SSM2211x Analog Devices, SSM2211x Datasheet - Page 9

• Manufacturer Part Number: SSM2211x
• Description: Low Distortion 1.5 Watt Audio Power Amplifier
• Manufacturer: Analog Devices

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Driving a speaker differentially from a bridged output offers an-
other advantage in that it eliminates the need for an output cou-
pling capacitor to the load. In a single supply application, the
quiescent voltage at the output is 1/2 of the supply voltage. If a
speaker were connected in a single ended configuration, a cou-
pling capacitor would be needed to prevent dc current from
flowing through the speaker. This capacitor would also need to
be large enough to prevent low frequency roll-off. The corner
frequency is given by:
Where
For an 8
capacitor would be needed, which is quite physically large and
costly. By connecting a speaker in a bridged output configura-
tion, the quiescent differential voltage across the speaker be-
comes 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
speaker is a function of the efficiency of the speaker. The effi-
ciency of a speaker 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 I shows a comparison of
the relative loudness of different sounds.
Source of Sound
Threshold of Pain
Heavy Street Traffic
Cabin of Jet Aircraft
Average Conversation
Average Home at Night
Quiet Recording Studio
Threshold of Hearing
It can easily be seen that 1 W of power into a speaker can pro-
duce quite a bit of acoustic energy.
Power Dissipation
Another important advantage in using a bridged output configu-
ration is the fact that bridged output amplifiers are more effi-
cient than single ended amplifiers in delivering power to a load.
Efficiency is defined as the ratio of power from the power supply
to the power delivered to the load
with a higher efficiency has less internal power dissipation,
which results in a lower die-to-case junction temperature, as
compared to an amplifier that is less efficient. This is important
when considering the amplifier device’s maximum power dissi-
pation rating versus ambient temperature. An internal power
dissipation versus output power equation can be derived to fully
understand this.
R
C
L
C
speaker and a corner frequency of 20 Hz, a 1000 F
Table I. Typical Sound Pressure Levels
is the speaker resistance and,
is the coupling capacitance
f
3
dB
2
R C
1
L
C
dB SPL
120
95
80
65
50
30
0
P
P
SY
L
. An amplifier
(4)
–9–
The internal power dissipation of the amplifier is the internal
voltage drop multiplied by the average value of the supply cur-
rent. An easier way to find internal power dissipation is to take
the difference between the power delivered by the supply voltage
source and the power delivered into the load. The waveform of
the supply current for a bridged output amplifier is shown in
Figure 40.
Figure 40. Bridged Amplifier Output Voltage and Supply
Current vs. Time
By integrating the supply current over a period T, then dividing
the result by T, I
peak output voltage and load resistance:
therefore power delivered by the supply, neglecting the bias cur-
rent for the device is,
Now, the power dissipated by the amplifier internally is simply
the difference between Equation 6 and Equation 3. The equa-
tion for internal power dissipated, P
power delivered to the load and load resistance is:
The graph of this equation is shown in Figure 41.
V
V
I
I
DD, PEAK
SY
OUT
PEAK
DD,AVG
P
DISS
I
DD AVG
P
can be found. Expressed in terms of
SY
T
2 2
T
2
V V
R
DD PEAK
V
2
L
DD
V
R
L
PEAK
R
DISS
L
P
, expressed in terms of
L
TIME
TIME
P
SSM2211
L
I
DD, AVG
(5)
(6)
(7)