Class-D PWM Dual Feedback for Supply-Variation Stability
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Solution Overview
Problem
Class-D switching audio amplifiers with continuous-time sigma-delta modulators face stability issues and bimodal modulation artifacts due to variations in power supply voltage, particularly in battery-operated systems, which affect modulator stability and introduce distortion.
Innovation Solution
The implementation of a dual feedback architecture where the second feedback signal is derived from a regulated internal voltage, rather than the unregulated output stage voltage, reduces the dependency on output voltage amplitude, thereby stabilizing the loop and mitigating bimodal behavior.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If feedback is acquired directly from the output terminals in battery-operated systems, then the system can operate with unregulated supply voltage, but modulator stability deteriorates and bimodal modulation artifacts occur due to 2:1 power supply voltage variation
Solution Approach 1:
A voltage regulation circuit is introduced as an intermediary between the unregulated power supply and the modulator feedback path. This regulator mediates the voltage variations, providing a stable reference voltage to the feedback network while allowing the output stage to operate from the unregulated supply, thus maintaining both adaptability and stability
Solution Approach 2:
The feedback path is segmented into two separate paths: one path feeds the output voltage signal to the summing nodes of the integrators, while another path provides a regulated voltage reference. This segmentation allows the system to handle unregulated supply variations without compromising modulator stability
2Device complexity
If direct feedback from output terminals is used, then circuit complexity is reduced, but audio fidelity deteriorates due to bimodal modulation artifacts
Solution Approach 1:
A voltage regulator is inserted as an intermediary component in the feedback path to provide a stable reference voltage. This adds minimal complexity while effectively eliminating bimodal modulation artifacts and improving audio fidelity by preventing feedback signal distortion during voltage transitions
3Use of energy by moving object
If unregulated supply voltage is used in the output stage, then energy efficiency is improved for battery operation, but artifact generation increases due to amplitude variations affecting the feedback signal
Solution Approach 1:
The voltage regulator serves as a mediator that isolates the modulator core from unregulated supply variations. It allows the output stage to efficiently operate from unregulated voltage while providing a stabilized reference to the feedback network, thereby maintaining energy efficiency without generating bimodal artifacts
Solution Approach 2:
The system changes the voltage parameter stability by introducing regulation only where needed in the feedback path, rather than regulating the entire power supply. This selective parameter control maintains energy efficiency while eliminating the harmful amplitude variations that cause artifacts
Data Source
AI summary
This document discusses, among other things, a modulator including a first integrator configured to receive an input signal and a first feedback signal from an output stage, a second integrator configured to receive an output of the first integrator and a second feedback signal, and a comparator configured to be coupled to a regulated supply voltage, to receive an output of the second integrator and a modulation signal, and to provide a pulse width modulated representation of the input signal. The output stage is configured to be coupled to an unregulated supply voltage, and the second feedback signal can include a representation of an output of the comparator configured to reduce artifacts in the pulse width modulated representation of the input signal induced by changes in an amplitude of the unregulated supply voltage.