SSM2250

–7–

REV. 0

The output coupling capacitor creates a high-pass filter with a

cutoff frequency of:

f

RC

dB

LC

−

=

3

1

2

π

(2)

Although a majority of headphones have around 80

Ω of resistance,

this resistance can vary between models and manufacturers. Head-

phone resistances are commonly between 32

Ω to 600 Ω. Using a

220

µF capacitor as shown in Figure 12, the worst-case –3 dB corner

frequency would be 22 Hz, with a 32

Ω headphone load. Smaller

output capacitors could be used at the expense of low frequency

response to the headphones.

An input coupling capacitor should be used to remove dc bias

from the inputs to the SSM2250. Again, the input coupling

capacitor in combination with the input resistor will create a

high-pass filter with a corner frequency of:

f

RC

dB

−

=

3

1

21 1

π

(3)

Using the values shown in Figure 2, where R1 = 20 k

Ω and

C1 = 1

µF, will create a corner frequency of 8 Hz. This is

acceptable, as the PC 99 audio requirement specifies the com-

puter audio system bandwidth to be 20 Hz to 20 kHz.

Pin 10 on the SSM2250 provides the proper bias voltage for the

amplifiers. A 0.1

µF capacitor should be connected here to

reduce sensitivity to noise on the power supply. A larger capaci-

tor can be used should more rejection from power supply noise

be required.

The SSM2250 has excellent phase margin and is stable even

under heavy loading. Therefore, a feedback capacitor in parallel

with R2 is not required, as it is in some competitors’ products.

Power Dissipation

An important advantage in using a bridged output configuration

is the fact that bridged output amplifiers are more efficient than

single-ended amplifiers in delivering power to a load.

OUTPUT POWER – W

1.5

0

0

0.25

1.25

1.0

0.75

0.5

0.25

0.5

0.75

1.0

1.25

1.5

Figure 13. Power Dissipation vs. Output Power in BTL Mode

P

V

R

DISS MAX

DD

L

,

=

2

2

2

π

(4)

Using Equation 4 and the power derating curve in Figure 15,

the maximum ambient temperature can be easily found. This

ensures that the SSM2250 will not exceed its maximum junc-

tion temperature of 150

°C.

The power dissipation for a single-ended output application where

an output coupling capacitor is used is shown in Figure 14.

OUTPUT POWER – W

0.35

0.3

0

0

0.4

0.1

0.2

0.3

0.2

0.15

0.1

0.05

0.25

Figure 14. Power Dissipation vs. Single-Ended Output

The maximum power dissipation for a single-ended output is:

P

V

R

DISS MAX

DD

L

,

=

2

2

2

π

(5)

Because the SSM2250 is designed to drive two single-ended

loads simultaneously, the worst-case maximum power dissipation

in SE Mode is twice the value of Equation 5.

A thorough mathematical explanation behind Equation 4 and

Equation 5 is given in the SSM2211 data sheet, which can be

downloaded at http://www.analog.com.

Example: Given worst-case stereo headphone loads of 32

Ω,

the maximum power dissipation of the SSM2250 in SE Mode

with a 5 V supply would be:

P

V

mW

DISS

MAX

,

=

5

232

79

2

2

πΩ

(6)

With an 8

Ω internal speaker attached, the maximum power

dissipation in BTL mode is (from Equation 4):

P

V

mW

DISS

MAX

,

=

×

()

=

25

8

633

2

2

πΩ

(7)

It can be easily seen that power dissipation from BTL Mode

operation is of greater concern than SE Mode.

Solving for Maximum Ambient Temperature

To protect the SSM2250 against thermal damage, the junction

temperature of the die should not exceed 150

°C. The maximum

allowable ambient temperature of the application can be easily

found by solving for the expected maximum power dissipation in

Equation 4 and Equation 5, and using Equation 8.