AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 5

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

Capacitors
Bypass capacitors
MOTOROLA
In regulator or reference circuits, the DC bias resistor must be placed as close
as possible to the active device to minimize decoupling effect (i.e. improve
transient response time).
In RC filter networks the inductive effect from the resistor must be considered
because the parasitic inductance of the wire wound resistor can easily cause
local oscillation.
Selecting the right capacitor is not easy due to their many types and behaviors.
Nontheless, the capacitor is one component that can solve many EMC
problems. The following sections describe the most common types, their
characteristics and uses.
Aluminium electrolytic capacitors are usually constructed by winding metal foils
spirally between a thin layer of dielectric, which gives high capacitance per unit
volume but increases internal inductance of the part.
Tantalum capacitors are made from a block of the dielectric with direct plate
and pin connections, which gives a lower internal inductance than aluminium
electrolytic capacitors.
Ceramic capacitors are constructed of multiple parallel metal plates within a
ceramic dielectric. The dominant parasitic is the inductance of the plate
structure and this usually dominates the impedance for most types in the lower
MHz region.
The difference in frequency response of different dielectric materials mean a
type of capacitor is more suited to one application than another. Aluminium and
tantalum electrolytic types dominate at the low frequency end, mainly in
reservoir and low frequency filtering applications. In the mid-frequency range
(from kHz to MHz) the ceramic capacitor dominates, for decoupling and higher
frequency filters. Special low-loss (usually higher cost) ceramic and mica
capacitors are available for very high frequency applications and microwave
circuits.
For best EMC performance, it is important to have a low ESR (equivalent series
resistance) value as this provides a higher attenuation to signals, especially
frequencies close to the self-resonant frequency of the capacitor in use.
The main function of the bypass capacitor is to create an AC shunt to remove
undesirable energy from entering susceptible areas. The bypass capacitor is
acting as a high frequency bypass source to reduce the transient circuit
demand on the power supply unit. Usually, the aluminium or tantalum capacitor
is a good choice for bypass capacitors, its value depends on the transient
current demand on the PCB, but it is usually in the range of 10 to 470 µ F. Larger
values are required on PCBs with a large number of integrated circuits, fast
switching circuits, and PSUs having long leads to the PCB.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
PART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES
AN2321/D
5