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MAX97000 Datasheet(PDF) 31 Page - Maxim Integrated Products |
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MAX97000 Datasheet(HTML) 31 Page - Maxim Integrated Products |
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31 / 33 page 
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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