Datasheet

SSM2211 Data Sheet
Rev. G | Page 16 of 24
APPLICATIONS INFORMATION
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 operational amplifier. The gain from the audio input
to the speaker is
I
F
V
R
R
A  2
(1)
The 2× factor results from Pin 8 having an opposite polarity of
Pin 5, providing twice the voltage swing to the speaker from the
bridged-output (BTL) configuration.
C
S
is a supply bypass capacitor used to provide power supply
filtering. Pin 2 is connected to Pin 3 to provide an offset voltage
for single-supply use, with C
B
providing a low ac impedance to
ground to enhance power-supply rejection. Because Pin 4 is a
virtual ac ground, the input impedance is equal to R
I
. C
C
is the
input coupling capacitor, which also creates a high-pass filter
with a corner frequency of
C
I
HP
CR
f


2
1
(2)
Because the SSM2211 has an excellent phase margin, a feedback
capacitor in parallel with R
F
to band limit the amplifier is not
required, as it is in some competitor 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 peak voltage of the signal and the
resistance of the load as
L
PK
L
R
V
P


2
2
(3)
By driving a load from a BTL configuration, the voltage swing
across the load doubles. Therefore, an advantage in using a BTL
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 power translates to a 12 dB increase in sound
pressure level from the speaker.
Driving a speaker differentially from a BTL 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 must be large enough to prevent low
frequency roll-off. The corner frequency is given by
C
L
CR
f



π2
1
dB3
(4)
where
R
L
is the speaker resistance and C
C
is the coupling
capacitance.
For an 8 Ω speaker and a corner frequency of 20 Hz, a 1000 μF
capacitor is needed, which is physically large and costly. By
connecting a speaker in a BTL 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 speaker 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 7 shows a comparison of the relative loudness of
different sounds.
Table 7. Typical Sound Pressure Levels (SPLs)
Source of Sound SPL (dB)
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
Consequently, Table 7 demonstrates that 1 W of power into a
speaker can produce quite a bit of acoustic energy.
SSM2211
A
UDIO
INPUT
SPEAKER
8V
R
F
C
S
5V
2
7
1
8
5
6
4
3
–
+
C
C
R
I
C
B
–
+
00358-042