The XSMT pin can also be used to monitor a system voltage, such as the 24-VDC LCD TV back light, or 12-VDC system supply using a voltage divider created with two resistors. See Figure 45.
|*||If the XSMT pin makes a transition from “1” to “0” over 6ms or more, the device switches into external undervoltage protection mode. This mode uses two trigger levels.|
|*||When the XSMT pin level reaches 2 V, soft mute process begins.|
|*||When the XSMT pin level reaches 1.2 V, analog mute engages, regardless of digital audio level, and analog shutdown begins. (DAC and related circuitry powers down).|
A timing diagram describing this is shown in Figure 46.
The XSMT input pin voltage range is from –0.3 V to DVDD + 0.3 V.The ratio of external resistors must produce a voltage within this input range. Any increase in power supply (such as power supply positive noise or ripple) can pull the XSMT pin higher than DVDD +0.3V.
For example, if the TAS576xM is monitoring a 12 V input, and dividing the voltage by 4, then the voltage at XSMT during ideal power supply conditions is 3 V. A voltage spike higher than 14.4 V causes a voltage greater than 3.6 V (DVDD+0.3) on the XSMT pin, potentially damaging the device. Providing the divider is set appropriately, any DC voltage can be monitored.