Datasheet
TEMP returns high once the junction temperature cools
below the set threshold minus the thermal hysteresis.
If TEMP is connected to either MUTE or SS, the audio
output resumes. The temperature threshold is set by the
TH0, TH1, and TH2 inputs as shown in Table 1. An RC
filter may be used to eliminate any transient at the TEMP
output as shown in Figure 3.
If TH2 = TH1 = TH0 = HIGH, it is likely that the MAX9709
enters thermal shutdown without tripping the thermal flag.
Gain Selection
The MAX9709 features four pin-selectable gain settings;
see Table 2.
Operating Modes
Fixed-Frequency Modulation (FFM) Mode
The MAX9709 features three switching frequencies in the
FFM mode (Table 3). In this mode, the frequency spec-
trum of the Class D output consists of the fundamental
switching frequency and its associated harmonics (see
the Wideband Output Spectrum graph in the Typical
Operating Characteristics). Select one of the three fixed
switching frequencies such that the harmonics do not
fall in a sensitive band. The switching frequency can be
changed any time without affecting audio reproduction.
Spread-Spectrum Modulation (SSM) Mode
The MAX9709 features a unique spread-spectrum (SSM)
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 FS1
= FS2 = high. In SSM mode, the switching frequency
varies randomly by ±4% around the center frequency
(200kHz). 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 pres-
ent at multiples of the switching frequency, the energy is
now spread over a bandwidth that increases with frequen-
cy. Above a few megahertz, the wideband spectrum looks
like white noise for EMI purposes. SSM mode reduces
EMI compared to fixed-frequency mode. This can also
help to randomize visual artifacts caused by radiated or
supply borne interference in displays.
Synchronous Switching Mode
The MAX9709 SYNCIN input allows the Class D ampli-
fier to switch at a frequency defined by an external clock
frequency. Synchronizing the amplifier with an external
clock source may confine the switching frequency to a
less sensitive band. The external clock frequency range is
from 600kHz to 1.2MHz and can have any duty cycle, but
the minimum pulse must be greater than 100ns.
SYNCOUT is an open-drain clock output for synchroniz-
ing external circuitry. Its frequency is four times the ampli-
fier’s switching frequency and it is active in either internal
or external oscillator mode.
Figure 3. An RC Filter Eliminates Transient During Switching
Table 1. MAX9709 Junction Temperature
Threshold Setting
Table 2. MAX9709 Gain Setting
Table 3. Switching Frequencies
JUNCTION
TEMPERATURE
(°C)
TH2 TH1 TH0
80 Low Low Low
90 Low Low High
100 Low High Low
110 Low High High
120 High Low Low
129 High Low High
139 High High Low
158 High High High
G1 G2 GAIN (dB)
Low High 22
High High 25
High Low 29.5
Low Low 36
FS1 FS2
SYNCOUT
FREQUENCY (kHz)
MODULATION
0 0 200 Fixed-frequency
0 1 250 Fixed-frequency
1 0 160 Fixed-frequency
1 1 200 ±4 Spread-spectrum
TEMP
0.1µF
10kΩ
10kΩ
V
DIGITAL
TO DIGITAL
INPUT
MAX9709 25W/50W, Filterless, Spread-Spectrum,
Stereo/Mono, Class D Amplier
www.maximintegrated.com
Maxim Integrated
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