MP1720 – 2.7W LOW EMI HIGH EFFICIENCY MONO CLASS-D AUDIO AMPLIFIER
OPERATION
The MP1720 is a high efficiency Class D audio
power amplifier, low cost which can operate well
without output LC filter, low EMI which can pass
FCC-Radiated Emissions Standards with 24inch
unshielded twisted-pair-speaker Cable without
output filter. It has fully differential input and
output. And this device can still be used with a
single-ended input; but for some noisy
environment the MP1720 should be used with
differential inputs to ensure maximum noise
rejection.
MP1720 Functional Block Diagram is shown as
Figure 1. The amplifier detects the input signal
and gets the input differential voltage, which
would be combined with the DC bias voltage to
generate the complementary voltage. The
complementary voltage is compared with the
sawtooth waveform. The output of the
comparators (U2, U3) would trip when the input
magnitude of the sawtooth exceeds the
complementary voltage. When no signal is
inputted, the output of both comparators are the
pulse with a fixed turn on time, and zero output is
got from the difference of two channel output
(V
OUTP
-V
OUTN
). As the larger positive signal is
inputted, the output duty of one comparator
would be larger while another remains the fixed
turn on time or less time, so larger output is got
from the difference of two channel output (V
OUTP
-
V
OUTN
). And there is opposite duty adjustment
when the larger negative signal is inputted.
2) External Clock Mode
The MP1720 CLK input allows the amplifier to be
synchronized to an external TTL clock, which can
push the spectral components of the switching
harmonics off sensitive frequency bands. This
external TTL clock frequency range is very wide
and frequency from 0.5MHz to 1.4MHz is
recommended.
Table 1—CLK Operating Modes
CLK=GND
CLK=FLOAT
CLK=Clocked
f
CLK
=1.0MHz
f
CLK
=1.3MHz
f
CLK
=external clock frequency
Input configuration
The MP1720 is a mono BTL Class-D amplifier
with differential inputs and outputs, and this
device can still be used with a single-ended input
1) Differential Input
For some noisy environment, the MP1720 should
be used with differential inputs. The fully
differential amplifier allows the inputs to be
biased at voltage other than mid-supply. The
input coupling capacitors are not required if the
design uses a differential source that is biased
from 0.2 V to 1.9V (please see TPC: THD+N vs.
Common mode voltage). Although the input
coupling capacitor is saved, the low frequency
noise would be amplified to the speaker.
If the input signal is not biased within the
recommended common-mode input range, the
input coupling capacitors are needed to pass
only the AC audio signal to the input of the
amplifier as a high pass filter.
CLK Operating Modes
The MP1720 offers two kinds of operate
frequency solution: internal fixed clock setting or
be synchronized to an external clock. Allowing
the switching frequency to be flexibly adjusted
can avoid the frequency or harmonics fall in
sensitive frequency bands.
1) Internal clock Mode
The MP1720 have two kinds of internal clock
selection. Setting CLK = DGND, the switching
frequency is 1.0MHz. Setting CLK = FLOAT, the
switching frequency is 1.3MHz.
2) Single-Ended Input
The MP1720 also can be configured as a single-
ended input amplifier. Input coupling capacitor is
needed to create a high-pass filter with the input
resistor and get the low frequency rejection.
Note: The voltage at each input pin (INP or INN)
should be not lower than -0.5V, so the input
signal voltage and the maximum output power
would be limited, especially for the low gain part.
For the high output power applications, the
higher gain versions or the differential input is
recommended.
MP1720 Rev. 1.01
8/9/2012
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