Zynq-7000 PCB Design Guide
28
UG933 (v1.8) November 7, 2014
Chapter 3:
Power Distribution System
L
MOUNT
= 0.8 nH (based on PCB mounting geometry)
To determine the effective in-system parasitic inductance (L
IS
), add the via parasitics:
L
IS
= L
SELF
+ L
MOUNT
= 0.9 nH + 0.8 nH
L
IS
= 1.7 nH
Equation 3-3
The values from the example are used to determine the mounted capacitor resonant
frequency (F
RIS
). Using
:
Equation 3-4
Equation 3-5
F
RSELF
is 53 MHz, but F
RIS
is lower at 38 MHz. The addition of mounting inductances shifts
the effective-frequency band down.
A decoupling capacitor is most effective at the narrow-frequency band around its resonant
frequency, and thus, the resonant frequency must be reviewed when choosing a capacitor
collection to build up a decoupling network. This being said, capacitors can be effective at
frequencies considerably higher and lower than their resonant frequency. Recall that
capacitors of differing values in the same package share the same inductance curve. As
shown in
, for any given frequency along the inductive portion of the curve, the
capacitors are equally effective.
Capacitor Anti-Resonance
One problem associated with combinations of capacitors in a PDS of an AP SoC is
anti-resonant spikes in the PDS aggregate impedance. The cause for these spikes is a bad
combination of energy storage elements in the PDS (intrinsic capacitances, discrete
capacitors, parasitic inductances, and power and ground planes).
Anti-resonance can arise between any two consecutive stages of a power distribution
system, such as between the high-frequency PCB capacitors and the PCB plane capacitance.
The inter-plane capacitance of the power and ground planes generally has
a high-Q factor.
If the high-frequency PCB capacitors also are high-Q, the crossover point between the
high-frequency discrete capacitors and the plane capacitance might exhibit a
high-impedance anti-resonance peak. If the AP SoC has a high transient current demand at
this frequency (as a stimulus), a large noise voltage can occur.
To correct this type of problem, the characteristics of the high-frequency discrete capacitors
or the characteristics of the V
CC
and ground planes must be changed, or AP SoC activity
shifted to a different frequency away from the resonance.
F
RIS
1
2
π
L
IS
C
-------------------------
=
F
RIS
1
2
π
1.7
9
–
×
10
H
(
)
0.01
6
–
×
10
F
(
)
⋅
-------------------------------------------------------------------------------------
38
6
×
10
Hz
=
=
