Class D Output Circuit With Square-Wave PWM and Stable PDM
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Solution Overview
Problem
Class D amplifiers using pulse width modulation (PWM) or pulse density modulation (PDM) suffer from distortion and noise due to deviations in carrier waveforms and variations in switching frequency, leading to suboptimal performance in output circuits.
Innovation Solution
The implementation of PWM and PDM output circuits that utilize a square wave signal instead of sawtooth or triangular waveforms, and adjustable reference voltages to stabilize switching frequencies, along with feedback mechanisms and dual-mode operation to support both PWM and PDM modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If sawtooth or triangular waveforms are used as carrier signals in PWM modulators, then the modulation process can be implemented, but distortion occurs due to deviations from ideal waveforms
Solution Approach 1:
The patent changes the waveform parameter from sawtooth/triangular to square wave, and changes the modulation approach from direct PWM comparison to PDM with variable duty cycle, thereby eliminating distortion caused by non-ideal carrier waveforms
Solution Approach 2:
The patent uses a simple square wave generator instead of complex sawtooth/triangular wave generators, sacrificing the ideal waveform shape for the benefit of using simple, easily generated square waves that are more robust to variations
2Ease of manufacture
If PDM modulators use fixed circuit components, then the design is simplified, but switching frequency varies widely due to IC process variations
Solution Approach 1:
The patent makes the duty cycle dynamic and variable rather than fixed, allowing the PDM modulator to adapt to process variations by adjusting the duty cycle to maintain stable switching frequency despite component tolerances
Solution Approach 2:
The patent changes the duty cycle parameter dynamically based on feedback, allowing the system to compensate for process variations and maintain stable switching frequency without requiring extremely precise fixed components
3Loss of energy
If class D amplifiers operate in triode and cut-off regions, then power efficiency is improved, but distortion and noise increase due to switching effects
Solution Approach 1:
The patent implements feedback mechanisms where the output signal is fed back to the PDM modulator to dynamically adjust the duty cycle, thereby compensating for switching distortion and noise while maintaining the high efficiency of class D operation
Solution Approach 2:
The patent uses an intermediate PDM modulation stage with variable duty cycle that acts as a buffer between the input signal and the class D amplifier switching, reducing the direct impact of switching distortion on the output
Data Source
AI summary
Output circuits using pulse width modulation (PWM) and/or pulse density modulation (PDM) are described. In one aspect, a PWM output circuit includes a PWM modulator that operates based on a square wave signal instead of a sawtooth or triangular wave signal. In another aspect, a PDM output circuit includes a PDM modulator that uses variable reference voltages to reduce variations in switching frequency. In yet another aspect, a dual-mode output circuit supports both PWM and PDM and includes a pulse modulator and a class D amplifier. The pulse modulator performs PWM on an input signal if a PWM mode is selected and performs PDM on the input signal if a PDM mode is selected. The class D amplifier receives a driver signal from the pulse modulator and generates an output signal.