Applicable to:
Multi-layer boards with power
distribution planes spaced ~0.3 mm or less
apart
General Guidelines
-
Multi-layer boards generally
employ two types of decoupling capacitor.
Large-valued "bulk" capacitors help to
minimize the impedance of the power bus at
low frequencies (e.g. below a few hundred
kHz). Smaller "local" capacitors reduce the
power bus impedance at higher frequencies
(e.g. up to ~ 100 MHz on boards with
closely spaced planes). At even higher
frequencies, the power bus impedance is
determined by the planes themselves.
-
Boards typically have one or
two large electrolytic bulk decoupling
capacitors or they may employ half a dozen
or more bulk decoupling capacitors in
smaller packages. Either approach is
effective and this decision is normally
made based on size, cost and board-area
constraints.
-
The total value of the bulk
decoupling is determined by the transient
power requirements of the active devices on
the board (See: "How much decoupling
capacitance do I need?") Generally, the
total bulk decoupling capacitance is 1 - 10
times the total local decoupling
capacitance connected to the power bus.
-
Local decoupling capacitors
are intended to be effective at higher
frequencies. The inductance of their
connection to the power distribution planes
is far more critical than their nominal
capacitance. Generally, smaller package
sizes can be connected to the planes with a
lower inductance than larger packages.
Therefore, local decoupling capacitors
should be as small as possible.
-
Choose the largest nominal
capacitance available in a given package
size. However, do not use capacitors that
have a nominal capacitance less than the
parallel plate capacitance that naturally
occurs between the power and power-return
planes [C=eA/d]. A board made with FR-4
material containing one pair of power
distribution planes spaced 0.25 mm (10
mils) apart has an interplane capacitance
of approximately 16 pF/cm2.
-
The location of the local
decoupling capacitors is not critical
because their performance is dominated by
the inductance of their connection to the
planes. At the frequencies where they are
effective they can be located anywhere
within the general vicinity of the active
devices [1].
-
The maximum frequency at
which the capacitors will be effective is
proportional to the square root of the
number of capacitors [1]. Therefore,
high-speed circuit boards often have many
local decoupling capacitors for every
active device on the board.
-
Connection inductance is
determined by the loop area formed by the
capacitor body, mounting pads, traces and
vias (See:
Estimating the connection inductance of a
decoupling capacitor.)
To minimize connection inductance:
-
Never use traces!
Locate the via adjacent to the
mounting pad.
-
If there is no room
for the via adjacent to the pad,
then move the whole capacitor.
Capacitor location doesn't matter,
but connection inductance is
critical.
-
Locate the two vias
as close together as possible.
-
Four vias (instead of
two) will cut the connection
inductance nearly in half.
-
Mount all of the
local decoupling capacitors on the
face of the board nearest to the
planes. Connection inductance is
nearly proportional to the distance
from the planes.
Examples
The figure below shows various
examples of local decoupling capacitor connections
to boards with closely spaced power distribution
planes. Connections with lower inductance will be
more effective at higher frequencies.
References
[1] T. H. Hubing, J. L. Drewniak, T.
P. Van Doren, and D. Hockanson, "Power bus
decoupling on multilayer printed circuit boards," IEEE Trans. on Electromagnetic Compatibility, vol.
37, no. 2, May 1995, pp. 155-166.
[2] M. Xu, T. Hubing, J. Chen, T. Van Doren, J. Drewniak and R. DuBroff, “Power-bus decoupling with embedded capacitance in printed circuit board design,” IEEE Trans. Electromag. Compat., vol. 45, no. 1, Feb. 2003, pp. 22-30..
|
