MAX9701 MAXIM [Maxim Integrated Products], MAX9701 Datasheet - Page 12

• Manufacturer Part Number: MAX9701
• Description: 1.3W, Filterless, Stereo Class D Audio Power Amplifier
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX9701 features a unique, patented spread-
spectrum mode that flattens the wideband spectral
components, improving EMI emissions that may be
radiated by the speaker and cables. This mode is
enabled by setting SYNC = V
mode, the switching frequency varies randomly by
±60kHz around the center frequency (1.2MHz). The
modulation scheme remains the same, but the period
of the sawtooth waveform changes from cycle to cycle
(Figure 2). Instead of a large amount of spectral energy
present at multiples of the switching frequency, the
energy is now spread over a bandwidth that increases
with frequency. Above a few megahertz, the wideband
spectrum looks like white noise for EMI purposes
(Figure 3). A proprietary amplifier topology ensures this
does not corrupt the noise floor in the audio bandwidth.
The SYNC input allows the MAX9701 to be synchronized
to a user-defined clock, or another Maxim class D ampli-
fier, creating a fully synchronous system, minimizing
clock intermodulation, and allocating spectral compo-
nents of the switching harmonics to insensitive frequency
bands. Applying a TTL clock signal between 1000kHz
and 1600kHz to SYNC synchronizes the MAX9701. The
period of the SYNC clock can be randomized, allowing
the MAX9701 to be synchronized to another Maxim class
D amplifier operating in SSM mode.
1.3W, Filterless, Stereo Class D Audio
Power Amplifier
Figure 3. MAX9701 with 76mm of Speaker Cable with TDK
Common-Mode Choke: TDK ACM4532-801-20-X
12
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
30
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60
80
100
Synchronous Switching Mode
120
Spread-Spectrum (SSM) Mode
140
FREQUENCY (MHz)
160
180
DD
200
(Table 1). In SSM
220
240
260
SYNC
280
300
SYNC_OUT allows several MAX9701s as well as other
class D amplifiers (such as the MAX9700) to be cas-
caded. The synchronized output minimizes interfer-
ence due to clock intermodulation caused by the
switching spread between single devices. Using
SYNC_OUT, the modulation scheme remains the same
and audio reproduction is not affected by changing the
switching frequency.
The MAX9701 uses Maxim’s unique, patented modulation
scheme that eliminates the LC filter required by traditional
class D amplifiers, improving efficiency, reducing compo-
nent count, conserving board space and system cost.
Conventional class D amplifiers output a 50% duty cycle,
180° out-of-phase square wave when no signal is pre-
sent. With no filter, the square wave appears across the
load as a DC voltage, resulting in finite load current,
which increases power consumption especially when
idling. When no signal is present at the input of the
MAX9701, the amplifiers will output an in-phase square
wave as shown in Figure 4. Because the MAX9701 drives
the speaker differentially, the two outputs cancel each
other, resulting in no net idle mode voltage across the
speaker, minimizing power consumption.
Efficiency of a class D amplifier is due to the switching
operation of the output stage transistors. In a class D
amplifier, the output transistors act as current-steering
switches and consume negligible additional power.
Any power loss associated with the class D output
stage is mostly due to the I*R loss of the MOSFET on-
resistance, and quiescent-current overhead.
Figure 4. MAX9701 Outputs with No Input Signal
V OUT_+ - V OUT_- = 0V
Filterless Modulation/Common-Mode Idle
V IN_ = 0V
OUT_+
OUT_-
Efficiency
SYNC_OUT