SSM2211

Rev. D | Page 15 of 24

TYPICAL APPLICATIONS

SSM2211

AUDIO

INPUT

SPEAKER

8V

5V

2

7

1

8

5

6

4

3

–

+

–

+

Figure 42. Typical Configuration

Figure 42 shows how the SSM2211 is connected in a typical

application. The SSM2211 can be configured for gain much like

a standard operational amplifier. The gain from the audio input

to the speaker is

I

F

V

R

R

A

×

= 2

(1)

The 2× factor comes from the fact that Pin 8 has the opposite

polarity of Pin 5, providing twice the voltage swing to the

speaker from the bridged-output configuration.

filtering. Pin 2 is connected to Pin 3 to provide an offset voltage

ground to help power supply rejection. Because Pin 4 is a

input coupling capacitor, which also creates a high-pass filter

with a corner frequency of

C

I

HP

C

R

f

×

π

=

2

1

(2)

Because the SSM2211 has an excellent phase margin, a feedback

required, as it is in some competitor products.

BRIDGED OUTPUT VS. SINGLE-ENDED OUTPUT

CONFIGURATIONS

The power delivered to a load with a sinusoidal signal can be

expressed in terms of the peak voltage of the signal and the

resistance of the load as

L

PK

L

R

V

P

×

=

2

2

(3)

By driving a load from a bridged-output configuration, the

voltage swing across the load doubles. Therefore, 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.

Driving a speaker differentially from a bridged output offers

another advantage in that it eliminates the need for an output

coupling capacitor to the load. In a single-supply application,

the quiescent voltage at the output is half of the supply voltage.

If a speaker is connected in a single-ended configuration, a

coupling capacitor is needed to prevent dc current from flowing

through the speaker. This capacitor also needs to be large

enough to prevent low frequency roll-off. The corner frequency

is given by

C

L

C

R

f

×

=

−

π

2

1

dB

3

(4)

capacitance.

For an 8 Ω speaker and a corner frequency of 20 Hz, a 1000 μF

capacitor would be needed, which is physically large and costly.

By connecting a speaker in a bridged-output configuration, the

quiescent differential voltage across the speaker becomes nearly

zero, eliminating the need for the coupling capacitor.

SPEAKER EFFICIENCY AND LOUDNESS

The effective loudness of 1 W of power delivered into an 8 Ω

speaker is a function of speaker efficiency. The efficiency is

typically rated as the sound pressure level (SPL) at 1 meter in

front of the speaker with 1 W of power applied to the speaker.

Most speakers are between 85 dB and 95 dB SPL at 1 meter at

1 W. Table 7 shows a comparison of the relative loudness of

different sounds.

Table 7. Typical Sound Pressure Levels

Source of Sound

dB SPL

Threshold of Pain

120

Heavy Street Traffic

95

Cabin of Jet Aircraft

80

Average Conversation

65

Average Home at Night

50

Quiet Recording Studio

30

Threshold of Hearing

0

It can easily be seen that 1 W of power into a speaker can

produce quite a bit of acoustic energy.