spreading bypass caps value for flat response. i would like to
offer some counter points to the discussions.

for core noise decoupling
in all of my analysis for core power decoupling, whether using wire
bond or c4 packages, the package inductance (bond wire or c4 bump/
distribution, core via and package pin) is so high that no external
decoupling can help or contribute to the initial transient. you
can connect an ideal source to the pin and it still won't help
the initial transient voltage drop. it seems to me other than
on die decoupling and on package decoupling capacitors, any
attempt to use external decoupling caps for >500MHz core noise
is not effective.
if you buy into the above observation, the function of external
caps are only used for resupply the depleted charges of the
on die or package decoupling caps. a mere few hundred pf from
the plane capacitance is not going to cut it. what you really
need is the bulkiest and lowest esl/esr cap you can place and
manufacture near your package. followed by even larger and
bulky cap further away until the power supply wakes up. i
have not seen any benefit from caps that is lower than 0.01uf
in suppressing core noise at all. think about it in another way,
if external decoupling can do the magic, why does sso happens ?
hint, the package inductance gets in the way.

for i/o noise decoupling
i believe its is more important to have tight coupling between
the signal traces and the power/ground return plane for its
return current NOT tight coupling between the power and ground
plane. remember, i/o current flow from power through the signal
trace or from signal trace to ground return NOT between i/o power
and ground plane.
if you buy into the above, for a finite number of layer stack
up in pcb, it makes more sense to sandwich the signals between
the power/gnd plane than pairing up the power plane and leave
the signals to dual strip layers.

as for the 1/4 wavelength placement etc. what you need to
realize is that by the time the noise goes through the package,
it is no longer a high speed single point source but rather a
lower speed drupe spread out across the package. 1/4 wavelength
and distance is meaningless given the size of the package.

chris

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