SSM2250RM View Datasheet(PDF) - Analog Devices
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SSM2250RM Datasheet PDF : 10 Pages
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SSM2250
PRODUCT OVERVIEW
The SSM2250 is a low distortion power amplifier that can drive a
set of stereo headphones or a single 8 Ω loudspeaker. It contains
three rail-to-rail output op amps, click and pop reduction biasing,
and all necessary switching circuitry. In SE (Single-Ended) Mode,
the device automatically mutes the internal 8 Ω speaker. In BTL
(Bridge-Tied Load) Mode, the internal speaker is activated.
The SSM2250 can operate from a 2.7 V to 5.5 V single supply.
The rail-to-rail outputs can be driven to within 400 mV of either
supply rail while supplying a sustained output current of 350 mA
into 8 Ω. The device is unity-gain stable and requires no exter-
nal compensation capacitors. The SSM2250 can be configured
for gains of up to 40␣ dB.
TYPICAL APPLICATION
In SE Mode, the device operates similar to a high current output,
dual op amp. A1 and A3 are independent amplifiers with a gain of
–R2/R1. The outputs of A1 and A3 are used to drive the external
headphones plugged into the headphone jack. Amplifier A2 is shut
down to a high output impedance state. This prevents current from
flowing through the 8 Ω internal speaker, thereby muting it.
Although the gains of A1 and A3 can be set independently, it is
recommended that the feedback and feedforward resistor around
both amplifiers be equal. This will prevent one channel from
becoming louder than the other.
In BTL mode, the current into the Right In pin is directed to
the input of A1. This effectively sums the Left and Right In
audio signals. The A2 amplifier is activated and configured with
a fixed gain of AV␣ =␣ –1. This produces a balanced output con-
figuration that drives the internal speaker. Because the BTL
output voltages swing opposite to each other, the gain to the
speaker in BTL mode is twice the gain of SE mode. The voltage
across the internal speaker can be written:
( ) VSPEAKER = VLEFT + VRIGHT
× 2 × R2
R1
(1)
The bridged output configuration offers the advantage of a more
efficient power transfer from the input to the speaker. Because
both outputs are symmetric, the dc voltage bias across the 8 Ω
internal speaker is zero. This eliminates the need for a coupling
capacitor at the output. In BTL mode, the A3 amplifier is shut
down to conserve power.
R2
20k⍀
In BTL Mode, the SSM2250 can achieve 1␣ W continuous output
into 8␣ Ω at ambient temperatures up to 40°C. The power derating
curve shown in Figure 15 should be observed for proper operation at
higher ambient temperatures. For a standard 14-lead TSSOP pack-
age, typical junction-to-ambient temperature thermal resistance (θJA)
is 180°C/W on a 2-layer board, and 140°C/W on a 4-layer board.
Internal Speaker/External Headphones Automatic Switching
Pin 4 on the SSM2250 controls the switching between BTL and
SE Modes. Logic low to Pin 4 activates BTL Mode, while logic
high activates SE Mode. The configuration shown in Figure 12
provides the appropriate logic voltages to Pin 4, muting the
internal speaker when headphones are plugged into the jack.
A stereo headphone jack with a normalizing pin is required for
the application. With no plug inserted, a mechanical spring
connects the normalizing pin to the output pin in the jack.
Once a plug is inserted, this connection is broken.
Referring to Figure 12, Pin 4 of the SSM2250 is connected to the
normalizing pin for the right channel output. This is the pin in
the headphone jack that will hit the ring on the headphone plug.
A 100 kΩ pull-up resistor to 5 V is also connected at this point.
With a headphone plug inserted, the normalizing pin disconnects
from the output pin, and Pin 4 is pulled up to 5 V, activating SE
Mode on the SSM2250. This mutes the internal speaker while
driving the stereo headphones.
Once the headphone plug is removed, the normalizing pin con-
nects to the output pin. This drives the voltage at Pin 4 to 50 mV,
as this point is pulled low by the 1 kΩ resistor now connected to
the node. The SSM2250 goes into BTL mode, deactivating the
right SE amplifier to prevent the occurrence of any false mode
switching.
It is important to connect Pin 4 and the 100 kΩ pull up resistor
to the normalizing pin for the right output in the headphone
jack. Connecting them to the left output normalizing pin will
result in improper operation from the device. The normalizing
pin to the left output in the headphone jack should be left open.
Coupling Capacitors
Output coupling capacitors are not required to drive the internal
speaker from the BTL outputs. However, coupling capacitors are
required between the amplifier’s SE outputs and the headphone
jack to drive external headphones. This prevents dc current from
flowing through the headphone speakers, whose resistances are
typically on the order of 80 Ω.
LEFT IN
SHUTDOWN
R1
20k⍀
1F
RIGHT IN
R1
20k⍀
1F
NC 1
2
14 NC
13
220F
+
3
12
SSM2250
– 1k⍀ NC
5V
BTL
OUT
4
11 10F
+
5
6
NC 7
10
9
8 NC
220F
+
1k⍀
5V
R2
20k⍀
NC = NO CONNECT
100k⍀
Figure 12. Typical Application
–6–
REV. 0
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