Inductor-Based Output Filters

Some applications will use the MAX9768 with a full

inductor-/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 L3 and L4, and capacitor C15 form the pri-

mary output filter. In addition to these primary filter

components, other components in the filter improve its

functionality. Capacitors C13 and C14, plus resistors

R6 and R7, form a Zobel at the output. A Zobel corrects

the output loading to compensate for the rising imped-

ance of the loudspeaker. Without a Zobel, the filter will

have a peak in its response near the cutoff frequency.

Capacitors C11 and C12 provide additional high-fre-

quency bypass to reduce radiated emissions.

Adjustable Gain

Gain-Setting Resistors

External feedback resistors set the gain of the

MAX9768. The output stage has an internal 20dB gain

in addition to the externally set gain. Set the maximum

the actual gain of the amplifier is dependent on the vol-

ume level setting. For example, with the volume control

set to +9.5dB, the amplifier gain would be 9.5dB +

The input amplifier can be configured into a variety of

circuits. The FB terminal is an actual operational ampli-

fier output, allowing the MAX9768 to be configured as a

summing amplifier, a filter, or an equalizer, for example.

A

R

R

VV

V

F

IN

/

= −

⎛

⎝

⎜

⎞

⎠

⎟

10

10W Mono Class D Speaker

Amplifier with Volume Control

______________________________________________________________________________________

21

BOOT_+

C1

0.1µF

OUT_+

L4

4

1, 2

C11

C15

R6

MAX9768

C13

L3

14, 18

15

BOOT_-

C2

0.1µF

OUT_-

C12

R7

C14

Figure 8. Output Filter for PWM Mode

Table 8. Suggested Values for LC filter

L3, L4 (µH)

C15 (µF)

C11, C12 (µF)

R6, R7 (Ω)

C13, C14 (µF)

6

15

0.33

0.01

7.5

0.68

8

22

0.22

0.01

10

0.47

12

33

0.1

0.01

15

0.33