The following is my design.
Input resistance:
The input resistance is mainly to determine the gain, that is, the output power, so it is necessary to determine the amplitude of the input signal and the amplitude of the speaker, and use the effective value calculation before and after. The speaker used in this design is 8R3W, the rated power is 3W, the rated voltage is 4.89V (effective value), and the maximum power is 4W.
We first determine the assignment of the input signal, and then determine the amplification factor according to the rated voltage of the speaker to obtain the value of RI.
The effective value of the input signal is designed to be about 600mv, and the effective value of the output signal is 4.89V. The amplification factor is 8.15 times. According to the RI calculation formula provided in the specification, AV=300K/RI,
RI takes 36K.
Input Capacitance:
After RI is determined, the second part determines the value of CI. CI and RI form a high-pass filter. The formula for calculating the -3dB corner frequency point is:
The size of this capacitor determines the low-frequency characteristics, and at the same time, it also affects the POP suppression performance of the switch. If the capacitance is large, the cut-off frequency is low, and pop sound is likely to appear, and the small capacitance affects the passband, 20hz-20K.
The cut-off frequency is 44HZ, and the calculated CI=0.1uf.
Output filtering:
The output waveform can directly drive the speaker to sound, because its waveform spectrum = audio frequency + PWM modulation waveform + harmonic component.
Essentially, a Class D audio power amplifier does not require an output filter, because the speaker itself is an LC filter. But in order to solve the EMI problem, it is recommended to use FB and capacitors to reduce radiation, just follow the reference design.
It is necessary to use the specifications of the main FB, calculate the output current according to the output power, and leave a margin for the rated current of the FB. That's it.
Ferrite bead filters are required for most applications. Ferrite filters reduce EMI at approximately 1 MHz and higher (FCC and
CE only tests for radiation above 30 MHz. When choosing a ferrite bead, please choose a high ferrite bead with high impedance at high frequencies and low impedance at low frequencies.
If you have low frequency (<1 MHz) emi sensitive circuits and/or have long wires, use an LC output filter from the amp to the speakers.
When the LC filter and ferrite bead filter are used at the same time, the LC filter should be placed as close as possible
ICs are followed by ferrite bead filters.
The following is TI's reference design: the typical value of the inductor is 33uH and the capacitor is 1uF. The cut-off frequency of this low-pass filter is 28K, and there is a certain margin for the audio frequency of 20K. This is considering the theoretical and practical errors, so as of The frequency is generally designed to be higher.
