taken from si list
it is helpful to think about this in the frequency domain.
A tantalum capacitor of any size will reach its lowest impedance in the
1000's to 10,000's of Hz. It will support the PDS impedance out to a point
where the ceramic capacitor network takes over, typically between a few
hundred KHz to no more than 10MHz. At 10MHz the spreading inductance of a
plane pair even on a thick PCB is still quite low. As a result, we can
pretty much locate the bulk capacitors anywhere on the board that is
convenient as far as the IC load current is concerned. This usually makes
it a good idea to concentrate them close to the voltage regulator module.
Using a number of smaller capacitors rather than a few large capacitors can
result in a lower cost network due to the mounted inductance of the
devices. Spreading devices around is only useful if there is significant
spreading resistance. This should be evaluated on a design by design basis.
The VRM maintains a low impedance out to a frequency that varies widely by
the design of the VRM. A bypass network should be engineered and not
built by cookbook rules. That means you need to know how the VRM response
is defined, with or without a particular bulk capacitor. Then from your
requirements, you can synthesize the rest of the network.
Steve