MP7740 – 15W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER
APPLICATION INFORMATION
COMPONENT SELECTION
The MP7740 uses a minimum number of external
components to complete a stereo Class D audio
amplifier. The circuit of Figure 1 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 MP7740 amplifier, set the gain
such that the maximum input signal results in the
maximum output voltage swing.
The maximum output voltage swing is ±V
DD
/2.
For a given input signal voltage, where V
IN
(pk) is
the peak input voltage, the maximum voltage
gain is:
A
V
(MAX)
=
V
DD
2
×
V
IN
(pk )
Table 1—Switching Frequency vs. V
DD
, Timing
Capacitor and Feedback Resistor (see Figure 1)
Gain
(V/V)
3.9
8.2
8.3
12.0
17.4
25.5
5.6
8.2
11.9
12.0
17.4
25.5
33.0
Gain
(dB)
11.8
18.3
18.4
21.6
24.8
28.1
15.0
18.3
21.5
21.6
24.8
28.1
30.4
R
FB
(kΩ)
39
82
39
120
82
120
56
82
56
120
82
120
330
R
IN
(kΩ)
10
10
4.7
10
4.7
4.7
10
10
4.7
10
4.7
4.7
10
C
INT
6.8nF
3.3nF
6.8nF
2.2nF
3.3nF
2.2nF
8.2nF
5.6nF
8.2nF
4.7nF
5.6nF
4.7nF
1.8nF
F
SW
660KHz
660KHz
660KHz
610KHz
660KHz
610KHz
670KHz
720KHz
670KHz
620KHz
720KHz
620KHz
700KHz
V
DD
(V)
12
12
12
12
12
12
24
24
24
24
24
24
24
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
a higher gain than A
V
(max) is required.
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
V
DD
, the timing capacitor C
INT
and the feedback
resistor R
FB
. Lower switching frequencies result
in 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 to 700KHz. Refer to the
Operating Specifications for recommended
values.
Choosing the LC Filter
The Inductor-Capacitor (LC) filter converts the
pulse train at SW to the output voltage that drives
the speaker.
The characteristic frequency of the LC filter
needs to be high enough to allow high frequency
audio to the output, yet needs to be low enough
to filter out high frequency products of the pulses
from SW. The characteristic frequency of the LC
filter is:
f
0
=
1
2
π
LC
The voltage ripple at the output is approximated
by the equation:
⎛
f
V
RIPPLE
≅
V
DD
× ⎜
0
⎜
f
⎝
SW
⎞
⎟
⎟
⎠
The quality factor (Q) of the LC filter is important.
If this is too low, output noise will increase, if this is
too high, then peaking may occur at high signal
frequencies reducing the passband flatness. The
circuit Q is set by the load resistance (speaker
resistance, typically 4Ω or 8Ω). The Q is
calculated as:
MP7740 Rev. 0.9
9/22/2009
www.MonolithicPower.com
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8
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