5 Additional considerations

Having achieved RTD with the TAS5431-Q1, there are two additional design considerations to review. The first is fault recovery. When a fault scenario is active, the Class-D amplifier should protect itself and any connected circuitry from damage, while also diagnosing the fault. Once the fault is removed, the device is expected to immediately detect that the fault has been removed and continue to play audio (if audio is being applied to the input). The TAS5431-Q1achieves fault recovery by continuously running its 229ms diagnostic cycle on repeat indefinitely. When a fault is removed, the diagnostic cycle determines that there is no longer a fault and allows the output stage to operate as normal. More details on this can be found in Section 7.3.5.1, “Load Diagnostics Sequence,” of the TAS5431-Q1 datasheet. The additional circuitry required to achieve RTD does not impact the device’s ability to recover from a fault and immediately play audio.

The second consideration is that, depending upon the total impedance of the load, the Class-D amplifier may have difficulty detecting the positive and negative output shorted condition. Each Class-D amplifier has an impedance threshold at which it will detect the positive output and negative output shorted together (a shorted speaker). In a given design, there may be a significant amount of impedance between the positive and negative outputs, especially when a short occurs close to a speaker, which can exceed the detection threshold of the Class-D amplifier. In vehicle systems, the cabling from the TCU module to the speaker (and back to the TCU module) can be as long as 10 to 12 meters. Figure 8 shows all the different impedances to consider.

Next, we will review an example analysis of how to consider the various impedances in the system and compare them to the TAS5431-Q1 detection threshold.

We can start by assuming 12 meters of 22AWG external cabling (0.053Ω/meter) totals 636mΩ of resistance, as shown in Equation 1:

(1)

Standard automotive connectors such as the Molex 34826-8160 specify approximately 20mΩ per connector, totaling 40mΩ for the two connectors.

For the inductors, it is important to find an automotive-grade, low direct current resistance (DCR) inductor such as the VAMV1009AA-220MM2 from Cyntec. The 22µH inductor has a maximum of 56mΩ, multiplied by 2 for each coil, totaling 112mΩ.

Finally, adding the 50mΩ for the current-sense resistor totals approximately 838mΩ of resistance between the positive and negative outputs, not including trace resistance.

According to the TAS5431-Q1 data sheet, the short-circuit detection threshold specification is 900 mΩ. Thus, to ensure the device identifies a short between the positive and negative outputs, the resistance between the two must be less than 900 mΩ. Based on the total calculated 838 mΩ of resistance, there is approximately 62 mΩ of trace impedance before the TAS5431-Q1may be susceptible to missing a short between the outputs.

Meeting the 900mΩ specification requires careful printed circuit board design and trace routing. You can also fine-tune other aspects of the design, such as using the “LC Filter Design” application report to select an alternative, lower-DCR inductor, or minimize the current-sense resistor value and select an INA180-Q1 with a larger gain setting.