SSM2250

–6–

REV. 0

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

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:

VV

V

R

R

SPEAKER

LEFT

RIGHT

=+

1

(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.

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 (

θ

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 k

Ω 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 k

Ω 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 k

Ω 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

1 F

NC

NC

R2

20k

SHUTDOWN

10 F

NC

NC

+

–

BTL

OUT

5V

220 F

+

1k

NC

1k

RIGHT IN

100k

5V

220 F

+

R1

20k

1

2

3

4

5

6

7

14

13

12

11

10

9

8

R1

20k

1 F

R2

20k

NC = NO CONNECT

SSM2250

Figure 12. Typical Application