SSM2250

Rev. A | Page 9 of 12

OUTPUT POWER (W)

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

0

0.3

0.2

0.1

0.4

The maximum power dissipation for a single-ended output is

L

DD

MAX

DISS

R

V

P

2

2

,

2π

=

(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 provided in the SSM2211 data sheet.

Example

Given worst-case stereo headphone loads of 32 Ω, the maxi-

mum power dissipation of the SSM2250 in SE mode with a 5 V

supply is

()

mW

V

P

MAX

DISS

79

32

2

5

2

2

,

=

Ω

π

=

(6)

With an 8 Ω internal speaker attached, the maximum power

dissipation in BTL mode is (from Equation 4)

()

mW

V

P

MAX

DISS

633

8

5

2

2

2

,

=

Ω

π

×

=

(7)

It can easily be 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.

14-lead TSSOP package on a 4-layer board is 140°C/W. To

ensure that the SSM2250 die junction temperature stays below

150°C, the maximum ambient temperature can be solved using

Equation 8.

(

C

W

W

C

C

P

C

T

MAX

DISS

JA

MAX

AMB

o

o

o

o

61

633

.

0

/

140

150

150

,

,

+

=

×

−

+

=

×

θ

−

+

=

)

(8)

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 17.

The y-axis corresponds to the expected maximum power

dissipation, and the x-axis is the corresponding maximum

ambient temperature. Either method returns the same answer.

AMBIENT TEMPERATURE (°C)

1.0

0.8

0.6

0.4

0.2

0

07

50

25

100

5

14-LEAD TSSOP

θ

10-LEAD MSOP

θ

FREE AIR

NO HEAT SINK

Figure 17. 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 17 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.