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An Ultralow-Power Overcurrent Protection Circuit for Micropower Class D Amplifiers

Although class D amplifiers (CDAs) are highly advantageous over their linear amplifier counterparts in terms of power efficiency, their power efficiency remains undesirably low at nominal operation conditions where the output power is ~26 dB lower than its peak output power (due to the large crest factor of the audio/speech signal and headroom for adjustment). This is particularly the case for power-critical micropower applications such as hearing instruments. At nominal conditions, we find that the overcurrent protection circuit (for the output stage) is unexpectedly the most power-dissipative block in micropower CDAs. In this brief, we propose a novel ultralow-power overcurrent protection circuit, which features 67% lower power dissipation compared to conventional overcurrent protection circuit without compromising the IC area. To further verify the advantages of the proposed overcurrent protection circuit, an ultralow-power bang-bang CDA is designed. We show that, by employing the proposed overcurrent protection circuit, the power efficiency of the bang-bang CDA is significantly improved from 10.5% to 23.7% for a 64-Ω load and from 1.8% to 4.7% for a 400-Ω load.