12W+12W, Low-EMI, Spread-Spectrum,

Stereo, Class D Amplifier

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9

Internal Regulator

The MAX9741 has an internal linear regulator, REG,

used to power the internal analog circuitry. The voltage

at REG is nominally 6V. Bypass REG to AGND with a

10nF capacitor, rated for at least 10V. REG is turned off

in shutdown.

Applications Information

Class D Amplifier Outputs

Class D amplifiers differ from analog amplifiers such as

Class AB in that their output waveform is composed of

high-frequency pulses from ground to the supply rail.

When viewed with an oscilloscope the audio signal will

not be seen; instead, the high-frequency pulses domi-

nate. To evaluate the output of a Class D amplifier

requires taking the difference from the positive and

negative outputs, then lowpass filtering the difference

to recover the amplified audio signal.

Ferrite Bead Output Filters

The MAX9741’s low-EMI output switching method

reduces the output filtering requirements when compared

to pure PWM Class D amplifiers. The outputs will contain

both differential and common-mode noise at the switch-

ing frequency and its harmonics. In many applications,

a simple ferrite bead filter (see the Simplified Block

Diagram) will allow the amplifier to pass FCC EMI limits.

Ferrite beads offer significant cost and size reductions

when compared to conventional inductors. The ferrite

bead type and capacitor value can be adjusted to tune

the rejection to match the speaker cable length.

Actual EMI test results for the MAX9741 are shown in

Figure 3. This shows the MAX9741, tested in a 10m ane-

choic EMC chamber. The MAX9741 test conditions

were: SSM mode, 0.5m cables on each side, 16dB gain,

18V supply voltage, both channels playing pink noise at

4W per channel into 8

Ω shielded speakers.

The graph of Figure 3 indicates peak readings. Actual

quasi peak readings per EN55022B specification will

be lower due to Maxim’s proprietary SSM mode. Table

2 lists select values, indicating the peak reading, the

quasi-peak reading, and the actual margin to

EN55022B specification.

FREQUENCY (MHz)

900

800

100

200

300

500

600

400

700

10

15

20

25

30

35

40

30

1000

Figure 3. EMI Measurement of MAX9741 in 10m Anechoic Chamber

Table 2. Peak and Quasi-Peak EMI Readings

FREQUENCY

(MHz)

PRELIMINARY PEAK

READING (dBµV/m)

QUASI PEAK READING

(dBµV/m)

EN55022B LIMIT

(dBµV/m)

ACTUAL MARGIN

(dBµV/m)

75.38

28.1

18.3

30.0

11.7

78.57

28.0

21.9

30.0

-8.1

83.18

26.6

20.6

30.0

-9.4