TPA032D02

10-W STEREO CLASS-D AUDIO POWER AMPLIFIER

SLOS243A – DECEMBER 1999 – REVISED MARCH 2000

17

POST OFFICE BOX 655303

APPLICATION INFORMATION

losses in a real-world class-D amplifier (continued)

Losses due to rise and fall times are called switching losses. A diagram of the output, showing switching losses,

is shown in Figure 9.

tSWon

+

tSWoff =

tSW

1

f

SW

Figure 9. Output Switching Losses

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.

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. Switching losses are dominated by conduction losses at the high output powers,

but should be considered at low powers. The switching losses are automatically taken into account if you

consider the quiescent current with the output filter and load.

class-D effect on power supply

Efficiency calculations are an important factor for proper power supply design in amplifier systems. Table 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 10-W per channel audio system with 4-

Ω loads and a 12-V supply

is almost 26 W. A similar linear amplifier such as the TPA032D02 has a maximum draw of greater than 50 W

under the same circumstances.

Table 2. Efficiency vs Output Power in 12-V 4-

Ω H-Bridge Systems

Output Power (W)

Efficiency (%)

Peak Voltage (V)

Internal Dissipation (W)

0.5

41.7

2

0.7

2

66.7

4

1.0

5

75.1

6.32

1.66

8

78

8

2.26

10

77.9

8.94†

2.84

† High peak voltages cause the THD to increase