[SI-LIST] Questions about interplane capacitance
- From: "Joel Brown" <joel@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 5 Mar 2008 10:41:07 -0800
Interplane capacitance is frequently cited as the only effective bypass
capacitance on a PCB at frequencies above 200 MHz.
I am currently working on a design which brings up some questions regarding
interplane capacitance.
1. Power planes normally carry "standard" voltage rails that are used
throughout a board such as +5V and +3.3V.
High speed ICs usually have core voltages that are local to the IC and are
provided by a local regulator which converts the standard rail to the core
voltage (example 3.3 to 1.8V).
The local core voltage is distributed on a plane area that is local to the
IC and therefore is small in area (0.25 sq in or less) which results in a
very small amount of interplane capacitance.
Is this very small amount of capicitance effective for bypassing the IC? I
am sure it depends somewhat on the current waveform being drawn by the IC
but this can only be estimated because semiconductor manufacturers do not
provide current consumption profile as a function of frequency. To make
matters worse, some ICs have several different VCC pins which the
manufacturer recommends connecting to separate networks of bypass caps and
ferrite beads. This cuts the power distributuion up even more resulting in
less (practically zero) interplane capacitance. It is somewhat ironic that
the the voltages such as +5V and +3.3V which are required at points across
the whole board and therefore have the most interplane capacitance are also
the voltages which have least requirement for interplane capacitance because
they do not directly supply high speed rails.
2. There has been a lot of emphasis on reducing the mounted inductance of
bypass capacitors. Even with this reduced inductance they are still only
effective up to several hundereds of MHz at which point the interplane
capacitance becomes the only bypass capacitance mechanism. However there is
inductance between the connection of the IC to the planes. This inductance
consists of vias and package inductance. I did look for some numbers for
package inductance and did not find much, it seems to be a closely held
secret. Also it is unknown how much bypass capacitnace is internal to the IC
package. Just for example if we assume 250pH for the vias and 500 pH for the
package, then the impedance at 500 MHz would be 2.36 Ohms. This seems rather
high for the interplane capacitance to be of much benefit.
In summary how much interplane capacitance is needed to be beneficial, and
why is it beneficial given the inductance in the vias and package?
Thanks - Joel
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