SSM2250

Rev. A | Page 7 of 12

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 reduc-

tion 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 a 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 external compensation capacitors. The SSM2250 can be

configured for gains of up to 40 dB.

TYPICAL APPLICATION

In SE mode, the device operates similarly 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 prevents 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 in and right in audio

signals. The A2 amplifier is activated and configured with a

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

()

1

2

2

R

R

V

V

V

RIGHT

LEFT

SPEAKER

×

×

+

=

(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 0. 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 17 should be observed for

proper operation at higher ambient temperatures. For a

standard 14-lead TSSOP package, typical junction-to-ambient

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 the BTL

mode and the SE mode. Logic low to Pin 4 activates BTL mode,

while logic high activates SE mode. The configuration shown in

Figure 14 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.

In Figure 14, Pin 4 of the SSM2250 is connected to the nor-

malizing pin for the right channel output. This pin, located in

the headphone jack, hits 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 discon-

nects 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

connects 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 results

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 about 80 Ω.