Texas Instruments TPA005D02 - Manual (Page 31)

The TPA005D02 Audio Power Amplifier Evaluation Module
3-17 Details
Given these plots, the efficiency of the class D device can be calculated and
compared to an ideal linear amplifier device. In the derivation below, a sine
wave of peak voltage (V
P
) is the output from an ideal class D and linear
amplifier and the efficiency is calculated.
V
L(rms)
+
V
P
2
Ǹ
CLASS D LINEAR
V
L(rms)
+
V
P
2
Ǹ
P
L
+ V
L
I
L
Average
ǒ
I
DD
Ǔ
+
I
L(rms)
V
L(rms)
V
DD
P
L
+
V
L(rms)
2
R
L
+
V
P
2
2 R
L
Average
ǒ
I
DD
Ǔ
+
2
p
V
P
R
L
P
SUP
+ V
DD
Average
ǒ
I
DD
Ǔ
P
SUP
+ V
DD
Average
ǒ
I
DD
Ǔ
+
V
DD
V
P
R
L
2
p
P
SUP
+
V
DD
I
L(rms)
V
L(rms)
V
DD
Efficiency
+ h +
P
L
P
SUP
Efficiency + h +
P
L
P
SUP
Efficiency
+ h +
V
DD
V
P
2
2R
L
2
p
V
P
R
L
Efficiency + h + 1 Efficiency + h + p
4
V
P
V
DD
In the ideal efficiency equations, assume that V
P
= V
DD
, which is the maximum
sine wave magnitude without clipping. Then, the highest efficiency that a linear
amplifier can have without clipping is 78.5%. A class D amplifier, however, can
ideally have an efficiency of 100% at all power levels.
The derivation above applies to an H-bridge as well as a half-bridge. An
H-bridge requires approximately twice the supply current but only requires half
the supply voltage to achieve the same output power—factors that cancel in
the efficiency calculation. The H-bridge circuit is shown in Figure 3–12.
Figure 3–12. H-Bridge Class D Output Stage
VDD
VOUT
L
CL
RL
IL IOUT
+ –
VA
M2
M1
VDD
L
CL
M4
M3