Operating Modes

Fixed-Frequency Modulation (FFM) Mode

The MAX9741 features three FFM modes with different

switching frequencies (Table 1). In FFM mode, the fre-

quency spectrum 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). The MAX9741

allows the switching frequency to be changed by

±35%, should the frequency of one or more of the har-

monics fall in a sensitive band. This can be done at any

time and does not affect audio reproduction.

Spread-Spectrum Modulation (SSM) Mode

A unique, proprietary spread-spectrum mode 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 =

H. In SSM mode, the switching frequency varies ran-

domly by ±7% around the center frequency (670kHz).

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 pre-

sent at multiples of the switching frequency, the energy

is now spread over a bandwidth that increases with fre-

quency. Above a few megahertz, the wideband spec-

trum looks like white noise for EMI purposes.

Efficiency

Efficiency of a Class D amplifier is attributed to the region

of operation of the output stage transistors. In a Class D

amplifier, the output transistors act as current-steering

switches and consume negligible additional power.

The theoretical best efficiency of a linear amplifier is

78%; however, that efficiency is only exhibited at peak

output powers. Under normal operating levels (typical

music reproduction levels), efficiency falls below 30%,

whereas the MAX9741 still exhibits > 78% efficiency

under the same conditions (Figure 1).

Shutdown

A shutdown mode reduces power consumption and

extends battery life. Driving SHDN low places the

device in low-power (0.2µA) shutdown mode. Connect

SHDN to a logic-high for normal operation.

Click-and-Pop Suppression

Comprehensive click-and-pop suppression eliminates

audible transients on startup and shutdown. While in

shutdown, the H-bridge is pulled to GND through 320k

Ω.

During startup, or power-up, the input amplifiers are

muted and an internal loop sets the modulator bias volt-

ages to the correct levels, preventing clicks and pops

when the H-bridge is subsequently enabled. Following

startup, a soft-start function gradually unmutes the input

amplifiers. The value of the soft-start capacitor has an

impact on the click/pop levels. For optimum performance,

CSS should be 470nF with a voltage rating of at least 7V.

Mute Function

The MAX9741 features a clickless/popless mute mode.

When the device is muted, the outputs stop switching,

muting the speaker. Mute only affects the output stage

and does not shut down the device. To mute the

MAX9741, drive SS to GND by using a MOSFET pull-

down (Figure 2). Driving SS to GND during the power-

up/down or shutdown/turn-on cycle optimizes

click-and-pop suppression.

12W+12W, Low-EMI, Spread-Spectrum,

Stereo, Class D Amplifier

8

_______________________________________________________________________________________

MAX9741

SS

0.47

µF

GPIO

MUTE SIGNAL

Figure 2. MAX9741 Mute Circuit

Table 1. Operating Modes

FS1

FS2

SWITCHING MODE

(kHz)

LL

670

LH

930

HL

470

HH

670

±7%

Figure 1. MAX9741 Efficiency vs. Class AB Efficiency

0

30

20

10

40

50

60

70

80

90

100

0

6

8

10 12 14 16 18

24

20

EFFICIENCY vs. OUTPUT POWER

OUTPUT POWER (W)

VDD = 15V

f = 1kHz

RL = 8

Ω

MAX9741

CLASS AB