2.3W, Ultra-Low-EMI, Filterless,

Class D Audio Amplifier

10

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improving EMI emissions by 5dB. 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 SSM

ing frequency varies randomly by ±120kHz around the

center frequency (1.22MHz). The modulation scheme

remains the same, but the period of the sawtooth wave-

form 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 the EMI Spectrum Diagram).

External Clock Mode

The SYNC input allows the MAX9705 to be synchronized

to a system clock moving the spectral components of the

switching harmonics to insensitive frequency bands.

Applying an external TTL clock of 800kHz to 2MHz to

SYNC synchronizes the switching frequency of the

MAX9705. The period of the SYNC clock can be ran-

domized, enabling the MAX9705 to be synchronized to

another MAX9705 operating in SSM mode.

Filterless Modulation/Common-Mode Idle

The MAX9705 uses Maxim’s unique, patented modula-

tion scheme that eliminates the LC filter required by

traditional Class D amplifiers, improving efficiency,

reducing component count, and conserving board

Figure 1. MAX9705 Outputs with an Input Signal Applied

OUT+

OUT-

SYNC INPUT

MODE

GND

FLOAT

±120kHz

Clocked

Table 1. Operating Modes