SSM2211_04 데이터 시트보기 (PDF) - Analog Devices
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SSM2211_04 Datasheet PDF : 20 Pages
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SSM2211
TYPICAL APPLICATION
RF
5V
AUDIO
INPUT
CC RI
CS
6
4–
5
SSM2211
3+
8
1
7
2
CB
–
SPEAKER
8V
+
Figure 42. Typical Configuration
Figure 42 shows how the SSM2211 is connected in a typical
application. The SSM2211 can be configured for gain much like
a standard op amp. The gain from the audio input to the
speaker is
AV
= 2×
RF
RI
(1)
The 2 × factor comes from the fact that Pin 8 has the opposite
polarity from Pin 5, providing twice the voltage swing to the
speaker from the bridged output configuration.
CS is a supply bypass capacitor to provide power supply
filtering. Pin 2 is connected to Pin 3 to provide an offset voltage
for single-supply use, with CB providing a low ac impedance to
ground to help power-supply rejection. Because Pin 4 is a
virtual ac ground, the input impedance is equal to RI. CC is the
input coupling capacitor which also creates a high-pass filter
with a corner frequency of
f HP
=
1
2πRI × CC
(2)
Because the SSM2211 has an excellent phase margin, a feedback
capacitor in parallel with RF to band limit the amplifier is not
required, as it is in some competitor’s products.
BRIDGED OUTPUT VS. SINGLE-ENDED OUTPUT
CONFIGURATIONS
The power delivered to a load with a sinusoidal signal can be
expressed in terms of the signal’s peak voltage and the resistance
of the load as
PL
=
VPK 2
2RL
(3)
By driving a load from a bridged output configuration, the
voltage swing across the load doubles. Thus, an advantage in
using a bridged output configuration becomes apparent from
Equation 3, as doubling the peak voltage results in four times
the power delivered to the load. In a typical application
operating from a 5 V supply, the maximum power that can be
delivered by the SSM2211 to an 8 Ω speaker in a single-ended
configuration is 250 mW. By driving this speaker with a bridged
output, 1 W of power can be delivered. This translates to a 12
dB increase in sound-pressure level from the speaker.
Driving a speaker differentially from a bridged output offers
another advantage in that it eliminates the need for an output
coupling capacitor to the load. In a single-supply application,
the quiescent voltage at the output is half of the supply voltage.
If a speaker is connected in a single-ended configuration, a
coupling capacitor is needed to prevent dc current from flowing
through the speaker. This capacitor also needs to be large
enough to prevent low frequency roll-off. The corner frequency
is given by
f − 3 dB
=
1
2πRLCC
(4)
where RL is the speaker resistance and CC is the coupling
capacitance.
For an 8 Ω speaker and a corner frequency of 20 Hz, a 1000 µF
capacitor would be needed, which is physically large and costly.
By connecting a speaker in a bridged output configuration, the
quiescent differential voltage across the speaker becomes nearly
zero, eliminating the need for the coupling capacitor.
SPEAKER EFFICIENCY AND LOUDNESS
The effective loudness of 1 W of power delivered into an 8 Ω
speaker is a function of the speaker’s efficiency. The efficiency is
typically rated as the sound pressure level (SPL) at 1 meter in
front of the speaker with 1 W of power applied to the speaker.
Most speakers are between 85 dB and 95 dB SPL at 1 meter at
1 W. Table 6 shows a comparison of the relative loudness of
different sounds.
Table 6. Typical Sound Pressure Levels
Source of Sound
dB SPL
Threshold of pain
120
Heavy street traffic
95
Cabin of jet aircraft
80
Average conversation
65
Average home at night
50
Quiet recording studio
30
Threshold of hearing
0
It can easily be seen that 1 W of power into a speaker can
produce quite a bit of acoustic energy.
Rev. C | Page 14 of 20
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