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MAX97000 Datasheet(PDF) 31 Page - Maxim Integrated Products

Part No. MAX97000
Description  Audio Subsystem with Mono Class D Speaker and Class H Headphone Amplifier
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX97000 Datasheet(HTML) 31 Page - Maxim Integrated Products

 
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Audio Subsystem with Mono Class D
Speaker and Class H Headphone Amplifier
31
Applications Information
Filterless Class D Operation
Traditional Class D amplifiers require an output filter
to recover the audio signal from the amplifier’s output.
The filters add cost, increase the solution size of the
amplifier, and can decrease efficiency and THD+N
performance. The traditional PWM scheme uses large
differential output swings (2 x VDD peak-to-peak) and
causes large ripple currents. Any parasitic resistance in
the filter components results in a loss of power, lowering
the efficiency.
The MAX97000 does not require an output filter. The
device relies on the inherent inductance of the speaker
coil and the natural filtering of both the speaker and
the human ear to recover the audio component of the
square-wave output. Eliminating the output filter results
in a smaller, less costly, more efficient solution.
Because the frequency of the MAX97000 output is well
beyond the bandwidth of most speakers, voice coil
movement due to the square-wave frequency is very
small. Although this movement is small, a speaker not
designed to handle the additional power can be dam-
aged. For optimum results, use a speaker with a series
inductance > 10FH. Typical 8I speakers exhibit series
inductances in the 20FH to 100FH range.
RF Susceptibility
GSM radios transmit using time-division multiple access
(TDMA) with 217Hz intervals. The result is an RF signal
with strong amplitude modulation at 217Hz and its har-
monics that is easily demodulated by audio amplifiers.
The MAX97000 is designed specifically to reject RF
signals; however, PCB layout has a large impact on the
susceptibility of the end product.
In RF applications, improvements to both layout and
component selection decrease the MAX97000’s suscep-
tibility to RF noise and prevent RF signals from being
demodulated into audible noise. Trace lengths should be
kept below 1/4 of the wavelength of the RF frequency of
interest. Minimizing the trace lengths prevents them from
functioning as antennas and coupling RF signals into the
MAX97000. The wavelength (
l) in meters is given by: l
= c/f where c = 3 x 108 m/s, and f = the RF frequency
of interest.
Route audio signals on middle layers of the PCB to allow
ground planes above and below shield them from RF
interference. Ideally the top and bottom layers of the
PCB should primarily be ground planes to create effec-
tive shielding.
Additional RF immunity can also be obtained from rely-
ing on the self-resonant frequency of capacitors as
it exhibits the frequency response similar to a notch
filter. Depending on the manufacturer, 10pF to 20pF
capacitors typically exhibit self-resonance at RF frequen-
cies. These capacitors when placed at the input pins
can effectively shunt the RF noise at the inputs of the
MAX97000. For these capacitors to be effective, they
must have a low-impedance, low-inductance path to the
ground plane. Do not use microvias to connect to the
ground plane as these vias do not conduct well at RF
frequencies.
Component Selection
Optional Ferrite Bead Filter
Additional EMI suppression can be achieved using a
filter constructed from a ferrite bead and a capacitor to
ground (Figure 14). Use a ferrite bead with low DC resis-
tance, high-frequency (> 600MHz) impedance between
100I and 600I, and rated for at least 1A. The capacitor
value varies based on the ferrite bead chosen and the
actual speaker lead length. Select a capacitor less than
1nF based on EMI performance.
Input Capacitor
An input capacitor, CIN, in conjunction with the input
impedance of the MAX97000 line inputs forms a high-
pass filter that removes the DC bias from an incoming
analog signal. The AC-coupling capacitor allows the
amplifier to automatically bias the signal to an optimum
DC level. Assuming zero-source impedance, the -3dB
point of the highpass filter is given by:
IN IN
1
f
2 R C
=
π
3dB
Choose CIN such that f-3dB is well below the lowest fre-
quency of interest. For best audio quality, use capacitors
whose dielectrics have low-voltage coefficients, such as
tantalum or aluminum electrolytic. Capacitors with high-
voltage coefficients, such as ceramics, may result in
increased distortion at low frequencies.
Figure 14. Optional Class D Ferrite Bead Filter
MAX97000
OUTP
OUTN


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