AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 6

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

AN2321/D
Decoupling capacitors
Capacitor
self-resonance
6
During active device switching, the high frequency switching noise created is
distributed along the power supply lines. The main function of the decoupling
capacitor is to provide a localized source of DC power for the active devices,
thus reducing the switching noise propagating across the board and decoupling
the noise to ground.
Ideally, the bypass and decoupling should be placed as close as possible to the
power supply inlet to help filter high frequency noise. The value of the
decoupling capacitor is approximately 1/100 to 1/1000 of the bypass capacitor.
For better EMC performance, decoupling capacitors should placed as close as
possible to each IC, because track impedance will reduce the effectiveness of
the decoupling function.
Ceramic capacitors are usually selected for decoupling; choosing a value
depends on the rise and fall times of the fastest signal. For example, with a
33MHz clock frequency, use 4.7nF to 100nF; with a 100MHz clock frequency,
use 10nF.
Apart from the capacitive value when choosing the decoupling capacitor, the
low ESR of the capacitor also affects its decoupling capabilities. For
decoupling, it is preferable to choose capacitors with a ESR value below 1 Ω .
The following briefly discusses how to choose the value of bypass and
decoupling capacitors based on their self-resonant frequency. In
capacitor remains capacitive up to its self-resonant frequency. After that, the
capacitor turns inductive, due to its lead length and trace inductance.
Table 1
one with standard 0.25 inch leads with interconnect inductance of 3.75nH and
the other surface mount with interconnect inductance of 1nH. We see that the
self-resonant frequency of the surface mount type is double that of the through-
hole type.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
lists the self-resonant frequency for two types of ceramic capacitors,
Figure 3. Impedance and Different Dielectric Materials
Go to: www.freescale.com
Impedance
(Ohms)
Capacitor works
as a capacitor
Capacitive
Self-resonant frequency
Frequency (MHz)
Capacitor works
as an inductor
Inductive
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Figure
MOTOROLA
3, the