MAX9744 datasheet(21/28 Pages) MAXIM

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Applications Information

Filterless Class D Operation

The MAX9744 meets common EMC radiation limits with-

out a filter when the speaker leads are less than approxi-

mately 10cm. Using lengths beyond 10cm is possible

verifying against the appropriate EMC standard.

For longer speaker wire lengths, up to approximately

1m, use a simple ferrite bead and capacitor filter to

meet EMC limits. Select a ferrite bead with 100

Ω to

600

Ω impedance, and rated for at least 3A. The capaci-

tor value varies based on the ferrite bead chosen and

the actual speaker lead length. Select the capacitor

value based on EMC performance. See Figure 7 for the

correct connections of these components.

When evaluating the device without a filter or a ferrite

bead filter, include a series inductor (68µH for 8

Ω load

and 33µH for 4

Ω load) to model the actual loudspeak-

er’s behavior. Omitting this inductor reduces the effi-

ciency, the THD+N performance, and the output power

of the MAX9744.

Inductor-Based Output Filters

Some applications use the MAX9744 with a full induc-

tor/capacitor-based (LC) output filter. This is common

for longer speaker lead lengths and to gain increased

margin to EMC limits. Select the PWM output mode and

use fixed-frequency modulation mode for best audio

performance. See Figure 8 for the correct connections

of these components.

The component selection is based on the load imped-

ance of the speaker. Table 8 lists suggested values for

a variety of load impedances.

Inductors L1 and L2 and capacitor C1 form the primary

output filter. In addition to these primary filter compo-

nents, other components in the filter improve its func-

tionality. Capacitors C4 and C5 plus resistors R1 and

R2 form a Zobel at the output. A Zobel corrects the out-

put loading to compensate for the rising impedance of

the loudspeaker. Without a Zobel, the filter has a peak

in its response near the cutoff frequency. Capacitors

C2 and C3 provide common-mode noise suppression

to reduce radiated emissions.

20W Stereo Class D Speaker Amplifier

with Volume Control

______________________________________________________________________________________

RL (

Ω)

L1, L2

(µH)

C1 (µF)

C2, C3

(µF)

R1, R2

(

Ω)

C4, C5

(µF)

0.47

0.33

0.22

0.33

0.22

Table 8. Suggested Values for LC filter

BOOT_+

CBOOT

0.1

μF

CFILT

470pF

CFILT

470pF

OUT_+

BOOT_-

CBOOT

0.1

μF

OUT_-

MAX9744

Figure 7. Ferrite Bead Filter

BOOT+

CBOOT

0.1

μF

OUT+

1, 2

MAX9744

14, 18

BOOT-

CBOOT

0.1

μF

OUT-

Figure 8. Output Filter for PWM Mode

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