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
Operating Modes
Fixed-Frequency Modulation (FFM) Mode
The MAX9741 features three FFM modes with different
switching frequencies (Table 1). In FFM mode, the fre-
quency spectrum of the Class D output consists of the
fundamental switching frequency and its associated
harmonics (see the Wideband Output Spectrum graph
in the Typical Operating Characteristics). The MAX9741
allows the switching frequency to be changed by
±35%, should the frequency of one or more of the har-
monics fall in a sensitive band. This can be done at any
time and does not affect audio reproduction.
Table 1. Operating Modes
FS1
FS2
SWITCHING MODE
(kHz)
L
L
670
L
H
930
H
L
470
H
H
670 ±7%
Spread-Spectrum Modulation (SSM) Mode
A unique, proprietary spread-spectrum mode flattens
the wideband spectral components, improving EMI
emissions that may be radiated by the speaker and
cables. This mode is enabled by setting FS1 = FS2 =
H. In SSM mode, the switching frequency varies ran-
domly by ±7% around the center frequency (670kHz).
The modulation scheme remains the same, but the
period of the triangle waveform changes from cycle to
cycle. Instead of a large amount of spectral energy pre-
sent at multiples of the switching frequency, the energy
is now spread over a bandwidth that increases with fre-
quency. Above a few megahertz, the wideband spec-
trum looks like white noise for EMI purposes.
Efficiency
Efficiency of a Class D amplifier is attributed to the region
of operation of the output stage transistors. In a Class D
amplifier, the output transistors act as current-steering
switches and consume negligible additional power.
The theoretical best efficiency of a linear amplifier is
78%; however, that efficiency is only exhibited at peak
output powers. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9741 still exhibits > 78% efficiency
under the same conditions (Figure 1).
Shutdown
A shutdown mode reduces power consumption and
extends battery life. Driving SHDN low places the
device in low-power (0.2µA) shutdown mode. Connect
SHDN to a logic-high for normal operation.
Click-and-Pop Suppression
Comprehensive click-and-pop suppression eliminates
audible transients on startup and shutdown. While in
shutdown, the H-bridge is pulled to GND through 320kΩ.
During startup, or power-up, the input amplifiers are
muted and an internal loop sets the modulator bias volt-
ages to the correct levels, preventing clicks and pops
when the H-bridge is subsequently enabled. Following
startup, a soft-start function gradually unmutes the input
amplifiers. The value of the soft-start capacitor has an
impact on the click/pop levels. For optimum performance,
CSS should be 470nF with a voltage rating of at least 7V.
Mute Function
The MAX9741 features a clickless/popless mute mode.
When the device is muted, the outputs stop switching,
muting the speaker. Mute only affects the output stage
and does not shut down the device. To mute the
MAX9741, drive SS to GND by using a MOSFET pull-
down (Figure 2). Driving SS to GND during the power-
up/down or shutdown/turn-on cycle optimizes
click-and-pop suppression.
100
90
80
70
60
50
40
30
20
10
0
0
EFFICIENCY vs. OUTPUT POWER
MAX9741
CLASS AB
VDD = 15V
f = 1kHz
RL = 8Ω
2 4 6 8 10 12 14 16 18 20
OUTPUT POWER (W)
Figure 1. MAX9741 Efficiency vs. Class AB Efficiency
GPIO
MUTE SIGNAL
0.47µF
SS
MAX9741
Figure 2. MAX9741 Mute Circuit
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