The LM4991LD/NOPB: That 3W figure is the max output power per channel at 10% THD — in a real portable speaker or alert tone circuit, expect clean output up to about 2W before distortion climbs noticeably. The 8-WSON package with exposed pad (4x4mm footprint) pulls heat through the PCB land pattern. For sustained 3W output into 3Ω, the thermal pad needs a decent copper pour and vias to an inner ground plane — otherwise the die temperature climbs past the 85°C operating limit on a warm day. The shutdown pin lets a GPIO or a power-management IC kill the amplifier current draw when no audio is needed, which matters for battery life in a portable device.
Depop, shutdown, thermal protection — what each feature actually does
Depop (pop-reduction circuitry) suppresses the audible thump when the amplifier turns on or off — without it, the DC offset at the output couples through the speaker and sounds like a relay clicking. The shutdown feature pulls the quiescent current to near zero, so a microcontroller can gate the amplifier between audio bursts without draining the battery. Thermal protection folds back the output drive if the die temperature exceeds the safe limit, preventing destruction from a sustained shorted output or inadequate heatsinking.