Diagnosing Automotive Faults with Class D Audio Amplifiers(8)

Continuous Diagnostics

The MAX13300/MAX13301/MAX13302 constantly monitor critical performance and safety parameters such as output offset voltages, output clipping, thermal faults, and under- and overvoltage conditions. The results are reported and continuously updated in the status registers (STAT and OSTAT_).

 

Offset Diagnostic

Run the offset diagnostic to determine if there is an offset between the differential outputs. To run this diagnostic, place the device in play mode and apply no input signals. The results of this diagnostic are reported in the OSTAT3./VOS[3:0] (offset voltage indicator) bits. VOS_ indicates whether the offset voltage is less or greater than the offset voltage threshold. The threshold is typically 800mV for the MAX13300/MAX13302 and typically 1.0V for the MAX13301.

 

Clipping Diagnostic

Use the clipping diagnostic to detect clipping outputs. Program the CTRL1.CLVL[1:0] (clip level) bits for a threshold of either 2%, 4%, 6%, or 10% THD. Clip indication is provided by the OSTAT0./CLIP[3:0] (clip indicator) bits. These bits are set to 1 only during the times when an overdriven output is actually clipping. A clipping output indictor is available for each output; OSTAT0./CLIP[3] is for output 3, etc. The open-drain outputs, active-low CL0, and active-low MUTE_CL1, also provide clip indication.

 

Thermal Warning Diagnostic

If the junction temperature exceeds the programmed temperature limit, then a temperature fault is set immediately. The device does not act upon a temperature warning to the programmed limit. The temperature warning self-clears when the temperature drops below the threshold. If the junction temperature exceeds the maximum junction temperature of +150°C, the device disables all channels. The digital interface remains active and the contents of the registers are unchanged. When the die temperature drops below +140°C, normal operation is restored. The programmed temperature limit is set by software from +110°C to +140°C in 10°C increments.

 

Charge-Pump Undervoltage Diagnostic

The MAX13300/MAX13301 drive the high-side FETs with the aid of a charge pump. The charge pump charges the hold capacitor, C_CHOLD, to 5V at the end of each switching cycle. When the voltage on C_CHOLD falls below 3.45V, the device asserts the STAT./CPUV (charge pump undervoltage indicator) bit and three-states all outputs. The device deasserts the bit only after voltage on the hold capacitor rises above 3.75V.

 

Undervoltage Diagnostic

An undervoltage monitor detects low voltages on PVDD (< 6V). During an undervoltage condition, the device three-states all outputs, sets the STATE./UV (undervoltage indicator) bit to 0, and asserts the open-drain output active-low FLT_OT.

 

Overvoltage Diagnostic

The MAX13300/MAX13301/MAX13302 detect overvoltage and load-dump conditions on PVDD and protect the DMOS devices from damage. During an overvoltage condition, the device sets the STAT./OV (overvoltage indicator) bit to 0, asserts the open-drain output active-low FLT/OT, and is latched into standby mode. All differential outputs are regulated to ½ VPVDD to minimize the drain-source voltage of the low- and high-side FETs and to prevent breakdown. Once the overvoltage condition is removed, bring the device out of standby mode by clearing the CTRL.STBY (standby) bit. The MAX13300/MAX13301 can withstand 50V load-dump voltage spikes. Battery charger voltages from 26V to 35V can be withstood for up to 1 hour. Figure 7 illustrates the behavior of the device during a load dump.

 

Conclusion

The diagnostics capabilities built into the MAX13300/MAX13301/MAX13302 devices allow the devices to meet OEM diagnostic requirements. Flexibility built into specific tests such as load and tweeter diagnostics allow OEM-specific thresholds to be used.