Diagnosing Automotive Faults with Class D Audio Amplifiers(4)

Short-to-Ground Diagnostic Procedure

This diagnostic detects a short to ground of less than 1kΩ on any output. The diagnostic for short to battery and ground is done at the same time and in the same diagnostic mode. None of the results are latched, so the OSTAT2 register must be read while running this diagnostic to obtain a valid status.

If the load is present, a short on either of the differential outputs results in a short on the other output. Therefore, the status register only indicates which channel's output is shorted and not which of its differential outputs is shorted. The I²C status register can indicate, for example, that output 1 is shorted to battery, but it cannot differentiate between an OUT1+ and OUT1- short to battery.

To determine if a short to ground or battery exists on any of the outputs, three-state all outputs through I²C. Run the short-to-ground/battery diagnostic by setting the CTRL3.SDET (short-to-ground/battery diagnostic enable) bit to 1. During the diagnostic, the device shunts a 3kΩ resistor across each differential output to ground and sources 2mA of current to each output from PVDD. Under normal conditions, 6V is developed at each output. An output voltage > 6V is interpreted as a short to battery. An output voltage < 6V is interpreted as a short to ground. Results of the diagnostic are reported in the OSTAT2.SBAT[3:0] (short-to-battery indicator) and OSTAT2.SGND[3:0] (short to ground) bits.

Because no latch is set, a short to ground or battery does not prevent the device from powering up. Therefore, the microcontroller can enable the device into a short, although this configuration is discouraged. Should the device be enabled into a short, the real-time overcurrent will latch the shorted channel off. The MAX13300/MAX13301 offer real-time protection for short to battery, short to ground, and shorted load to prevent damage to the device.

 

Step-by-Step Procedure (Figure 5)

1. The outputs must be in three-state mode with the charge pump active (CTRL2.STBY = 0). If starting from MUTE or PLAY mode, follow the normal shutdown procedure to prevent click-and-pop.
2. Enable the short-to-ground diagnostic by setting CTRL3.SDET = 1.
3. Wait a minimum of 200µs.
4. Read the OSTAT2.SGND[3:0] bits for results.
5. Disable short-to-ground diagnostics by setting CTRL3.SDET = 0.

Figure 5. Short-to-ground diagnostic procedure.