ssm2211z-eval Analog Devices, Inc., ssm2211z-eval Datasheet - Page 16

• Manufacturer Part Number: ssm2211z-eval
• Description: Low Distortion 1.5 Watt Audio Power Amplifier
• Manufacturer: Analog Devices, Inc.

SSM2211
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
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.
By integrating the supply current over a period, T, then dividing
the result by T, I
output voltage and load resistance
Therefore, power delivered by the supply, neglecting the bias
current for the device, is
Figure 43. Bridged Amplifier Output Voltage and Supply Current vs. Time
η
I
P
DD
SY
=
I
,
P
DD, PEAK
AVG
=
P
SY
L
V
2
V
PEAK
OUT
V
I
=
SY
DD
2
DD,AVG
π
V
π
×
R
PEAK
R
V
L
L
PEAK
can be found. Expressed in terms of peak
T
T
TIME
TIME
I
DD, AVG
Rev. D | Page 16 of 24
(5)
(6)
The power dissipated by the amplifier internally is simply the
difference between Equation 6 and Equation 3. The equation
for internal power dissipated, P
delivered to the load and load resistance, is
The graph of this equation is shown in Figure 44.
Because the efficiency of a bridged-output amplifier (Equation 3
divided by Equation 6) increases with the square root of P
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
and occurs when
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
The graph of Equation 10 is shown in Figure 45.
P
∂
P
∂
DISS
P
∂
DISS
P
Figure 44. Power Dissipation vs. Output Power with V
1.5
1.0
0.5
P
DISS
DISS
0
L
,
0
MAX
=
=
=
2
2
=
V
2
π
DD
2
2
2
V
π
π
V
DD
×
= 5V
×
R
π
2
R
DD
R
V
V
R
L
L
×
L
L
DD
DD
2
V
PEAK
×
0.5
×
OUTPUT POWER (W)
P
R
L
L
P
= 16Ω
−
L
1
DISS
2
−
−
P
, expressed in terms of power
1
L
=
0
1.0
R
L
R
= 8Ω
L
= 4Ω
DD
= 5 V
1.5
L
, the
(10)
(7)
(8)
(9)