SSM2250

Rev. A | Page 10 of 12

SUPPLY VOLTAGE (V)

1.6

0.8

1.0

1.2

1.4

0.6

0.4

0.2

0

1.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

Use Figure 18 to find the minimum supply voltage needed to

achieve a specified maximum undistorted output power.

The output power in SE mode is exactly one-fourth the

equivalent output power in BTL mode. This is because twice the

voltage swing across the two BTL outputs results in 4 × the

power delivered to the load. Figure 19 shows the maximum

output power in SE mode vs. supply voltage for various

headphone loads.

SUPPLY VOLTAGE (V)

100

25

50

75

0

1.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

Example

An application requires only 500 mW to be output in BTL

mode into an 8 Ω speaker. By inspection, the minimum supply

voltage required is 3.3 V.

Speaker Efficiency and Loudness

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

speaker is a function of the efficiency of the speaker. The

efficiency of a speaker is typically rated at 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 one meter at 1 W of power. Table 5 shows a com-

parison of the relative loudness of different sounds.

Table 5. 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.

Shutdown Feature

The SSM2250 can be put into a low power consumption

mode, the SSM2250 has low supply current of 60 μA.

Pin 3 should be connected to ground for normal operation.

all outputs into a high impedance state, effectively muting the

SSM2250. A pull-up or pull-down resistor is not required.

Pin 3 should never be left floating, as this could produce unpre-

dictable results.

PC99-Compliant Computer Audio Reference Design

The schematic shown in Figure 20 is a reference design for a

complete audio system in a computer. The design is compliant

with the PC99 standard for computer audio.

The AD1881A is an AC’97, version 2.1, audio codec, available

from Analog Devices. The stereo output from the AD1881A is

coupled into the SSM2250, which is used to drive a mono

internal speaker and stereo headphones. The internal speaker

switching is controlled by the SSM2250 through the normaliz-

ing pin on the headphone jack. The AD1881A controls the

shutdown pin on the SSM2250, and is activated through the

power management software drivers installed on the computer.

For more information, refer to the AD1881A data sheet.