SSM2211_02 데이터 시트보기 (PDF) - Analog Devices
부품명
상세내역
제조사
SSM2211_02 Datasheet PDF : 16 Pages
| |||
SSM2211
TYPICAL APPLICATION
RF
5V
AUDIO
INPUT
CC RI
CS
6
4–
5
SSM2211
3+
8
1
7
2
CB
–
SPEAKER
8V
+
Figure 2. Typical Configuration
Figure 2 shows how the SSM2211 would be 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 2ϫ factor comes from the fact that Pin 8 is opposite polar-
ity 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 filter-
ing. 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:
1
fHP = 2 pRI ¥ CC
(2)
Because the SSM2211 has an excellent phase margin, a feed-
back 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 ex-
pressed in terms of the signal’s peak voltage and the resistance
of the load:
2
PL
=
VPK
2 RL
(3)
By driving a load from a bridged output configuration, the volt-
age swing across the load doubles. 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 W 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 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 half 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:
1
f-3dB = 2 p RLCC
(4)
Where RL is the speaker resistance and CC is the coupling
capacitance
For an 8 W speaker and a corner frequency of 20 Hz, a 1000 mF
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 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 W
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:
ÊËÁh
=
PL
PSY
ˆ
¯˜
An amplifier with a higher efficiency has less internal power dis-
sipation, which results in a lower die-to-case junction tempera-
REV. B
–9–
Share Link: 