SSM2211_04 데이터 시트보기 (PDF) - Analog Devices
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SSM2211_04 Datasheet PDF : 20 Pages
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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 the power delivered to the load:
η = PL
PSY
An amplifier with a higher efficiency has less internal power
dissipation, which results in a lower die-to-case junction temp-
erature, as compared to an amplifier that is less efficient. This is
important when considering the amplifier’s 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.
VOUT
VPEAK
T
ISY
TIME
IDD, PEAK
IDD, AVG
T
TIME
Figure 43. Bridged Amplifier Output Voltage and Supply Current vs. Time
By integrating the supply current over a period T, then dividing
the result by T, IDD,AVG can be found. Expressed in terms of peak
output voltage and load resistance
I DD,AVG
=
2VPEAK
πRL
(5)
Therefore power delivered by the supply, neglecting the bias
current for the device is
PSY
=
2 VDDVPEAK
πRL
(6)
SSM2211
The power dissipated by the amplifier internally is simply the
difference between Equation 6 and Equation 3. The equation
for internal power dissipated, PDISS, expressed in terms of power
delivered to the load and load resistance is
2
PDISS =
2 × VDDVPEAK
πRL
(7)
The graph of this equation is shown in Figure 44.
1.5
VDD = 5V
RL = 4Ω
1.0
0.5
RL = 8Ω
RL = 16Ω
0
0
0.5
1.0
1.5
OUTPUT POWER (W)
Figure 44. Power Dissipation vs. Output Power with VDD = 5 V
Because the efficiency of a bridged output amplifier (Equation 3
divided by Equation 6) increases with the square root of PL, the
power dissipated internally by the device stays relatively flat,
and actually decreases with higher output power. The maxi-
mum power dissipation of the device can be found by differ-
entiating Equation 7 with respect to load power, and setting the
derivative equal to zero. This yields
∂PDISS =
∂PL
2 × VDD
πRL
−1
PL 2
−1 =
0
(8)
And occurs when
PDISS ,MAX
=
2 VDD 2
π 2RL
(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
∂PDISS
=
2
2
π
× VDD
RL
PL − PL
(10)
The graph of Equation 10 is shown in Figure 45.
Rev. C | Page 15 of 20
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