Switched Feedback Resistor Compensation for Class-D Slew Rate Limits
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Solution Overview
Problem
Class-D amplifiers with feedback loops face slew rate limitations, particularly in high-frequency switching applications, which can lead to distortions and noise, and existing solutions complicate the design by requiring modifications to the integrator stage to achieve higher slew rates.
Innovation Solution
A compensation current is provided at the output of the integrator stage using a compensation circuit with an inverter and a switched resistor, matching the magnitude of the feedback current, without modifying the integrator design, to alleviate slew rate limitations and reduce total harmonic distortion (THD).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the integrator stage is modified to provide higher slew rate, then the slew rate limitation is improved, but the design complexity increases
Solution Approach 1:
A compensation circuit is introduced as an intermediary element between the feedback path and the integrator output. This compensation circuit includes a compensation resistor and compensation capacitor that generate a compensation current to offset the feedback current, thereby improving the effective slew rate without modifying the integrator stage itself. The intermediary compensation circuit resolves the contradiction by providing slew rate enhancement while keeping the integrator design simple and unchanged.
2Speed
If high-frequency switching is implemented, then the switching speed is improved, but distortion and noise increase due to slew rate limitations
Solution Approach 1:
The compensation circuit applies preliminary anti-action by generating a compensation current that preemptively counteracts the harmful feedback current before it can cause distortion and noise. The compensation current is designed to match and oppose the feedback current waveform, thereby canceling out its detrimental effects on high-frequency switching performance. This allows high-frequency operation without the usual penalty of increased distortion and noise.
3Reliability
If the feedback current magnitude is increased, then the feedback effectiveness is improved, but the slew rate limitation worsens
Solution Approach 1:
The compensation circuit implements the anti-weight principle by introducing a compensation current that acts as a counterweight to the feedback current. The compensation current is generated with equal magnitude but opposite polarity to the feedback current, effectively canceling its negative impact on slew rate. This allows the feedback current to maintain its high magnitude for effective feedback while the compensation current simultaneously offsets its detrimental effect on switching speed, resolving the contradiction between feedback effectiveness and slew rate.
Data Source
AI summary
A class-D driver circuit includes a feedback loop including an input integrator stage, a switched modulator, and an output driver stage. A feedback resistor connects an output terminal of the output driver stage with an input node of the input integrator stage to provide a feedback current. The class-D driver circuit also includes a compensation circuit configured to provide a compensation current to an output node of the input integrator stage to relieve a slew rate limitation of the feedback loop, the compensation current having a magnitude based on the magnitude of the feedback current.