A 0.96mA quiescent current, 0.0032% THD+N, 1.45W Class-D audio amplifier with area-efficient PWM-residual-aliasing reduction

Low quiescent current (I_Q) is critical for Class-D audio amplifiers in mobile devices to extend battery usage time [1], since typical audio signals have a high crest factor of 10 to 20dB. In addition, low distortion is also important for audio fidelity. Distortion sources in closed-loop Class-D amplifiers can be classified into two types. One is attributed to the nonlinearities of PWM modulators and power stages, while the other is due to the aliasing of fed-back PWM high-frequency residuals, the latter of which comprises phase-error and duty-cycle-error distortions [2]. Figure 3.6.1 shows 2^nd-order closed-loop amplifiers and existing techniques for enhancing an amplifier's linearity. Increasing the loop filter order to obtain a higher in-band loop gain by using more integrators [3] or the single-amplifier-biquad [4] suppresses all aforementioned distortions except for the phase-error distortion, which can be suppressed by adding a phase-error-free PWM modulator [2]. However, these techniques increase I_Q and/or die area due to the additional active circuits and/or several resistors and capacitors. Since phase-error distortion, as well as duty-cycle-error distortion, is caused by the fed-back PWM high-frequency residuals aliasing with the reference triangular wave V_TRI, a uniform PWM [5] with a sample-and-hold circuit implemented before the PWM modulation reduces the PWM residuals via an equivalent notch filtering. However, loop stability is affected by the notch filtering unless the PWM switching frequency f_SW is increased, but doing so increases power consumption [4]. Though the technique in [1] uses a feed-forward path with a replicated loop filter to eliminate the PWM residuals without affecting loop stability, the replicated loop filter increases both I_Q and area.