max9744 Maxim Integrated Products, Inc., max9744 Datasheet - Page 14

• Manufacturer Part Number: max9744
• Description: 20w Stereo Class D Speaker Amplifier With Volume Control
• Manufacturer: Maxim Integrated Products, Inc.

20W Stereo Class D Speaker Amplifier
with Volume Control
The MAX9744 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
spectrum mode, the switching frequency varies ran-
domly by ±7.5kHz around the center frequency
(300kHz). 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 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 pur-
poses. A proprietary amplifier topology ensures this
does not corrupt the noise floor in the audio bandwidth.
The SYNC input allows the MAX9744 to be synchro-
nized to an external clock or another Maxim Class D
amplifier, creating a fully synchronous system. This
minimizes clock intermodulation and allocates spectral
components of the switching harmonics to insensitive
frequency bands. Applying a clock signal between
1MHz and 1.6MHz to SYNC synchronizes the
MAX9744. The MAX9744 Class D amplifier operates at
1/4 of the SYNC frequency. For example, if SYNC is
1.6MHz, the Class D amplifier operates at 400kHz.
The external SYNC signal can be any CMOS clock
source with a 40% to 60% duty cycle. Spread-spectrum
clocks work well to reduce EMI; therefore, the
SYNCOUT signal from another MAX9744 in spread-
spectrum mode is an excellent SYNC input.
Table 1. Operating Modes
14
GND
Unconnected
V
Clocked
DD
SYNC
______________________________________________________________________________________
Fixed-frequency
modulation
Fixed-frequency
modulation
Spread-spectrum
modulation
EXT
Spread-Spectrum Modulation Mode
MODE
DD
External Clock Mode
(Table 1). In spread-
f
SYNC
1200 ±30
1000 to
1200
1440
1600
(kHz) f
300 ±7.5
SW
250 to
300
360
400
(kHz)
SYNCOUT allows several Maxim amplifiers to be cas-
caded (Figure 1). The synchronized output minimizes
interference due to clock intermodulation caused by the
switching spread between single devices. Using
SYNCOUT and SYNC does not affect the audio perfor-
mance of the MAX9744.
The MAX9744 features two output modulation schemes:
filterless modulation or classic PWM. The MAX9744 out-
put modulation schemes are selectable through
SCLK/PWM when the device is in analog mode (ADDR1
and ADDR2 = GND, Table 2) or through the I
face (Table 8). Maxim’s unique, filterless modulation
scheme eliminates the LC filter required by traditional
Class D amplifiers, reducing component count and con-
serving board space and system cost. Although the
MAX9744 meets FCC and other EMI limits with a low-
cost ferrite bead filter, many applications still may want
to use a full LC-filtered output. If using a full LC filter,
audio performance is best with the MAX9744 configured
for classic PWM output.
Switching between schemes, the output is not click-and-
pop protected. To have click-and-pop protection when
switching between output schemes, the device must
enter shutdown mode and be configured to the new out-
put scheme before the startup sequence is finished.
Figure 1. Cascading Two Amplifiers’ External Clock Mode
SYNC INPUT
Filterless Modulation/PWM Modulation
SYNC
SYNC
MAX9744
MAX9709
SYNCOUT
OUTL+
OUTR+
OUTL-
OUTR-
OUT+
OUT-
2
C inter-