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AN-1368
APPLICATION NOTE
One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Ferrite Bead Demystified
By Jefferson Eco and Aldrick Limjoco
INTRODUCTION
High resolution, high performance converters and radio frequency
(RF) systems require a low noise design of the power supply to
achieve optimum performance. Good filtering of power supply
noise and high frequency crosstalk reduction between analog and
digital domains is required, especially on mixed-signal converters
and transceivers.
Power supply noise is more evident with switching regulators,
which generate undesired output artifacts that can be harmful
to any noise sensitive system. However, due to their high power
conversion efficiency, switching regulators are often found in
portable devices where prolonged battery life is needed and in
systems where thermal limitations exist.
Similar digital and analog voltage rails of mixed-signal ICs are
often powered from different power domains. This approach
helps to prevent fast digital switching noise coupling onto the
sensitive analog supply rail and degrading the converter
performance, but it increases system level complexity and cost.
With proper high frequency isolation of supply domains, power
supplies can be shared between analog and digital domains,
simplifying the design and reducing the cost.
An effective method for filtering high frequency power supply
noise and cleanly sharing similar supply rails is the use of ferrite
beads. A ferrite bead is a passive device that filters high frequency
noise energy over a broad frequency range. It becomes resistive
over its intended frequency range and dissipates the noise energy in
the form of heat. The ferrite bead is connected in series with the
power supply rail and is often combined with capacitors to ground
on either side of the bead. This forms a low-pass filter network,
further reducing the high frequency power supply noise.
Figure 1 shows an example of a filtering scheme often used in
mixed-signal ICs to separate analog and digital supplies. The
scheme is suitable for any power distribution network; however,
understanding its effectiveness and limitations helps users to
avoid characteristics that may, for example, be detrimental in high
performance converter applications.
This application note discusses the important considerations that
system designers need to be aware of when using ferrite beads
in power supply systems, including response characteristics of
the ferrite bead, a simplified ferrite bead model and simulation, dc
current considerations, LC resonance effects, and damping
methods. An understanding of these elements can make the
system design approach fast and effective.
The Analog Devices, Inc., products used to demonstrate the
effects of ferrite beads as output filters are the 1.25 A, 1.2 MHz
synchronous step-down switching regulator (ADP2120) and the
2 A/1.2 A dc-to-dc switching regulator with independent positive
and negative outputs (ADP5071).
Figure 1. Sample Filter Scheme for a Mixed-Signal IC
POWER
SUPPLY
ANALOG
SUPPLY
DIGITAL
SUPPLY
FERRITE
BEAD
13359-001
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