Zynq-7000 PCB Design Guide
21
UG933 (v1.8) November 7, 2014
Chapter 3:
Power Distribution System
If tantalum capacitors are not available or cannot be used, low-ESR, low-inductance
electrolytic capacitors can be used, provided they have comparable ESR and ESL values.
Other new technologies with similar characteristics are also available (Os-Con, POSCAP, and
Polymer-Electrolytic SMT).
A real capacitor of any type then not only has capacitance characteristics but also
inductance and resistance characteristics.
shows the parasitic model of a real
capacitor. A real capacitor should be treated as an RLC circuit (a circuit consisting of a
resistor (R), an inductor (L), and a capacitor (C), connected in series).
shows a real capacitor’s impedance characteristic. Overlaid on this plot are
curves corresponding to the capacitor’s capacitance and parasitic inductance (ESL). These
two curves combine to form the RLC circuit’s total impedance characteristic, softened or
sharpened by the capacitor’s ESR.
As capacitive value is increased, the capacitive curve moves down and left. As parasitic
inductance is decreased, the inductive curve moves down and right. Because parasitic
inductance for capacitors in a specific package is fixed, the inductance curve for capacitors
in a specific package remains fixed.
X-Ref Target - Figure 3-4
Figure 3-4:
Parasitics of a Real, Non-Ideal Capacitor
C
ESL
ESR
UG933_c3_04_032811
X-Ref Target - Figure 3-5
Figure 3-5:
Contribution of Parasitics to Total Impedance Characteristics
Frequency
Impedance
Capacitive
Contribution (C)
Inductive
Contribution (ESL)
Total Impedance Characteristic
UG933_c3_05_032811
