Rainbow Electronics MAX9704 - Manual (Page 8)

MAX9703/MAX9704
Detailed Description
The MAX9703/MAX9704 filterless, class D audio power
amplifiers feature several improvements to switch-
mode amplifier technology. The MAX9703 is a mono
amplifier, the MAX9704 is a stereo amplifier. These
devices offer class AB performance with class D effi-
ciency, while occupying minimal board space. A
unique filterless modulation scheme and spread-spec-
trum switching mode create a compact, flexible, low-
noise, efficient audio power amplifier. The differential
input architecture reduces common-mode noise pick-
up, and can be used without input-coupling capacitors.
The devices can also be configured as a single-ended
input amplifier.
Comparators monitor the device inputs and compare
the complementary input voltages to the triangle wave-
form. The comparators trip when the input magnitude of
the triangle exceeds their corresponding input voltage.
Operating Modes
Fixed-Frequency Modulation (FFM) Mode
The MAX9703/MAX9704 feature three FFM modes with
different switching frequencies (Table 1). In FFM mode,
the frequency spectrum of the class D output consists
of the fundamental switching frequency and its associ-
ated harmonics (see the Wideband FFT graph in the
Typical Operating Characteristics). The MAX9703/
MAX9704 allow the switching frequency to be changed
by ±35%, should the frequency of one or more of the
harmonics fall in a sensitive band. This can be done at
any time and does not affect audio reproduction.
Spread-Spectrum Modulation (SSM) Mode
The MAX9703/MAX9704 feature a unique, patented
spread-spectrum mode that flattens the wideband
spectral components, improving EMI emissions that
may be radiated by the speaker and cables. This mode
is enabled by setting FS1 = FS2 = H. In SSM mode, the
switching frequency varies randomly by ±7% around
the center frequency (670kHz). The modulation scheme
remains the same, but the period of the triangle wave-
form changes from cycle to cycle. 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.
Efficiency
Efficiency of a class D amplifier is attributed to the
region of operation of the output stage transistors. In a
class D amplifier, the output transistors act as current-
steering switches and consume negligible additional
power. Any power loss associated with the class D out-
put stage is mostly due to the I*R loss of the MOSFET
on-resistance, and quiescent current overhead.
The theoretical best efficiency of a linear amplifier is
78%; however, that efficiency is only exhibited at peak
output powers. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9704 still exhibits >78% efficiency
under the same conditions (Figure 2).
15W, Filterless, Spread-Spectrum
Mono/Stereo Class D Amplifiers
8 _______________________________________________________________________________________
Table 1. Operating Modes
FS1 FS2
SWITCHING MODE
(kHz)
L L 670
L H 940
H L 470
H H 670 ±7%
Figure 1. MAX9704 Outputs with No Input Signal Applied
V IN = 0V
OUT-
OUT+