MAX9705 Maxim Integrated Products, MAX9705 Datasheet - Page 10

• Manufacturer Part Number: MAX9705
• Description: 2.3W Ultra-Low-EMI Filterless Class D Audio Amplifier
• Manufacturer: Maxim Integrated Products

Figure 1. MAX9705 Outputs with an Input Signal Applied
The MAX9705 features a unique, patented spread-spec-
trum 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
ing frequency varies randomly by ±120kHz around the
center frequency (1.22MHz). The modulation scheme
2.3W, Ultra-Low-EMI, Filterless,
Class D Audio Amplifier
Table 1. Operating Modes
10
SYNC INPUT
Clocked
______________________________________________________________________________________
GND
V
DD
Spread-Spectrum Modulation (SSM) Mode
V
V
V
IN+
OUT+
IN-
- V
FFM with f
SSM with f
FFM with f
OUT-
OUT+
OUT-
S
S
S
DD
t
ON(MIN)
= 1100kHz
= external clock frequency
= 1220kHz ±120kHz
. In SSM mode, the switch-
MODE
t
SW
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 ).
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.
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External Clock Mode