9.3.9 Battery Tracking AGC

The TAS2555 device monitors battery voltage and the audio signal to automatically decrease gain when the battery voltage is low and audio output power is high. This provides louder audio while preventing early shutdown at end-of-charge battery voltage levels. The battery tracking AGC starts to attenuate the signal once the voltage at the Class-D output exceeds V_LIM for a given battery voltage (VBAT). If the Class-D output voltage is below the V_LIM value, no attenuation occurs. If the Class-D output exceeds the V_LIM value the AGC starts to attack the signal and reduce the gain until the output is reduced to V_LIM. Once the signal returns below V_LIM plus some hysteresis the gain reduction decays. The V_LIM is constant above the user configurable inflection point. Below the inflection point the V_LIM is reduced by a user configurable slope in relation to the battery voltage. The attack time, decay time, inflection point and V_LIM/VBAT slope below the inflection point are user configurable. The parameters for the Battery Tracking AGC are part of the DSP core and can be set using thePurePath Console 3 Software TAS2555 Application software for the TAS2555 device part under the Device Control Tab. Below a VBAT level of 2.9 V, the boost will turn on to ensure correct operation but results in increased current consumption. The device is functional until the set brownout level is reached and the device shuts down. The minimum brownout voltage is 2.7 V.

Figure 24. VLIM versus Supply Voltage (VBAT)