SSM2211
For an application where R1 = 10 kΩ and C
C
= 0.22
µF,
the
midrail bypass capacitor, C
B
, should be at least 0.1
µF
to mini-
mize start-up popping noise.
SSM2211 Amplifier Design Example
Selecting
C
B
to be 2.2
µF
for a practical value of capacitor will
minimize start-up popping noise.
To summarize the final design:
V
DD
R1
R
F
C
C
C
B
Max. T
A
5V
20 kΩ
28 kΩ
2.2
µF
2.2
µF
+85°C
Given:
Maximum Output Power
Input Impedance
Load Impedance
Input Level
Bandwidth
1W
20 kΩ
8
Ω
1 V rms
20 Hz – 20 kHz
±
0.25 dB
Single Ended Applications
The configuration shown in Figure 39 will be used. The first
thing to determine is the minimum supply rail necessary to ob-
tain the specified maximum output power. From Figure 43, for
1 W of output power into an 8
Ω
load, the supply voltage must
be at least 4.6 V. A supply rail of 5 V can be easily obtained
from a voltage reference. The extra supply voltage will also al-
low the SSM2211 to reproduce peaks in excess of 1 W without
clipping the signal. With V
DD
= 5 V and R
L
= 8
Ω,
Equation 9
shows that the maximum power dissipation for the SSM2211 is
633 mW. From the power derating curve in Figure 28, the am-
bient temperature must be less than +85°C.
The required gain of the amplifier can be determined from
Equation 17:
A
V
=
P
L
R
L
V
IN
,
rms
=
2
.
8
(17)
There are applications where driving a speaker differentially is
not practical. An example would be a pair of stereo speakers
where the minus terminal of both speakers is connected to
ground. Figure 45 shows how this can be accomplished.
10k
+5V
6
4
5
10k
AUDIO
INPUT
0.47 F
SSM2211
3
7
2
470 F
250mW
SPEAKER
(8 )
1 8
0.1 F
Figure 45. A Single Ended Output Application
R
F
A
V
=
, or
R
F
=
1
.
4
×
R
1. Since the de-
R
1
2
sired input impedance is 20 kΩ, R1 = 20 kΩ and R2 = 28 kΩ.
From Equation 1,
The final design step is to select the input capacitor. Because add-
ing an input capacitor, C
C
, high pass filter, the corner frequency
needs to be far enough away for the design to meet the bandwidth
criteria. For a 1st order filter to achieve a passband response
within 0.25 dB, the corner frequency should be at least 4.14 times
away from the passband frequency. So, (4.14 f
HP
) < 20 Hz.
Using Equation 2, the minimum size of input capacitor can be
found:
C
C
>
1
20
Hz
2
π
20
k
Ω
4
.
14
It is not necessary to connect a dummy load to the unused output
to help stabilize the output. The 470
µF
coupling capacitor cre-
ates a high pass frequency cutoff as given in Equation 4 of 42 Hz,
which is acceptable for most computer speaker applications.
The overall gain for a single ended output configuration is
A
V
= R
F
/R1, which for this example is equal to 1.
Driving Two Speakers Single Endedly
It is possible to drive two speakers single endedly with both out-
puts of the SSM2211.
20k
(
)
+5V
(18)
20k
AUDIO
INPUT
1 F
3
7
2
6
4
5
470 F
LEFT
SPEAKER
(8 )
So
C
C
> 1.65
µF.
Using a 2.2
µF
is a practical choice for C
C
.
The gain-bandwidth product for each internal amplifier in the
SSM2211 is 4 MHz. Because 4 MHz is much greater than
4.14 20 kHz, the design will meet the upper frequency band-
width criteria. The SSM2211 could also be configured for higher
differential gains without running into bandwidth limitations.
Equation 16 shows an appropriate value for C
B
to reduce start-
up popping noise:
SSM2211
1 8
470 F
0.1 F
RIGHT
SPEAKER
(8 )
Figure 46. SSM2211 Used as a Dual Speaker Amplifier
C
B
(
2
.
2
µ
F
)(
20
k
Ω
)
=
1
.
76
µ
F
>
25
k
Ω
(19)
Each speaker is driven by a single ended output. The trade-off
is that only 250 mW sustained power can be put into each
speaker. Also, a coupling capacitor must be connected in series
with each of the speakers to prevent large DC currents from
flowing through the 8
Ω
speakers. These coupling capacitors
–12–
REV. 0
AD [ ANALOG DEVICES ]
