MP1720 – 2.7W LOW EMI HIGH EFFICIENCY MONO CLASS-D AUDIO AMPLIFIER
APPLICATION INFORMATION
COMPONENT SELECTION
The MP1720 uses a minimum number of external
components to complete a fully bridged Class D
audio amplifier. Use the following sections to
customize the amplifier for your particular
application.
1) Input Coupling Capacitors (C
IN
)
The MP1720 is a mono BTL Class-D amplifier
with differential outputs and inputs. If the input
signal is not biased within the recommended
common-mode input range or if using a single-
ended source, the input coupling capacitors are
used to pass only the AC audio signal to the
input of the amplifier as a high pass filter. 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:
2) Power Supply Decoupling Capacitor (C
S
)
The class-D audio amplifier requires adequate
power supply decoupling to ensure the efficiency
is high and total harmonic distortion (THD+N) is
low. To carry the higher frequency transient
current, spikes, or digital hash on the line, a good
low
ESR ceramic capacitor is necessary. Place a 1μF
decoupling capacitor as close as possible to the
device VCC lead. It is very important for the
efficiency of the class-D amplifier, because any
resistance or inductance in the trace between the
capacitor and the device may cause a loss. A
10μF or greater capacitor placed near the audio
power amplifier would also help for filtering lower-
frequency noise
f
IN
½
1
2
R
IN
C
IN
(2)
R
IN
is 15kΩ for MP1720. Speakers in wireless
handsets usually can’t respond well to low
frequencies, so for this application the corner
frequency can be set to block the low frequencies.
The input coupling capacitance is calculated as:
C
IN
½
1
2
R
IN
f
IN
(3)
If the corner frequency is within the audible band,
the capacitors should have a tolerance of ±10%
or better, because any mismatch in capacitance
causes an impedance mismatch at the corner
frequency and below.
Tantalum or aluminum electrolytic capacitor with
low-voltage coefficients is recommended, or the
largest package ceramics capacitor to minimize
voltage coefficient effects, for example X7R
dielectrics is better than Y5V or Z5U.
MP1720 Rev. 1.01
8/9/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
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