User Manual

The TPA005D02 Audio Power Amplifier Evaluation Module
3-18
Details
3.2.4.2 Losses in a Real-World Class D Amplifier
Losses make class D amplifiers nonideal, and reduce the efficiency below
100%. These losses are due to the output transistors having a nonzero
R
DS(on)
, and rise and fall times that are greater than zero.
The loss due to a nonzero R
DS(on)
is called conduction loss, and is the power
lost in the output transistors at nonswitching times, when the transistor is ON
(saturated). Any R
DS(on)
above 0 Ω causes conduction loss. Figure 3–13
shows an H-bridge output circuit simplified for conduction loss analysis and
can be used to determine new efficiencies with conduction losses included.
Figure 3–13. Output Transistor Simplification for Conduction Loss Calculation
V
DD
= 5 V
R
L
4 Ω
0.31 Ω
0.31 Ω
R
DS(on)
R
DS(off)
R
DS(off)
R
DS(on)
5 MΩ
5 MΩ
The power supplied, P
SUP
, is determined to be the power output to the load
plus the power lost in the transistors, assuming that there are always two
transistors on.
Efficiency
I
2
R
L
I
2
2R
DS(on)
I
2
R
L
Efficiency
P
L
P
SUP
Efficiency
R
L
2R
DS(on)
R
L
Efficiency 95% at all output levels R
DS(on)
0.1, R
L
4
Efficiency 87% at all output levels R
DS(on)
0.31, R
L
4