PWM Feedback ADC for Low-Distortion Class-D Modulators
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Solution Overview
Problem
Class D amplifier circuits with digital PWM modulators face distortion issues due to analogue effects like transistor on-resistance, rise and fall times, propagation delays, and power supply ripples, which require closed-loop operation with negative feedback to suppress distortion, but this increases circuit complexity and area requirements.
Innovation Solution
Implementing a PWM-based ADC in the feedback path with a controller that phase and frequency matches the PWM carrier of the ADC to the output PWM signal, allowing for reduced filtering needs and smaller anti-alias filters, thereby minimizing distortion and circuit size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If closed-loop operation with negative feedback is used to suppress distortion, then distortion is reduced, but circuit complexity and area requirements increase
Solution Approach 1:
The patent changes the operating parameters of the feedback system by using PWM-based ADC that matches the PWM carrier frequency and phase, enabling the feedback loop to operate effectively with reduced filtering requirements and smaller circuit area while maintaining distortion suppression performance
2Reliability
If closed-loop operation with negative feedback is used to suppress distortion, then distortion is reduced, but circuit area requirements increase
Solution Approach 1:
The patent modifies the feedback system parameters by implementing PWM-based ADC with carrier matching, which reduces the need for large anti-alias filters and other filtering components, thereby reducing the overall circuit area while maintaining effective distortion suppression through closed-loop operation
Solution Approach 2:
The patent extracts and eliminates the need for large anti-alias filters and extensive filtering components from the circuit by using PWM-based ADC that processes the feedback signal directly in the PWM domain, reducing circuit area while maintaining distortion suppression capabilities
3Measurement precision
If large filters are used in the feedback path, then filtering performance is improved, but circuit area and feedback loop delay increase
Solution Approach 1:
The patent replaces the traditional mechanical/filter-based anti-alias filtering system with a PWM-based digital processing approach, where the PWM ADC directly processes the feedback signal in the PWM domain, eliminating the need for large physical filters while maintaining effective filtering performance and reducing circuit area and delay
4Measurement precision
If large filters are used in the feedback path, then filtering performance is improved, but feedback loop delay increases
Solution Approach 1:
The patent replaces large physical filters with PWM-based digital processing in the feedback path, which processes signals in the PWM domain without requiring long signal paths through multiple filter stages, thereby maintaining filtering performance while significantly reducing feedback loop delay
Data Source
AI summary
This application describes time-encoding modulator circuitry (200), and in particular a PWM modulator suitable for use for a class-D amplifier. A forward signal path receives a digital input signal (Din) and outputs an output PWM signal (Sout) and includes a first PWM modulator (101). A feedback path provides feedback to an input of the first PWM modulator (101). The feedback path includes an ADC (203) which receive a first PWM signal (Sa) derived from the output PWM signal. The ADC (203) includes a second PWM modulator (401) which generates a second PWM signal (Sb) based on the first PWM signal. A controller (201) controls the second PWM modulator such that a PWM carrier of the second PWM signal is phase and frequency matched to a PWM carrier of the output PWM signal.