max9773etgt Maxim Integrated Products, Inc., max9773etgt Datasheet - Page 12

• Manufacturer Part Number: max9773etgt
• Description: Max9773 1.8w, Filterless, Ultra-low Emi, Stereo Class D Audio Power Amplifier
• Manufacturer: Maxim Integrated Products, Inc.

1.8W, Filterless, Ultra-Low EMI,
Stereo Class D Audio Power Amplifier
The MAX9773 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 connecting SYNC to 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 MAX9773 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 2000kHz to SYNC synchronizes the MAX9773. The
period of the SYNC clock can be randomized, allowing
Figure 3. EMI Spectrum of MAX9773 with 6in of Twisted-Pair Speaker Cable with TDK Ferrite Beads MPZ1608S300A
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40
35
30
25
20
15
10
5
30
Synchronous Switching Mode
Spread-Spectrum (SSM) Mode
60
80
DD
(Table 1). In SSM
100
120
SYNC
140
FREQUENCY (MHz)
160
the MAX9773 to be synchronized to another Maxim Class
D amplifier operating in SSM mode.
SYNC_OUT allows several MAX9773s 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 MAX9773 uses Maxim’s unique, patented modulation
scheme that eliminates the LC filter required by traditional
Class D amplifiers, improving efficiency, reducing com-
ponent 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
MAX9773, the amplifiers output an in-phase square wave
as shown in Figure 4. Because the MAX9773 drives the
speaker differentially, the two outputs cancel each other,
resulting in no net idle mode voltage across the speaker,
minimizing power consumption.
Filterless Modulation/Common-Mode Idle
180
200
220
240
260
280
SYNC_OUT
300