Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual page 23

Requirements for choosing decoupling capacitors:
•
•
Surface-mount chip capacitors are the smallest capacitors available and are a good choice
for discrete decoupling capacitors:
•
•
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.
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).
X-Ref Target - Figure 2-5
Figure 2-6
dashed-line 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.
Spartan-6 FPGA PCB Design and Pin Planning
UG393 (v1.1) April 29, 2010
For a specific capacitance value, choose the smallest package available.
- or -
For a specific package size (essentially a fixed inductance value), choose the highest
capacitance value available in that package.
For values from 100 µF to very small values such as 0.01 µF, X7R or X5R type
capacitors are usually used. These capacitors have a low parasitic inductance and a
low ESR, with an acceptable temperature characteristic.
For larger values, such as 100 µF to 1000 µF, tantalum capacitors are used. These
capacitors have a low parasitic inductance and a medium ESR, giving them a low Q
factor and consequently a very wide range of effective frequencies.
Figure 2-5: Parasitics of a Real, Non-Ideal Capacitor
shows a real capacitor's impedance characteristic. Overlaid on this plot are
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Figure 2-5 shows the parasitic model of a real
ESR
ESL
C
ug393_c2_05_091809
Basic PDS Principles
23