MAX9775 datasheet(20/38 Pages) AD

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Operating Modes

Fixed-Frequency Modulation

The MAX9775/MAX9776 feature a fixed-frequency

modulation mode with a 1.1MHz switching frequency,

set through the I2C interface (Table 2). In fixed-frequen-

cy modulation mode, the frequency spectrum of the

Class D output consists of the fundamental switching

frequency and its associated harmonics (see the

Wideband Output Spectrum Fixed-Frequency Mode

graph in the

Typical Operating Characteristics).

Spread-Spectrum Modulation

The MAX9775/MAX9776 feature a unique spread-spec-

trum modulation that flattens the wideband spectral com-

ponents. Proprietary techniques ensure that the

cycle-to-cycle variation of the switching period does not

degrade audio reproduction or efficiency (see the

Typical Operating Characteristics). Select spread-spec-

trum modulation mode through the I2C interface (Table

2). In spread-spectrum modulation mode, the switching

frequency varies randomly by ±30kHz around the center

frequency (1.16MHz). The modulation scheme remains

the same, but the period of the sawtooth waveform

changes from cycle to cycle (Figure 2). 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 (see Figure 3).

2 x 1.5W, Stereo Class D Audio Subsystem

with DirectDrive Headphone Amplifier

______________________________________________________________________________________

VOUT+ - VOUT-

tSW

VIN-

VIN+

OUT+

OUT-

tON(MIN)

Figure 2. Output with an Input Signal Applied (Spread-Spectrum Modulation Mode)

MAX9775 Datasheet(PDF) 20 Page - Analog Devices