4 Achieving RTD

To detect a positive output short to a negative output when no audio is present, the audio source (codec or processor) must mix in a baseline signal with the voice or audio signal (Figure 3). As an example, a baseline 5Hz 200mV peak-to-peak signal is well below the audio frequency range (20Hz to 20kHz) and has enough amplitude (200mV peak to peak) to produce the required excitation to detect a positive output short to a negative output.

Applying a low-frequency baseline signal to the input of the TAS5431-Q1ensures that the device will hit its overcurrent limit when a positive output shorts to a negative output. The diagnostic assessment mode integrated into the TAS5431-Q1will then detect the positive and negative voltages and report the fault in the register map. Simply implementing this waveform changes the summary of diagnostics, as shown in Table 2.

Implementing a current-sense resistor, a current-sense amplifier, and a resistor-capacitor (RC) filter (Figure 4) in conjunction with the same 5Hz, 200mV peak-to-peak baseline signal enables the system to detect a speaker or open-load condition.

The baseline signal generates current through the 50mΩ current-sense resistor, regardless of whether audio is present, which the INA180-Q1 converts to a proportional voltage. Although the baseline signal is bipolar (goes both positive and negative), the unidirectional nature of the INA180-Q1 automatically half-wave-rectifies the output voltage and vastly reduces the post-filtering requirements to a simple RC filter. The resulting DC output from the RC filter is driven into a microcontroller general-purpose input/output or analog-to-digital converter (ADC) for analysis (Figure 5). During an open-load failure, current will cease to flow through the current-sense resistor, forcing the output of the INA180-Q1 to 0 V. The INA180-Q1 output crossing a pre-determined threshold triggers an alert to disconnect the speaker (Figure 6).

Final verification of the circuit (Figure 7) with audio superimposed on the baseline signal yields the intended results during normal operation. The INA180-Q1 converts load current traveling through a 50mΩ resistor to a half-wave-rectified voltage. The post-filtered result is a DC voltage of 1V, which is greater than the pre-selected threshold. All designs must be tuned to the intended load characteristics and the INA180-Q1 is available in several additional gain options. The INA180-Q1 also features a large common-mode input voltage range (26 V), ensuring that no damage will occur during a shorted battery fault, which typically induces up to 16 V on the lines.

With the additional circuitry, the TAS5431-Q1 is finally able to achieve full RTD, as shown in Table 3.