Rainbow Electronics MAX9770 - Manual (Page 12)

fundamental switching frequency and its associated
harmonics (see the Wideband FFT graph in the Typical
Operating Characteristics). The MAX9770 allows the
switching frequency to be changed by +32% should
the frequency of one or more harmonics fall in a sensi-
tive band. This can be done during operation and does
not affect audio reproduction.
Spread-Spectrum Modulation (SSM) Mode
The MAX9770 features a unique, patented spread-
spectrum mode that flattens the wideband spectral
components, improving EMI emissions radiated by the
speaker and cables by 5dB. Proprietary techniques
ensure that the cycle-to-cycle variation of the switching
period does not degrade audio reproduction or effi-
ciency (see the Typical Operating Characteristics).
Select SSM mode by setting SYNC = V
DD . In SSM
mode, the switching frequency 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 MHz, the wideband spec-
trum looks like white noise for EMI purposes (Figure 3).
MAX9770
1.2W Low-EMI, Filterless, Mono Class D Amplifier
with Stereo DirectDrive Headphone Amplifiers
12 ______________________________________________________________________________________
V
OUT+
- V
OUT-
t SW t SW t SW t SW
V IN-
V IN+
OUT+
OUT-
t ON(MIN)
Figure 2. MAX9770 Output with an Input Signal Applied (SSM mode)