3 Schematic and Design

The fault diagnostic circuitry presented within this application note is intended to address the four major types of faults which can occur on the speaker output lines. The circuitry provided is widely applicable and can be applied to virtually any amplifier circuit, provided the amplifier in question includes a manual shutdown control which drives the outputs of the amplifier to a high impedance state.

Comparators are used to measure the voltages on either side of the load against voltage thresholds set by the resistor dividers. Additionally, a pair of operational amplifiers serves to measure the differential voltage across the load. The output lines of the comparators are pulled low when the voltages pass the thresholds, indicating the occurrence of a fault condition. By examining the outputs of the pins, this is possible to determine which type of fault has occurred. The following truth table gives the various fault conditions indicated by various output states.

The various fault detection mechanisms are similar in design. During operation of the diagnostic circuit, the outputs of the amplifier are placed into a high impedance state. In the event the output circuit is shorted to PVDD, the voltages throughout the circuit can rise to the level of PVDD, provided the amplifier outputs are high impedance to prevent current flow. Likewise, a short to GND can pull all voltages in the circuit to ground, provided that the amplifier outputs are high impedance. These cases pull the voltages in the diagnostic circuit up or down, respectively, past the referenced voltage thresholds set by the resistor divider networks; this causes the corresponding comparator outputs to be pulled to ground. In the event of an open circuit, the pullup and pull-down resistances within the diagnostic circuitry pull both voltages past their thresholds, pulling both the PVDD and GND detection pins low. By contrast, the short-circuit diagnostics rely on a differential amplifier to compare the differential voltage across the load to a threshold voltage; the output of the corresponding pin is pulled low when the amplified differential voltage falls beneath this threshold.

Additionally, dual PMOS switching circuits (see Figure 3-2) were implemented to isolate the diagnostics from the speaker circuit when the speaker is in use. This decreases power consumption and prevents the speaker pop caused by the biasing within the diagnostic circuitry. This also isolates the diagnostics when not in use, preventing them from contributing additional THD and noise to the audio signals.