MP7748 – 2X20W STEREO SINGLE ENDED CLASS D AUDIO AMPLIFIER

MP7748 Rev. 1.0

www.MonolithicPower.com

13

1/28/2011

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© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

Component Selection

The MP7748 uses a minimum number of external

components to complete a stereo SE or mono

BTL Class D audio amplifier. The circuit of Figure

3 (stereo SE application circuit) and figure 4

(mono BLT application circuit) are optimized for a

24V power supply. This circuit 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 MP7748 amplifier, set the gain

such that the maximum input signal results in the

maximum output voltage swing.

For single-ended (SE) output configuration, the

bridge-tied-load (BTL) output configuration, the

peak input voltage, the maximum voltage gain is:

)

PK

(

IN

)

PK

(

OUT

V

V

V

)

MAX

(

A

=

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

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. When used to drive

stereo speakers in single-ended configuration, it

is recommended to set right channel idle

switching frequency larger than left channel’s

with 50k Hz difference by using different timing

table 1 for recommended SE output configuration

design, and table 2 for recommended BTL output

configuration design.

Table 1—Switching Frequency Setting For SE

Output Configuration

(V)

Gain

(V/V)

(kΩ)

(kΩ)

Left

channel

Right

channel

(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

Table 2 - Switching Frequency Setting For

BTL Output Configuration

(V)

Gain

(V/V)

(kΩ)

(kΩ)

(nF)

(kHz)

12

10

100

10

3.9

400

12

20

100

5

3.3

400

12

30

121

4.02

2.7

400

24

10

100

10

3.9

580

24

20

121

6.04

3.9

480

24

30

121

4.02

3.9

480

32

10

100

10

4.7

580

32

20

121

6.04

3.9

560

32

30

162

5.36

3.3

500