Datasheet

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 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 fre-
quency 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 amplifier 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