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Class AB amplifier. All power devices in a Class D amplifier are operated in
on/off mode. The term usually applies to devices intended to reproduce
signals with a bandwidth well below the pulse switching frequency which in
our case is about 200kHz.
The MAX9708 converts the input signal to a sequence of pulses whose
averaged value is directly proportional to the instantaneous amplitude of the
signal. The frequency of the pulses is typically ten or more times the highest
frequency of interest in the input signal. The pulse frequency in the Power
Amp is around 200kHz or 10 times 20kHz. The output of such an amplifier
contains unwanted spectral components (that is, the pulse frequency and its
harmonics) which must be removed by a passive filter. The resulting filtered
signal is then an amplified replica of the input. This filtering is handled by the
patented technology of the Maxim MAX9807 amplifier chip and the speaker.
The design uses a unique filterless modulation and spread-spectrum switching
mode. This eliminates the need for large inductors and capacitors on the
output.
Typical class D amplifiers use a simple modulation method called 'Fixed-
Frequency Modulation' (FFM). This simply means that a switching frequency
is selected and that is the frequency at which the output is switched. The
MAX9708 can use this method but it also has a special method called
'Spread-Spectrum Modulation' (SSM). With this method the switching
frequency is actually changed around a center frequency, in this case +- 4%
around 200kHz. Using this method the MAX9708 is able to reduce the noise
generated with typical class D amplifiers and the need for large filters
described above is minimized. How this is accomplished is beyond this
manual. If you are interested you can find information on the web by
searching for 'spread spectrum' and also check out the Maxim web site at
"www.maxim-ic.com" and go to the data sheet for the MAX9708.
The main advantage of a class D amplifier is power efficiency. Because the
output pulses have a fixed amplitude the output devices, which are MOSFETS
in the case of the MAX9708, are switched either on or off rather than operated
in linear mode. This means that very little power is dissipated by the
transistors except during the very short interval between the on and off states.
The MAX9708 is capable of achieving up to 87% efficiency. The wasted
power, the remaining 13%, is low because the instantaneous power dissipated
in the transistor is the product of voltage and current, and one or the other is
almost always close to zero. The lower losses permit the use of a smaller heat
sink while the power supply requirements are lessened too.
The input signal(s) are applied to U7 though C57 and C59. Once the signal
reaches the MAX9708 there isn't a lot more to say other than it amplifies the
signal and applies it to the output terminals at T1.
SETUP AN
UAM4SYS • 9
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