herunterladen
© Semiconductor Components Industries, LLC, 2007
March, 2007 − Rev. 1
1 Publication Order Number:
AND8245/D
AND8245/D
Design Considerations for
ESD/EMI Filters: II
Low Pass Filters for
Audio Filter Applications
Prepared by: Ryan Hurley
ON Semiconductor
Applications Engineer
Background
For some time now portable electronic devices that have
some sort of audio function have needed some type of
Electro Static Discharge (ESD) protection. These ESD
protection devices would be used to protect the audio
input/output jacks and sometimes be located at the speakers
or microphone in the portable device itself. These ESD
protection devices provided nothing more than basic ESD
protection from ESD that the device could expect from
normal use.
In portable devices such as cell phones, there is the added
concern with regard to the audio lines. Not only do they still
need ESD protection, there are quite often requirements to
filter out Electromagnetic Interference (EMI). The concern
is that radiated or conducted EMI could be generated at the
audio input out output and then transmitted to the higher
frequency RF lines causing distortion. Or, RF transmission,
radiated or conducted, could affect the audio lines.
If a discrete solution is used for both ESD protection and
EMI filtering, the number of components could be three or
more parts per audio line. That would consist of a single ESD
component for ESD protection and an inductor/Resistor and
capacitor combination to provide EMI filtering. For
increased EMI filtering, more components are needed. Also,
an EMI filter composed of discrete components quite often
has some undesirable parasitics that may result in poor
filtering. More importantly, a discrete solution requires a
significant amount of board space. When board space is at
a premium, the last thing needed is a large number of surface
mount components.
One solution would be to integrate the ESD and EMI
filtering functions on to a single piece of silicon. Doing this
would not only reduce the number of components used for
each audio line, but would reduce unwanted parasitics,
especially ground path inductance. For audio input
applications at microphones, there are parts such as the
NUF2114 and NUF2116, each capable of providing ESD
protection and EMI filtering for two audio lines using an RC
“Pi” filter configuration. For audio output that may require
an LC “Pi” filter, there is the NUF2441 and NUF2070, that
each can support two lines, and the NUF4220 with four lines
of protection.
Not all integrated ESD/EMI filters are the same. There are
cases where using a series resistor in place of an inductor
may be just fine, especially on the audio input side near the
microphone where the network impedances are high. How
many audio lines are there that need ESD protection and
EMI filtering? What voltage of protection is needed for ESD
protection? Would a unidirectional Transient Voltage
Suppressor (TVS) diode be needed or would a bidirectional
TVS diode be required? These are just a few of the
considerations when choosing an ESD/EMI filter.
Types of Audio Applications
With regard to portable handheld electronic devices, there
are two general areas where an ESD/EMI filter could be
used. There is the input audio after the microphone, and then
there is the audio output before the speaker. Each of these has
its own requirements, particularly when considering the
network impedance.
For today’s cell phones, there are four areas where an
audio ESD/EMI Filter may be placed. The most obvious
place is next to the hands free connector. This is generally
where an Electrostatic Discharge is most likely to occur.
This is because the phone itself is incased in plastic and can
hold some charge and the hands free set can hold another. If
there is no ESD protection at this port, it is likely ESD will
damage the more sensitive internal components.
APPLICATION NOTE
http://onsemi.com
Verzeichnis
