MP7731 – 5W – 30W CLASS D MONO BRIDGED AUDIO AMPLIFIER

MP7731 Rev. 1.8

www.MonolithicPower.com

6

9/25/2006

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© 2006 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

The MP7731 uses a minimum number of external

components to complete a fully bridged Class D

audio amplifier. The circuit in Figure 1 shows a

typical application. Use the following sections to

customize the amplifier for your particular

application.

Setting the Voltage Gain

The voltage gain sets the output voltage swing

for a given input voltage swing and is set by the

following equation:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

4

1

V

R

R

2

A

Where:

R1 = R8

R4 = R6

The maximum output voltage swing is limited by

the power supply. The MP7731 is a bridged

amplifier and the output load is driven differentially.

Each side of the load is limited to a maximum

peak-to-peak voltage swing of approximately

the MP7731 amplifier, set the amplifier gain

such that the maximum peak-to-peak input

signal results in at least the maximum peak-to-

peak output voltage swing.

Setting the Switching Frequency

The idle switching frequency (the switching

frequency with no audio input signal) is a

C4, C6 and C10 and resistors R1 and R8. Lower

switching frequencies result in more inductor

ripple, causing more quiescent output voltage

ripple, increasing the output noise.. Higher

switching frequencies result in more power loss.

The optimum quiescent switching frequency is

approximately 400KHz-600KHz. C6 and C14

are typically 1pF to 2.2pF. C10 is used to

program the idle switching frequency.

Choosing the LC Filter

Two identical LC filters are required in the

typical application. The inductor-capacitor (LC)

filter is a second order filter that converts the

pulse train at SW (Pins 14, 19) to the output

differential signal that drives the speaker.

Typical values for the LC filters are shown in

Figure 1. The characteristic frequency of the LC

filter needs to be high enough to allow high

frequency audio to reach the output, yet needs

to be low enough to filter out high frequency

contents of the pulses from SW.

The characteristic frequency of the LC filter is:

LC

2

1

π

=

Where:

L = L1 + L2, L1 = L2

⎟

⎠

⎞

⎜

⎝

⎛

+

×

+

=

17

C

2

C

17

C

2

C

9

C

C

,

C2 = C17

The quality factor (Q) of the LC filter is

important. If this is too low, output noise will

increase. If the Q factor is too high, then

peaking may occur at high signal frequencies

reducing the pass-band flatness.

The Q is calculated as:

C

L

R

Q

=

Where R is the load (speaker) resistance.

Use an LC filter with a Q between 0.7 and 2.0.

The actual output ripple and noise is greatly

affected by the type of inductor and capacitor

used in the LC filter. Use a film capacitor and

an inductor with sufficient power handling

capability to supply the output current to the

load. The inductor should exhibit soft saturation

characteristics. If the inductor exhibits hard

saturation, it should operate well below the

saturation current. Gapped ferrite, MPP, Powdered

Iron, or similar type toroidal cores are

recommended. If open or shielded bobbin