20W/40W, Filterless, Spread-Spectrum,

Mono/Stereo, Class D Amplifier

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13

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.

Gain Selection

The MAX9708 features four pin-selectable gain settings;

see Table 2.

Operating Modes

Fixed-Frequency Modulation (FFM) Mode

The MAX9708 features three switching frequencies in

the FFM mode (Table 3). In this mode, the frequency

spectrum of the Class D output consists of the funda-

mental switching frequency and its associated harmon-

ics (see the Wideband Output Spectrum graph in the

Typical Operating Characteristics). Select one of the

three fixed switching frequencies such that the harmon-

ics do not fall in a sensitive band. The switching fre-

quency can be changed at any time without affecting

audio reproduction.

Spread-Spectrum Modulation (SSM) Mode

The MAX9708 features a unique, patented spread-

spectrum (SSM) mode that flattens the wideband spec-

tral 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 wave-

form 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. SSM mode reduces EMI com-

pared to fixed-frequency mode. This can also help to

randomize visual artifacts caused by radiated or sup-

ply-borne interference in displays.

Synchronous Switching Mode

The MAX9708 SYNCIN input allows the Class D amplifi-

er 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 synchro-

nizing external circuitry. Its frequency is four times the

amplifier’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. MAX9708 Junction Temperature

Threshold Setting

TEMP

0.1

µF

10k

Ω

10k

Ω

TO DIGITAL

INPUT

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

150

High

High

High

Table 2. MAX9708 Gain Setting

G1

G2

GAIN (dB)

Low

High

22

High

High

25

High

Low

29.5

Low

Low

36

Table 3. Switching Frequencies

FS1

FS2

SYNCOUT

FREQUENCY (kHz)

MODULATION

00

200

Fixed-Frequency

01

250

Fixed-Frequency

10

160

Fixed-Frequency

11

200 ±4

Spread-Spectrum