MP1740 –3W HIGH EFFCIENCY MONO CLASS-D AUDIO AMPLIFIER
MP1740 Rev. 1.0 www.MonolithicPower.com 9
1/5/2012 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved
APPLICATION INFORMATION
COMPONENT SELECTION
The MP1740 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. Setting the Voltage Gain
The voltage gain sets the output voltage swing
for a given input voltage swing and is set by the
input resistors (R
I
) as the following equation:
I
150k
GAIN=2
R
×
(1)
The maximum output voltage swing is limited by
the power supply. To achieve the maximum
output power of the MP1740 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. Lower gain
allows the MP1740 to operate at its best,
because a higher voltage at the input for the
same output power making the inputs less
susceptible to noise. So setting voltage gain to
2V/V may achieve optimal performance.
2. Input Coupling Capacitors (C
I
)
The MP1740 is a mono BTL Class-D amplifier
with differential inputs and outputs. This device
can still be used with a single-ended input; but for
some noisy environment (such as a wireless
handset) the MP1740 should be used with
differential inputs to ensure maximum noise
rejection.
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 0.5 V to VCC - 0.8 V (as shown in
Recommended Operating Conditions). 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. The input capacitors create a high-pass
filter with the input resistor. Choose an input
coupling capacitor such that the corner frequency
f
C
is less than the desired pass-band frequency.
The formula for the corner frequency is:
C
II
1
f=
2RC
(2)
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:
I
IC
1
C=
2Rf
(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.
3. 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) 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 1F
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
10F or greater capacitor placed near the audio
power amplifier would also help for filtering lower-
frequency noise.
4. Input Resistors (R
I
)
In fully differential amplifiers, please use the
matching input resistors. Matching is very
important for the balance of the output on the
reference voltage (more important than overall
tolerance). The higher matching, the batter
CMRR and PSRR performance. Therefore, it is
recommended to use 1% precision resistors or
better to keep the performance optimized. Place
the input resistors as close to the MP1740 as
possible to limit noise injection on the high-
impedance nodes.
