Class D Amplifier Startup Circuit for Pop Noise Reduction
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Solution Overview
Problem
Class D amplifiers experience popping and clicking sounds during power up, power down, or abrupt changes due to the coupling of output signals with the speaker, which current solutions like muting and enable circuits complicate and cause audio signal interruptions or delays.
Innovation Solution
A method and circuit that increase the duty cycle of a pulse train to charge an output capacitor, reducing switching sounds by gating and filtering the signal, with the duty cycle adjustment occurring over a period corresponding to the filtering frequency to diminish switching sounds produced by the speaker.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If muting and enable circuits are used to reduce switching sounds, then switching sounds are reduced, but the system becomes more complicated and audio signal interruptions occur
Solution Approach 1:
The patent extracts the harmful switching sound generation mechanism by separating the audio signal path from the power-up/power-down transition path. During startup, the audio signal is blocked while the output capacitor charges through a dedicated startup circuit, preventing the coupling that causes popping sounds. This extraction approach reduces switching sounds without requiring complex muting circuits.
Solution Approach 2:
The patent applies preliminary action by pre-charging the output capacitor through a startup circuit before the audio signal is connected to the speaker. The duty cycle is gradually increased from 0% to 50% during this preliminary phase, ensuring the capacitor is charged to the appropriate voltage level before normal audio operation begins, thereby preventing popping sounds during power-up.
2Object-affected harmful factors
If muting and enable circuits are used to reduce switching sounds, then switching sounds are reduced, but audio signal interruptions and delays occur
Solution Approach 1:
The startup circuit performs the capacitor charging action in advance during the power-up sequence, gradually increasing the duty cycle from 0% to 50% before full audio operation begins. This preliminary charging prevents popping sounds without requiring post-startup muting or enable circuits that would cause audio signal interruptions or delays.
3Object-affected harmful factors
If the duty cycle is increased to charge the output capacitor, then switching sounds are reduced, but power dissipation increases
Solution Approach 1:
The patent applies partial action by increasing the duty cycle only to 50% during the startup phase, which is sufficient to charge the output capacitor to the required voltage level. This partial duty cycle increase provides enough charging current to prevent popping sounds while avoiding the excessive power dissipation that would result from using a 100% duty cycle, thus optimizing the balance between noise reduction and energy efficiency.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces switching sounds detectable by the human ear by minimizing power transfer to the speaker, ensuring that switching sounds do not correspond to the continuous signal, thereby improving the startup process of class D amplifiers without causing audio signal interruptions.
Implementation Method 1
driving an output signal from the gated signal to charge an output capacitor
Implementation Method 2
The filter translates the modified amplified signal back into a continuous signal. A typical filter is an LC filter, for example. The resulting amplified continuous signal may be provided to a speaker and translated into sound. The benefits of filters include minimizing electromagnetic interference ("EMI") and power dissipation in the amplified signal.
Data Source
AI summary
In one embodiment the present invention includes a method for starting up a class D amplifier. The method comprises increasing, gating, and driving. The increasing includes increasing a duty cycle of a pulse train from a first duty cycle to a second duty cycle. The gating includes gating a signal based on the increasing of the duty cycle. The gating results in a gated signal. The driving includes driving an output signal from the gated signal to charge an output capacitor. The output capacitor is coupled to a speaker. The increasing of the duty cycle contributes to the charging of the output capacitor such that switching sounds detectable by the human ear are reduced.