SSM2211P 查看數據表（PDF） - Analog Devices
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SSM2211P Datasheet PDF : 16 Pages
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SSM2211
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:
f−3dB
=
2π
1
RLCC
(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 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 Ω
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.
Table I. 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 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
η
=
PL
PSY
.
An
amplifier
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.
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.
VOUT
VPEAK
T
ISY
TIME
IDD, PEAK
IDD, AVG
T
TIME
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, IDD,AVG can be found. Expressed in terms of
peak output voltage and load resistance:
IDD,
AVG
=
2VPEAK
π RL
(5)
therefore power delivered by the supply, neglecting the bias cur-
rent for the device is,
PSY
=
2 VDDVPEAK
π RL
(6)
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, PDISS, expressed in terms of
power delivered to the load and load resistance is:
PDISS = 2 2 ×VDD PL − PL
(7)
π RL
The graph of this equation is shown in Figure 41.
REV. 0
–9–
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