Texas Instruments TPA005D02 - Manual (Page 33)

The TPA005D02 Audio Power Amplifier Evaluation Module
3-19 Details
Losses due to rise and fall times are called switching losses. A plot of the
output, showing switching losses, is shown in Figure 3–14.
Figure 3–14. Output Switching Losses
tSWon + tSWoff = tSW
1
f
SW
Rise and fall times are greater than zero for several reasons. One is that the
output transistors cannot switch instantaneously because (assuming a
MOSFET) the channel from drain to source requires a specific period of time
to form. Another is that transistor gate-source capacitance and parasitic
resistance in traces form RC time constants that also increase rise and fall
times.
The switching power loss formula below with the following values (V
DD
= 5V,
t
SW
= 50 ns, f
SW
= 250 kHz, R
DS(on)
= 310 m
Ω,
R
L
= 4
Ω
) yields a switching power
loss of 4.4 mW at all output powers.
P
SW
+
1
2
t
SW
f
SW
ǒ
V
DD
2R
DS(on)
R
L
)
2R
DS(on)
Ǔ
2
2R
DS(on)
Switching losses are constant at all output power levels, which means that
switching losses can be ignored at high power levels in most cases. At low
power levels, however, switching losses must be taken into account when
calculating efficiency.
3.2.4.3 Class D Effect on Power Supply
Efficiency calculations are an important factor for proper power supply design
in amplifier systems. Table 3–2 shows class D efficiency at a range of output
power levels (per channel) with a 1-kHz sine wave input. The maximum power
supply draw from a stereo 1-W per channel audio system with 8-
Ω
loads and
a 5-V supply is almost 2.7 W. A similar linear amplifier such as the TPA0202
has a maximum draw of 3.25 W under the same circumstances.
Table 3–2. Efficiency vs Output Power in 5-V 8-
Ω
H-Bridge Systems
Output Power (W) Efficiency (%) Peak Voltage (V) Internal Dissipation (W)
0.25 63.4 2 0.145
0.5 73 2.83 0.183
0.75 77.1 3.46 0.222
1 79.3 4 0.314
1.25 80.6 4.47† 0.3
† High peak voltages cause the THD to increase