Datasheet

MAX9705
2.3W, Ultra-Low-EMI, Filterless,
Class D Audio Amplifier
10 ______________________________________________________________________________________
Spread-Spectrum Modulation (SSM) Mode
The MAX9705 features a unique spread-spectrum mode
that flattens the wideband spectral components, 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 mode by
setting SYNC = V
DD
. In SSM mode, the switching fre-
quency varies randomly by ±120kHz around the center
frequency (1.22MHz). 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 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.
Figure 1. MAX9705 Outputs with an Input Signal Applied
OUT+
OUT-
V
IN-
V
IN+
V
OUT+
- V
OUT-
t
ON(MIN)
t
SW
SYNC INPUT
MODE
GND FFM with f
S
= 1100kHz
V
DD
SSM with f
S
= 1220kHz ±120kHz
Clocked FFM with f
S
= external clock frequency
Table 1. Operating Modes