The BYPASS in the Class AB power amplifier IC indicates the reference voltage and functions as the reference voltage of the amplifier. Now CST8002D/LM4863 LM4871 / LM4890, for example, between discipline and to BYPASS the choice of reference voltage capacitance opening time on the device and switch machine "tick and crackle" corresponding relation.

     Reference voltage

    For any power amplifier, a suitable reference voltage is critical to reducing noise. When a 5V voltage regulator is used, a 10uF and a 0.1uF filter capacitor are usually used in parallel to stabilize the voltage regulator's output, thereby reducing noise and improving the transient response of the power supply. However, even if these two capacitors are present, a 1.0uF tantalum reference capacitor still needs to be connected between the power pin and the ground. Ceramic capacitors cannot be used to replace tantalum capacitors, otherwise it will cause oscillation of the output signal. Minimize the length of the wires and circuits used to connect the capacitor between the power pin and the ground. A 1uF capacitor is connected between the reference voltage pin and ground to improve the stability of the internal bias voltage and the PSRR of the amplifier. With the increase of this capacitance value, the improvement of PSRR is more obvious. However, if the capacitance value is too large, it will also increase the starting time and introduce the ticking sound phenomenon of the amplifier. The choice of the reference capacitance value depends on the following factors, including the requirements for the PSRR, the ticking phenomenon (which can be understood here as, the selection of the appropriate peripheral circuit), the system cost, and size constraints.

     Functional optimization to avoid the ticking phenomenon
    CST8002D/LM4863 / LM4871 / LM4890 contains make open or shut off the transient value or to minimize the tick and crackle of the circuit. The on-off mode in this discussion refers to the on-off mode of the power supply voltage. When the power supply voltage to the final value, gradually CST8002D/LM4863 / LM4871 / LM4890 internal amplifier is configured to unity gain buffer, internal current source load a voltage under the constraint of a linear way to BYPASS the pin. Theoretically, the input and output voltages vary with the voltage applied to the BYPASS pin. Until the voltage applied to the BYPASS pin rises to VDD/2, the gain of the internal amplifier remains generally stable. Once the voltage to the BYPASS pin is stable, the entire device is in full operation. Although the current of the BYPASS pin cannot be changed, a change in the reference capacitance value can change the device's on time and a lot of "clicks and pops". In any case, a trade-off arises from this phenomenon: as the reference capacitance increases, the opening voltage increases, and the relationship between the two is linear. Below are the different reference capacitance values and their corresponding opening times.

     In order to eliminate noise, all capacitors must be discharged before opening. Closing the switch quickly does not discharge the capacitor completely but causes "ticking and popping noises". In single-terminal operation mode, the output is connected to the load via COUT, usually this capacitor is larger, and it is discharged through an internal resistance of 20kΩ. The duration of the discharge time of COUT depends on its size. In order to reduce the influence of the transient response in single-terminal mode, an external resistance of 1kΩ ~ 5kΩ in parallel with the internal 20kΩ resistance is configured, but this resistance will increase the static current.

    To sum up, it is necessary to strike a balance between the start time of the device and the "click and crack sound" of the switching machine, so as to select the appropriate capacitor value between the BYPASS and the ground.