9 Protection

Protection features in Class-D audio amplifiers cover everything from electricity to heat. A standard protection feature is over-temperature shutdown, in which the amplifier turns off if the temperature rises beyond a set threshold. At too high of a temperature, the IC could sustain permanent damage or even present a safety hazard if it combusts. Heat is a common point of contention in audio systems, since even the best Class-D amplifier are only around 90% efficient. This means roughly 10% of the power dissipates into the system as EMI and heat. Put this in a confined space and the problem is obvious, especially for applications like portable wireless speakers which may be exposed to the sun. Advanced thermal management systems like thermal foldback automatically dial back the gain over time as the temperature increases, allowing the device to cool off. Instead of overheating and shutting off, the volume is decreased to continue playing.

Along the same lines, some devices implement cycle-by-cycle current control (CB3C) as opposing to latching-shutdown. Latching-shutdown performs an immediate shut down of output in the case of excess output current. Usually devices featuring CB3C offer both options. Since CB3C limits the output current for each switching cycle, it prevents premature shutdown due to temporarily high output current. This may be caused by high level music transients and a drop of the real speaker’s load impedance. To understand CB3C in more detail, visit section 9.4.1.1 of the TPA3244 datasheet, which explains this feature in depth.

Under-voltage and over-voltage protection are two other common protection features that protect the device in the case of surges, brownouts, or even power-up/down. A less self-explanatory protection feature is DC speaker protection. This feature is not as common, but effectively a DC protection scheme prevents a speaker from excess DC current in case one speaker terminal is accidentally shorted to the chassis ground while the other is connected to the amplifier. This could occur if the end user assumes that the negative speaker output terminal is the same as ground. In BTL configuration, this terminal-to-ground short would result in a DC voltage of PVDD/2 across the speaker, which could destroy the speaker. DC speaker protection detects any imbalance between the two outputs of a bridge-tied load and shuts down in the event of an imbalance exceeding a programmed threshold. Speaking of shorts, yet another important feature that protects the devices from permanent damage is pin-to-pin short circuit protection (PPSC). PPSC checks that there are no shorts at the output pins of the amplifier during startup. These protection features are not only useful, but can even save PCB space and BOM cost since they are integrated into the amplifier as opposed to external protection circuitry.