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MP7720

20W Class D Mono Single Ended Audio Amplifer

Monolithic Power Systems 

MP7720 – 20W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER

APPLICATION INFORMATION

COMPONENT SELECTION

The MP7720 uses a minimum number of

external components to complete a 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 MP7720 amplifier, set the gain

such that the maximum input signal results in

the maximum output voltage swing.

The maximum output voltage swing is ±VDD/2.

For a given input signal voltage, where VIN(pk)

is the peak input voltage, the maximum voltage

gain is:

A V (MAX)

=

VDD

2 × VIN(pk)

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 a higher gain than AV (max) is

required.

Setting the Switching Frequency

The idle switching frequency is a function of

VDD, the capacitor C3 and the feedback resistor

R4. Lower switching frequencies result 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.

Table 1—Switching Frequency vs. Integrating

Capacitor and Feedback Resistor (see Figure 1)

Gain Gain R4 R1

(V/V) (dB) (kΩ) (kΩ)

C3

FSW

VDD

(V)

3.9 15.0 39 10 6.8nF 660KHz 12

8.2 18.3 82 10 3.3nF 660KHz 12

8.3 21.5 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. 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:

f0

=

1

2π(LC)

1

2

The voltage ripple at the output is approximated

by the equation:

VRIPPLE

≅

VDD

×

⎜⎜⎝⎛

f0

fSW

⎟⎟⎠⎞

MP7720 Rev. 1.9

www.MonolithicPower.com

6

3/13/2006

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