MP7731 – 5W – 30W CLASS D MONO BRIDGED AUDIO AMPLIFIER
MP7731 Rev. 2.0 www.MonolithicPower.com 7
10/3/2008 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
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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
voltage (V
DD
/2). The input coupling capacitor
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
frequency (f
IN
) is less than the desired pass-
band frequency.
The formula for the corner frequency is:
()
RC
f
IN
π
2
1
=
Where:
R = R4 = R6
C = C7 = C12
Where f
IN
is the -3db cutoff frequency, R4 and
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
ceramic capacitors at the MP7731 V
DD
supply
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
pin 18 (V
DD1
) to prevent the voltage at SW1
from swinging excessively about V
DD
. Place
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.
V
DD bypass capacitors (C5 and C11) must be
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.
