Datasheet

MAX9744
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 100Ω to
600Ω 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 8Ω load
and 33µH for 4Ω 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
______________________________________________________________________________________ 21
R
L
(Ω)
L1, L2
(µH)
C1 (µF)
C2, C3
(µF)
R1, R2
(Ω)
C4, C5
(µF)
4 10 0.47 0.47 10 0.47
6 15 0.33 0.22 15 0.33
8 22 0.22 0.22 22 0.22
Table 8. Suggested Values for LC filter
BOOT_+
C
BOOT
0.1μF
C
FILT
470pF
C
FILT
470pF
OUT_+
BOOT_-
C
BOOT
0.1μF
OUT_-
MAX9744
Figure 7. Ferrite Bead Filter
BOOT+
C
BOOT
0.1μF
OUT+
L1
4
1, 2
C2
C1
R1
MAX9744
C4
L2
14, 18
15
BOOT-
C
BOOT
0.1μF
OUT-
C3
R2
R
L
C5
Figure 8. Output Filter for PWM Mode