MP7740 – 15W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER

MP7740 Rev. 0.9

www.MonolithicPower.com

8

9/22/2009

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© 2009 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

The MP7740 uses a minimum number of external

components to complete a stereo Class D audio

amplifier. The circuit of Figure 1 should be

suitable for most applications, and 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 MP7740 amplifier, set the gain

such that the maximum input signal results in the

maximum output voltage swing.

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 levels, and so

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. Refer to the

Operating

Specifications

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

12.0

21.6

120

10

2.2nF

610KHz

12

17.4

24.8

82

4.7

3.3nF

660KHz

12

25.5

28.1

120

4.7

2.2nF

610KHz

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

12.0

21.6

120

10

4.7nF

620KHz

24

17.4

24.8

82

4.7

5.6nF

720KHz

24

25.5

28.1

120

4.7

4.7nF

620KHz

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.

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

The quality factor (Q) of the LC filter is important.

If this is too low, output noise will increase, if this is

too high, then peaking may occur at high signal

frequencies reducing the passband flatness. The

circuit Q is set by the load resistance (speaker

resistance, typically 4Ω or 8Ω). The Q is

calculated as: