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

MP7731 Rev. 1.8

www.MonolithicPower.com

7

9/25/2006

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ferrite cores are used, make sure that the start

windings of each inductor line up (all starts

going toward SW pin, or all starts going toward

the output) to prevent crosstalk or other

channel-to-channel interference.

Input Coupling Capacitor

The input coupling capacitors, C7 and C12, are

used to pass only the AC audio signal to the

input of the amplifier. In a typical system

application, the source input signal is typically

centered around the circuit ground, while the

MP7731 input is at half the power supply

transmits the AC signal from the source to the

MP7731 while blocking the DC voltage. This

input coupling capacitor creates a low-pass filter

with the input resistor of the MP7731. Choose

an input coupling capacitor such that the corner

band frequency.

The formula for the corner frequency is:

()

LC

2

1

π

=

Where:

L = L1 = L2

C = C2 = C17

R6 are the input resistors and C7 and C12 are

the input AC coupling capacitors.

Power Source

For maximum output power, the amplifier circuit

requires a regulated external power source to

supply the power to the amplifier. The higher

the power supply voltage, the more power can

be delivered to a given load resistance. However,

if the power source voltage exceeds the maximum

operating voltage of 18V, the MP7731 may sustain

damage. The power supply rejection of the MP7731

is excellent. However, noise at the power supply

can get to the output, so care must be taken to

minimize power supply noise within the pass-

band frequencies. Bypass the power supply

pins with a large electrolytic capacitor (typically

aluminum electrolytic) along with smaller 1µF

pins.

Circuit Layout

Proper circuit layout is critical for optimum

performance and low output distortion and

noise. Place the following components as close

to the MP7731 as possible:

1. Power Supply Bypass, C5, C11.

C5 and C11 carry the transient current for

the switching power stage. Place a 1uF

power supply bypass capacitor as close to

Pin 18 (VDD1) and Pin 20 (PGND1) as

possible. Also place a 1uF power supply

bypass capacitor as close to Pin 13 (VDD2)

and Pin 15 (PGND2) as possible.

2. Output Catch Diodes, D2, D3, D5, and D6.

These diodes carry the current over the

dead-time while both MOSFET switches

are off. Place D3 between Pin 19 (SW1)

and Pin 20 (PGND1) to prevent the voltage

at SW1 from swinging excessively below

ground, and place D2 between SW1 and

D6 and D5 similarly to minimize the under-

shoot and over-shoot of SW2 node.

3. Input Modulator Capacitor, C10.

C10 is used to set the amplifier switching

frequency. Place C10 as close to the

differential inputs, Pin 2 and Pin 3, as

possible to reduce distortion and noise.

Electro-Magnetic Interference (EMI)

Due to the switching nature of the Class-D

amplifier, care must be taken to minimize the

effects of electromagnetic interference from the

amplifier. However, with proper component

selection and careful attention to circuit layout,

the effects of the EMI due to the amplifier

switching can be minimized. The power inductors

are a potential source of radiated emissions. For

the best EMI performance, use shielded

inductors, since the magnetic field is well

contained inside the core. On the system

printed circuit board, trace loops that carry

rapidly changing currents need to be minimized.

placed as close to the MP7731 as possible.

Nodes that carry rapidly changing voltage, such

as SW1 and SW2, must be made as small as

possible. If sensitive traces run near SW1 or

SW2, place a ground shield between the traces.