Datasheet

Detailed Description
The MAX9703/MAX9704 filterless, Class D audio power
amplifiers feature several improvements to switchmode
amplifier technology. The MAX9703 is a mono amplifier,
the MAX9704 is a stereo amplifier. These devices offer
Class AB performance with Class D efficiency, while occu-
pying minimal board space. A unique filterless modulation
scheme and spread-spectrum switching mode create a
compact, flexible, lownoise, efficient audio power ampli-
fier. The differential input architecture reduces common-
mode noise pickup, and can be used without input-cou-
pling 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 waveform.
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 associated
harmonics (see the Wideband Output Spectrum (FFM
Mode) 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 spread-spec-
trum mode that flattens the wideband spectral compo-
nents, 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 waveform changes from cycle to
cycle. Instead of a large amount of spectral energy pres-
ent at multiples of the switching frequency, the energy
is now spread over a bandwidth that increases with fre-
quency. Above a few megahertz, the wideband spectrum
looks like white noise for EMI purposes (see Figure 1).
Efciency
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 currentsteering
switches and consume negligible additional power. Any
power loss associated with the Class D output stage is
mostly due to the I
2
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).
Table 1. Operating Modes
FS1 FS2
SWITCHING MODE
(kHz)
L L 670
L H 940
H L 470
H H 670 ±7%
MAX9703/MAX9704 10W Stereo/15W Mono, Filterless,
Spread-Spectrum, Class D Ampliers
www.maximintegrated.com
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