MAX9741 View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX9741 Datasheet PDF : 16 Pages
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12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
Ferrite beads are available from many manufacturers.
Table 3 lists some manufacturers who make ferrite
beads and other products suitable for use with Class D
amplifiers.
Although they offer a low cost and small size, ferrite
bead filters slightly increase distortion and slightly
reduce efficiency. If the audio performance of the ferrite
bead filters does not meet the system requirements, then
a full inductor/capacitor (LC) filter should be considered.
Inductor/Capacitor Output Filters
Using a full inductor and capacitor (LC) output filter
provides significant attenuation of the fundamental
switching energy.
Select inductors rated for the expected RMS current
load. For example, if using a Class D amplifier up to
10W into 8Ω, the inductor should be rated for 1.25A
RMS or more. Furthermore, the inductor should maintain
a constant inductance value across the expected cur-
rent range. Inductors which change in value as a func-
tion of current will cause harmonic distortion.
The output capacitors can also affect audio perfor-
mance. Ceramic capacitors are often selected for their
size and cost advantage, but they cause distortion. If
the application constraints dictate ceramic capacitors,
selecting higher voltage rating and larger package size
mitigates some of the shortcomings. Best performance
is obtained with plastic film capacitors, but these are
larger and more expensive.
Filterless Operation
In some cases, a Class D amplifier can be used without
an output filter. The intrinsic inductance of the loud-
speaker stores energy from the high-speed PWM pulses,
Table 3. Filter Component Suppliers
SUPPLIER
Murata
Taiyo Yuden
TDK
Fairrite
Coilcraft
Sumida
PRODUCT
Ferrite beads,
capacitors
Ferrite beads,
capacitors
Ferrite beads,
capacitors
Ferrite beads
Inductors
Inductors
Panasonic
Inductors
WEBSITE
www.murata.com
www.t-yuden.com
www.tdk.co.jp/tetop01
www.fair-rite.com
www.coilcraft.com
www.sumida.com
www.panasonic.com/indu
strial/components
converting these into power in the audible frequency
range. Filterless operation requires the Class D amplifi-
er to be very close to the speaker. Distances greater
than a few centimeters must be evaluated for EMC
compliance.
Gain Selection
Table 4 shows the suggested gain settings to attain a
maximum output power from a given peak input voltage
and given load.
Output Offset
Unlike a Class AB amplifier, the output offset voltage of
Class D amplifiers does not noticeably increase quies-
cent current draw when a load is applied. This is due to
the power conversion of the Class D amplifier. For
example, an 8mVDC offset across an 8Ω load results in
1mA extra current consumption in a Class AB device.
In the Class D case, an 8mV offset into 8Ω equates
to an additional power drain of 8µW. Due to the high
efficiency of the Class D amplifier, this represents an
additional quiescent current draw of: 8µW / (VDD / 100 ✕
η), which is in the order of a few microamps.
Input Amplifier
Differential Input
The MAX9741 features a differential input structure, mak-
ing them compatible with many CODECs, and offering
improved noise immunity over a single-ended input ampli-
fier. In devices such as PCs, noisy digital signals can be
picked up by the amplifier’s input traces. The signals
appear at the amplifiers’ inputs as common-mode noise. A
differential input amplifier amplifies the difference of the
two inputs, any signal common to both inputs is canceled.
Table 4. Gain Settings
G1
G2
0
0
0
1
1
0
1
1
GAIN (dB)
29.6
19.1
13
16
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