SSM2250

–8–

REV. 0

Continuing from the previous example, the

θ

14-lead TSSOP package on a 4-layer board is 140

°C/W. To ensure

the SSM2250 die junction temperature stays below 150

°C, the

maximum ambient temperature can be solved using Equation 8.

T

150 C

P

AMB, MAX

JA

DISS, MAX

=+

° −

×

θ

=+

° −

°

×

()

150

140

0 633

C

C/W

W

.

(8)

=+ °

61 C

So the maximum ambient temperature must remain below 61

°C

to protect the device against thermal damage.

Another method for finding the maximum allowable ambient

temperature is to use the power derating curve in Figure 15.

The y-axis corresponds to the expected maximum power dissi-

pation, and the x-axis is the corresponding maximum ambient

temperature. Either method will return the same answer.

AMBIENT TEMPERATURE – C

1.0

0.8

0

0

100

25

50

75

0.6

0.4

0.2

14-LEAD TSSOP

10-LEAD MSOP

FREE AIR

NO HEAT SINK

Figure 15. Maximum Power Dissipation vs. Ambient

Temperature

Maximum Output Power

The maximum amount of power that can be delivered to a

speaker is a function of the supply voltage and the resistance of

the speaker. Figure 15 shows the maximum BTL output power

possible from the SSM2250. Maximum output power is defined

as the point at which the output has greater than 1% distortion.

SUPPLY VOLTAGE – V

1.5

5.0

2.0

2.5

3.0

3.5

4.0

4.5

1.6

0

1.4

0.8

0.6

0.4

0.2

1.2

1.0

To find the minimum supply voltage needed to achieve a speci-

fied maximum undistorted output power, simply use Figure 16.

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 17 shows the maximum output power in SE mode vs.

supply voltage for various headphone loads.

SUPPLY VOLTAGE – V

1.5

5.0

2.0

2.5

3.0

3.5

4.0

4.5

100

0

50

25

75

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 I shows a comparison of the relative

loudness of different sounds.

Table I. 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 be easily 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 shut-

SSM2250 has low supply current of 60

µA.

Pin 3 should be connected to ground for normal operation. Con-

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 unpredictable results.

PC 99 Compliant Computer Audio Reference Design

The schematic shown in Figure 18 is a reference design for a

complete audio system in a computer. The design is compliant

with the PC 99 standard for computer audio.