9.3.6 Adjustable Amplifier Gain and Switching Frequency Selection

The voltage divider between the GVDD_REG pin and the SPK_GAIN/FREQ pin is used to set the gain and switching frequency of the amplifier. Upon start-up of the device, the voltage presented on the SPK_GAIN/FREQ pin is digitized and then decoded into a 3-bit word which is interpreted inside the TAS5782M device to correspond to a given gain and switching frequency. In order to change the SPK_GAIN or switching frequency of the amplifier, the PVDD must be cycled off and on while the new voltage level is present on the SPK_GAIN/FREQ pin.

Because the amplifier adds gain to both the signal and the noise present in the audio signal, the lowest gain setting that can meet voltage-limited output power targets should be used. Using the lowest gain setting ensures that the power target can be reached while minimizing the idle channel noise of the system. The switching frequency selection affects three important operating characteristics of the device. The three affected characteristics are the power dissipation in the device, the power dissipation in the inductor, and the target output filter for the application.

Higher switching frequencies typically result in slightly higher power dissipation in the TAS5782M device and lower dissipation in the inductor in the system, due to decreased ripple current through the inductor and increased charging and discharging current in device and parasitic capacitances. Switching at the higher of the available switching frequencies will result in lower overall dissipation in the system and lower operating temperature of the inductors. However, the thermally limited power output of the device can be decreased in this situation, because some of the TAS5782M device thermal headroom will be absorbed by the higher switching frequency. Conversely inductor heating can be reduced by using the higher switching frequency to reduce the ripple current.

Another advantage of increasing the switching frequency is that the higher frequency carrier signal can be filtered by an L-C filter with a higher corner frequency, leading to physically smaller components. Use the highest switching frequency that continues to meet the thermally limited power targets for the application. If thermal constraints require heat reduction in the TAS5782M device, use a lower switching rate.

The switching frequency of the speaker amplifier is dependent on an internal synchronizing signal, (f_SYNC), which is synchronous with the sample rate. The rate of the synchronizing signal is also dependent on the sample rate. Refer to Table 13 below for details regarding how the sample rates correlate to the synchronizing signal.

Table 14 summarizes the de-code of the voltage presented to the SPK_GAIN/FREQ pin. The voltage presented to the SPK_GAIN/FREQ pin is latched in upon startup of the device. Subsequent changes require power cycling the device. A gain setting of 20 dB is recommended for nominal supply voltages of 13 V and lower, while a gain of 26 dB is recommended for supply voltages up to 26.4 V. Table 14 shows the voltage required at the SPK_GAIN/FREQ pin for various gain and switching scenarios as well some example resistor values for meeting the voltage range requirements.