SSM2211

Rev. D | Page 16 of 24

POWER DISSIPATION

Another important advantage in using a bridged-output config-

uration is the fact that bridged-output amplifiers are more

efficient than single-ended amplifiers in delivering power to a

load. Efficiency is defined as the ratio of power from the power

supply to power delivered to the load

SY

L

P

P

=

η

An amplifier with a higher efficiency has less internal power

dissipation, which results in a lower die-to-case junction tem-

perature as compared to an amplifier that is less efficient. This is

important when considering the amplifier maximum power

dissipation rating vs. ambient temperature. An internal power

dissipation vs. output power equation can be derived to fully

understand this.

The internal power dissipation of the amplifier is the internal

voltage drop multiplied by the average value of the supply

current. An easier way to find internal power dissipation is to

measure the difference between the power delivered by the

supply voltage source and the power delivered into the load.

The waveform of the supply current for a bridged-output

amplifier is shown in Figure 43.

T

T

TIME

TIME

Figure 43. Bridged Amplifier Output Voltage and Supply Current vs. Time

By integrating the supply current over a period, T, then dividing

output voltage and load resistance

L

PEAK

AVG

DD

R

V

I

π

2

,

=

(5)

Therefore, power delivered by the supply, neglecting the bias

current for the device, is

L

PEAK

DD

SY

R

V

V

P

π

×

=

2

(6)

The power dissipated by the amplifier internally is simply the

difference between Equation 6 and Equation 3. The equation

delivered to the load and load resistance, is

L

PEAK

DD

DISS

R

V

V

P

π

×

=

2

2

(7)

The graph of this equation is shown in Figure 44.

OUTPUT POWER (W)

1.5

0

0

1.5

0.5

1.0

1.0

0.5

Because the efficiency of a bridged-output amplifier (Equation 3

power dissipated internally by the device stays relatively flat and

actually decreases with higher output power. The maximum

power dissipation of the device can be found by differentiating

Equation 7 with respect to load power and setting the derivative

equal to zero. This yields

0

1

2

2

1

=

−

×

π

×

=

∂

∂

−

L

L

DD

L

DISS

P

R

V

P

P

(8)

and occurs when

L

DD

MAX

DISS

R

V

P

2

2

,

π

2

=

(9)

Using Equation 9 and the power derating curve in Figure 31,

the maximum ambient temperature can be found easily. This

ensures that the SSM2211 does not exceed its maximum

junction temperature of 150°C. The power dissipation for a

single-ended output application where the load is capacitively

coupled is given by

L

L

L

DD

DISS

P

P

R

V

P

−

×

π

×

=

∂

2

2

(10)

The graph of Equation 10 is shown in Figure 45.