MP7731 – 5W – 30W CLASS D MONO BRIDGED AUDIO AMPLIFIER
MP7731 Rev. 2.0 www.MonolithicPower.com 6
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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
2A
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
V
DD
. To achieve the maximum output power of
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
function of the supply voltage, V
DD
, the capacitors
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:
LC2
1
f
0
π
=
Where:
L = L1 + L2, L1 = L2
⎟
⎠
⎞
⎜
⎝
⎛
+
×
+=
17C2C
17C2C
9CC
, 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
