Rainbow Electronics MAX9768 - Manual (Page 13)

Operating Modes
Fixed-Frequency Mode
The MAX9768 features two fixed-frequency modes:
300kHz and 360kHz. Connect SYNC to GND to select
300kHz switching frequency; leave SYNC unconnected
to select 360kHz switching frequency. The frequency
spectrum of the MAX9768 consists of the fundamental
switching frequency and its associated harmonics (see
the Wideband Output Spectrum graphs in the
Typical
Operating Characteristics ). For applications where
exact spectrum placement of the switching fundamen-
tal is important, program the switching frequency so the
harmonics do not fall within a sensitive frequency band
(Table 1). Audio reproduction is not affected by chang-
ing the switching frequency.
Spread-Spectrum Mode
The MAX9768 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
DD (Table 1). In SSM
mode, the switching frequency varies randomly 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 purposes. A proprietary amplifi-
er topology ensures this does not corrupt the noise
floor in the audio bandwidth.
External Clock Mode
The SYNC input allows the MAX9768 to be synchro-
nized to an external clock, or another Maxim Class D
amplifier, creating a fully synchronous system, minimiz-
ing clock intermodulation, and allocating spectral com-
ponents of the switching harmonics to insensitive
frequency bands. Applying a clock signal between
1MHz and 1.6MHz to SYNC synchronizes the
MAX9768. The Class D switching frequency is equal to
one-fourth the SYNC input frequency.
SYNCOUT is equal to the SYNC input frequency and
allows several Maxim amplifiers to be cascaded. The
synchronized output minimizes interference due to
clock intermodulation caused by the switching spread
between single devices. The modulation scheme
remains the same when using SYNCOUT, and audio
reproduction is not affected (Figure 1). Current flowing
between SYNCOUT of a master device and SYNC of a
slave device is low as the SYNC input is high imped-
ance (typically 200kΩ).
MAX9768
10W Mono Class D Speaker
Amplifier with Volume Control
______________________________________________________________________________________ 13
Table 1. Operating Modes
SYNC OSCILLATOR FREQUENCY (kHz) CLASS D FREQUENCY (kHz)
GND Fixed-frequency modulation with f OSC = 1200 Fixed-frequency modulation with f OSC = 300
Unconnected Fixed-frequency modulation with f OSC = 1440 Fixed-frequency modulation with f OSC = 360
V DD Spread-spectrum modulation with f OSC = 1200 ±30 Spread-spectrum modulation with f OSC = 300 ±7.5
Clocked
Fixed-frequency modulation with f OSC = external clock
frequency
Fixed-frequency modulation with f OSC = external clock
frequency / 4
SYNCOUT
OUT+
OUT-
OUT+
OUT-
SYNC
MAX9768
MAX9768
Figure 1. Cascading Two Amplifiers