MP7721 – 2 x 10W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER

MP7721 Rev. 1.0

www.MonolithicPower.com

6

5/16/2007

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TM

APPLICATION INFORMATION

COMPONENT SELECTION

The MP7721 uses a minimum number of

external components to complete a stereo

Class D audio amplifier. The circuit of Figure 1

is optimized for a 24V power supply and a 1.5V

RMS maximum input signal. This circuit should

be suitable for most applications. However, if

this circuit is not suitable, use the following

sections to determine how to customize the

amplifier for a particular application.

Setting the Voltage Gain

The maximum output voltage swing is limited by

the power supply. To achieve the maximum

power out of the MP7721 amplifier, the gain

resistors should be set such that the maximum

input signal results in an output voltage swing

that reaches the supply limit (clipping). The

maximum output voltage swing at clipping is

peak input voltage, the voltage gain is:

)

pk

(

V

2

V

)

MAX

(

A

IN

DD

V

×

=

The voltage gain setting results in the peak

output voltage approaching its maximum for the

maximum input signal. There are applications

where it is desirable to allow the amplifier to

overdrive slightly, allowing the THD to increase

at higher power levels (as the output signal

continues to go further into clipping), and so a

Setting the Switching Frequency

The idle switching frequency (the switching

frequency when no audio input is present) is a

function of several variables: the supply voltage

frequencies for CH1 and CH2 are independent

of each other and are a function of their own

associated components. The proper setting of

the “idle frequency” is important for obtaining

optimum performance. If the frequency is set

too high, the result will be more power loss and

high distortion, while setting the idle switching

frequency too low results in more inductor ripple,

causing more output voltage ripple with

increased the output noise.

The optimum quiescent switching frequency is

approximately 700KHz to 800KHz. Refer to

Table 1 for recommended values.

Capacitor and Feedback Resistor (see Figure 1)

Gain

(V/V)

Gain

(dB)

(kΩ)

(kΩ)

(V)

3.9

11.8

39

10

6.8nF

660KHz

12

8.2

18.3

82

10

3.3nF

660KHz

12

8.3

18.4

39

4.7

6.8nF

660KHz

12

17.4

24.8

82

4.7

3.3nF

660KHz

12

5.6

15.0

56

10

8.2nF

670KHz

24

8.2

18.3

82

10

5.6nF

720KHz

24

11.9

21.5

56

4.7

8.2nF

670KHz

24

17.4

24.8

82

4.7

5.6nF

720KHz

24

33.0

30.4

330

10

1.8nF

700KHz

24

Choosing the LC Filter

The Inductor-Capacitor (LC) filter converts the

pulse train at SW to the output voltage that

drives the speaker. Typical values for the LC

filter are a 10µH inductor and a 0.47µF

capacitor.

The characteristic frequency of the LC filter

needs to be high enough to allow high

frequency audio to the output, yet needs to be

low enough to filter out high frequency products

of the pulses from SW. The characteristic

frequency of the LC filter is:

LC

2

1

π

=

The voltage ripple at the output is approximated

by the equation:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

≅

SW

0

DD

RIPPLE

f

f

V

V