Rainbow Electronics MAX9768 - Manual (Page 20)

MAX9768
10W Mono Class D Speaker
Amplifier with Volume Control
20 ______________________________________________________________________________________
Applications Information
Filterless Class D Operation
The MAX9768 can be operated without a filter and
meet common EMC radiation limits when the speaker
leads are less than approximately 10cm. Lengths
beyond 10cm are possible but should be verified
against the appropriate EMC standard. Select the filter-
less modulation mode with spread-spectrum modula-
tion mode for best performance.
For longer speaker wire lengths, a simple ferrite bead
and capacitor-based filter can be used to meet EMC
limits. See Figure 7 for the correct connections of these
components. Select a ferrite bead with 100Ω to 600Ω
impedance, and rated for at least 1.5A. The capacitor
value will vary based on the ferrite bead chosen and
the actual speaker lead length. Select the capacitor
value based on EMC performance.
When doing bench evaluation without a filter or a ferrite
bead filter, include a series inductor (68µH for 8Ω load)
to model the actual loudspeaker’s behavior. If this
inductance is omitted, the MAX9768 will have reduced
efficiency and output power, as well as worse THD+N
performance.
Table 7. Setting Class D Output Modulation Scheme
D7 (MSB) D6 D5 D4 D3 D2 D1 D0 (LSB) FUNCTION
1 1 0 1 0 1 0 1 Classic PWM
1 1 0 1 0 1 1 0 FILTERLESS MODULATION*
BOOT_+
C1
0.1
μF
C9
330pF
C10
330pF
OUT_+
BOOT_-
C2
0.1
μF
OUT_-
MAX9768
Figure 7. Ferrite Bead Filter
* Power-on default.