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Semiconductor Components Industries, LLC, 2003
August, 2003 − Rev. 0
1 Publication Order Number:
AND8131/D
AND8131/D
New Low Resistance
Analog Switches Permit
Speaker Switching in Cell
Phones
Prepared by: Fred Zlotnick
ON Semiconductor
INTRODUCTION
CMOS Analog switches have been available from the
semiconductor industry for more than 30 years. The
venerable MC14066 is an example of a still popular device
in the “4000” family. The MC14066 exhibits 500 Ω series
resistance (R
ON
).Switches of this style are still popular and
useful for controlling high impedance audio and similar
signals. In the past few years the industry has begun to offer
switches in the 10−20 Ω R
ON
range, and in 2003 switches
with R
ON
values of ≤ 1.0 Ω became available. The
NLAS4684 and NLAS4685 are two examples of switches
with R
ON
≤ 1.0 Ω, capable of switching an audio output
signal that is directly connected to a speaker.
In the cell phone marketplace, it is virtually mandatory to
include a jack for external microphone/speaker connections.
Many places in the world do not permit operation of a cell
phone when driving, except in a hands−free mode. It is very
common to see people walking and talking into a lapel
mounted microphone, with the earpiece placed in the users
ear. It is potentially dangerous to have the ring tones that are
generated at high levels directed into the earpiece. The use
of a mechanical switch to disable the speaker and direct the
signal to the earpiece is therefore ruled out. This design gets
further strained, when one adds all the new features
consumers are demanding. MP3 stereo audio playback and
FM−stereo radio are two such desirable features. The
commuter riding the metro or suburban train would want to
have MP3 to have music playback, of his choice, and FM
radio for news. With a rechargeable battery, and a high
power microprocessor on board, all that is needed to add
MP3/FM functionality is an external memory card and
software, and possibly an FM tuner chip. For MP3/FM the
user will likely demand stereo playback. Both MP3 and
commercial FM radio require a stereo audio amplifier. If the
designer had to switch the routing between the stereo
earpiece and the internal speaker, there are two choices:
a. two stereo audio output amplifiers
b. one stereo amplifier and an electronic switch
At first blush, it might appear that two amplifiers might be
the desirable way to go. with The advent of some new
switching technology will show this is not the case. If the
designer were to use two stereo amplifiers, he needs to steer
the signals at low level to the appropriate amplifier. He also
needs to Enable/Disable the appropriate amplifier, to reduce
the standby current. ON Semiconductor supplies a small
stereo amplifier capable of > 100 mW output, that can be
used for this purpose. The NCP2809 is delivered in a
Micro−10 surface mount package that is 3x5 mm, occupying
15 mm
2
of board space. A second amplifier would occupy
an additional 15 mm
2
of space. The signals need to be routed
to the appropriate amplifier, and a pair of NLAS4599
devices in 2x2 mm packages could be used. In addition, an
inverter is needed to permit the Enable/Disable function to
be done with one line from a micro−controller.
On Semiconductor offers two new low impedance
switches that exhibit less than 1.0 Ω R
ON
. The NLAS4684
is a dual analog switch with a typical R
ON
of 0.5 Ω. It is
available as a flip−chip or bumped die device. In this form
it occupies only 3 mm
2
of board space.
Circuitry: the following diagram shows a cell phone based
on two stereo amplifiers and no switch low R
ON
switch
versus one stereo amplifier and the NLAS4684.
APPLICATION NOTE
http://onsemi.com
