3 Summary

This application note addresses critical design considerations for Class-D amplifiers when driving highly inductive loads such as subwoofers, LRAs, haptic coils, and when using LC-EMI filters at the amplifier output. Highly inductive actuators exhibit significant phase lag between voltage and current, leading to two primary fault conditions: Boost Over-Voltage (OV) faults caused by reverse inductive kickback current, and Over-Current (OC) shutdowns due to insufficient Y-Bridge switching hysteresis.

This application note covers TI Class-D devices with integrated boost converters (like TAS2572, TAS2574, TAS2562, TAS2563 orTAS2564) and those with external PVDD supplies (like TAS2764, TAS2770, TAS2780 or TAS2781). It provides an electrical characterization of various actuator types ranging from microspeakers (≤200µH) to highly inductive subwoofers (≤50mH) and haptic coils.

Two practical solutions are presented for Boost OV mitigation: (1) Calculating and implementing sufficient boost capacitance to absorb inductive energy without exceeding OV thresholds, with a provided design calculator tool, and (2) Adding a Zener diode voltage clamp to sink reverse current. For Y-Bridge OC prevention, the note establishes minimum hysteresis requirements to ensure adequate current decay before bridge transitions.

The note also addresses OC issues when using LC-EMI filters on Class-D outputs, providing ripple current calculation methods and design trade-offs between Y-Bridge operation, filter component values, and system efficiency. These quantitative design guidelines enable robust amplifier operation across diverse inductive loads without triggering protection faults.