MP7742 – 2 x 15W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER

MP7742 Rev. 0.91

www.MonolithicPower.com

8

1/25/2010

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© 2010 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

The MP7742 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. 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 MP7742 amplifier, set the gain

such that the maximum input signal results in

the maximum output voltage swing.

is the peak input voltage, the maximum voltage

gain is:

)

pk

(

V

2

V

)

MAX

(

A

IN

DD

V

×

=

This voltage gain setting results in the peak

output voltage approaching it’s maximum for

the maximum input signal. In some cases the

amplifier is allowed to overdrive slightly,

allowing the THD to increase at high power

required.

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

in more inductor ripple, causing more quiescent

output voltage ripple and increasing the output

noise

and

distortion.

Higher

switching

frequencies result in more power loss. The

optimum quiescent switching frequency is

approximately 600KHz

to 700KHz. It is

recommended

to

set

right

channel

idol

switching frequency larger than left channel’s

with 50kHz difference by using different timing

capacitor

Refer

to

the

Operating

Specifications for recommended values.

Capacitor and Feedback Resistor (see Figure 1)

Left channel

Right channel

(V)

Gain

(V/V)

(kΩ)

(kΩ)

(nF)

(kHz)

(nF)

(kHz)

12

5.6

56

10

4.7

560

3.3

700

12

8.2

39

4.7

5.6

620

4.7

700

12

12.0

56.4

4.7

4.7

530

3.3

670

12

17.6

56.4

3.2

4.7

530

3.3

670

12

25.5

56.4

2.2

4.7

530

3.3

670

12

30

60

2

4.7

520

3.3

650

24

5.6

56

10

10

540

8.2

650

24

8.2

82

10

5.6

610

4.7

690

24

12.0

120

10

4.7

530

3.3

660

24

17.4

82

4.7

5.6

610

4.7

690

24

25.5

120

4.7

4.7

530

3.3

660

24

30

120

4

4.7

530

3.3

660

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 shown in Figure 1, 10µH inductor and

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