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MAX9877AERP Datasheet(PDF) 28 Page - Maxim Integrated Products |
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MAX9877AERP Datasheet(HTML) 28 Page - Maxim Integrated Products |
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28 / 31 page  Increasing the value of C1 reduces the charge-pump out- put resistance to an extent. Above 1µF, the on-resistance of the switches and the ESR of C1 and C2 dominate. Output Holding Capacitor (C2) The output capacitor value and ESR directly affect the ripple at VSS. Increasing the value of C2 reduces output ripple. Likewise, decreasing the ESR of C2 reduces both ripple and output resistance. Lower capacitance values can be used in systems with low maximum output power levels. See the Output Power vs. Load Resistance graph in the Typical Operating Characteristics. PVDD Bulk Capacitor (C3) In addition to the recommended PVDD bypass capaci- tance, bulk capacitance equal to C3 should be used. Place the bulk capacitor as close to the device as possible. Supply Bypassing, Layout, and Grounding Proper layout and grounding are essential for optimum performance. Use wide traces for the power-supply inputs and amplifier outputs to minimize losses due to parasitic trace resistance. Wide traces also aid in mov- ing heat away from the package. Proper grounding improves audio performance, minimizes crosstalk between channels, and prevents any switching noise from coupling into the audio signal. Connect PGND and GND together at a single point on the PCB. Route all traces that carry switching transients away from GND and the traces/components in the audio signal path. Connect PVDD to a 2.7V to 5.25V source. Bypass PVDD to the PGND pin with a 1µF ceramic capacitor. Additional bulk capacitance should be used to prevent power-supply pumping. Place the bypass capacitors as close to the MAX9877A as possible. Connect VDD to PVDD. Bypass VDD to GND with a 1µF capacitor. Place the bypass capacitors as close to the MAX9877A as possible. 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 that is easily demodulated by audio amplifiers. Figure 14 shows the susceptibility of the MAX9877A to a trans- mitting GSM radio placed in close proximity. Although there is measurable noise at 217Hz and its harmonics, the noise is well below the threshold of hearing using typical headphones. In RF applications, improvements to both layout and component selection decreases the MAX9877A’s sus- ceptibility to RF noise and prevent RF signals from being demodulated into audible noise. Trace lengths should be kept below 1/4 the wavelength of the RF fre- quency of interest. Minimizing the trace lengths pre- vents them from functioning as antennas and coupling RF signals into the MAX9877A. The wavelength λ in meters is given by: λ = 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 capaci- tors typically exhibit self resonance at RF frequencies. These capacitors, when placed at the input pins, can effectively shunt the RF noise at the inputs of the MAX9877A. 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. Low RF Susceptibility, Mono Audio Subsystem with DirectDrive Headphone Amplifier 28 ______________________________________________________________________________________ RF SUSCEPTIBILITY FREQUENCY (Hz) 10k 1k 100 -130 -110 -90 -70 -50 -30 -10 -150 10 100k THRESHOLD OF HEARING MAX9877A NOISE FLOOR Figure 14. MAX9877A Susceptibility to a GSM Cell Phone Radio |
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