SSM2211
SSM2211 PRODUCT OVERVIEW
TYPICAL APPLICATION
R
F
+5V
The SSM2211 is a low distortion speaker amplifier that can run
from a 1.7 V to 5.5 V supply. It consists of a rail-to-rail input
and a differential output that can be driven within 400 mV of
either supply rail while supplying a sustained output current of
350 mA. The SSM2211 is unity-gain stable, requiring no exter-
nal compensation capacitors, and can be configured for gains of
up to 40 dB. Figure 38 shows the simplified schematic.
20k
V
DD
6
C
C
AUDIO
INPUT
R
I
6
4
C
S
5
SSM2211
3
7
2
C
B
1 8
SPEAKER
8
V
IN
20k
4
A1
SSM2211
V
O1
Figure 39. A Typical Configuration
5
3
50k
50k
50k
Figure 39 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:
V
O2
2
A2
8
50k
A
V
=
2
×
0.1 F
7
BIAS
CONTROL
1
SHUTDOWN
R
F
R
I
(1)
The 2 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.
C
S
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 C
B
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 R
I
. C
C
is the input
coupling capacitor which also creates a high-pass filter with a
corner frequency of:
Figure 38. Simplified Schematic
Pin 4 and Pin 3 are the inverting and noninverting terminals to A1.
An offset voltage is provided at Pin 2, which should be connected
to Pin 3 for use in single supply applications. The output of A1
appears at Pin 5. A second op amp, A2, is configured with a fixed
gain of A
V
= –1 and produces an inverted replica of Pin 5 at Pin 8.
The SSM2211 outputs at Pins 5 and 8 produce a bridged configu-
ration output to which a speaker can be connected. This bridge
configuration offers the advantage of a more efficient power trans-
fer from the input to the speaker. Because both outputs are sym-
metric, the dc bias at Pins 5 and 8 are exactly equal, resulting in
zero dc differential voltage across the outputs. This eliminates the
need for a coupling capacitor at the output.
The SSM2211 can achieve 1 W continuous output into 8
Ω,
even
at ambient temperatures up to +85°C. This is due to a propri-
etary SOIC package from Analog Devices that makes use of an
internal structure called a Thermal Coastline. The Thermal
Coastline provides a more efficient heat dissipation from the die
than in standard SOIC packages. This increase in heat dissipation
allows the device to operate in higher ambient temperatures or at
higher continuous output currents without overheating the die.
For a standard SOIC package, typical junction to ambient tem-
perature thermal resistance (
JA
) is +158°C/W. In a Thermal
Coastline SOIC package,
JA
is +98°C/W. Simply put, a die in a
Thermal Coastline package will not get as hot as a die in a stan-
dard SOIC package at the same current output.
Because of the large amounts of power dissipated in a speaker
amplifier, competitor’s parts operating from a 5 V supply can
only drive 1 W into 8
Ω
in ambient temperatures less than
+44°C, or +111°F. With the Thermal Coastline SOIC package,
the SSM2211 can drive an 8
Ω
speaker with 1 W from a 5 V
supply with ambient temperatures as high as +85°C (+185°F),
without a heat sink or forced air flow.
f
HP
=
1
2
π
R
I
×
C
C
(2)
Because the SSM2211 has an excellent phase margin, a feed-
back capacitor in parallel with R
F
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:
V
P
L
=
PK
2
R
L
2
(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
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 translates to a 12 dB increase in
sound pressure level from the speaker.
–8–
REV. 0
AD [ ANALOG DEVICES ]
