Class-D Current Sensing Calibration Using a Common-Mode Pilot Tone
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Solution Overview
Problem
Class-D driver stages in audio devices are sensitive to common-mode leakage current, leading to significant non-zero sense voltage even in the absence of a transducer, which affects accurate current measurement.
Innovation Solution
Incorporating a pilot tone injection circuitry into the differential pulse-width modulation driving signal at a specific frequency to isolate and compensate for common-mode leakage by modulating the common-mode offset of the triangular waveform, allowing the sense resistor to capture common-mode leakage gain at the pilot tone frequency, separate from the desired signal frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a Class-D driver stage with sense resistor is used to measure output current, then current measurement capability is provided, but common-mode leakage current causes significant non-zero sense voltage even in the absence of transducer
Solution Approach 1:
The patent converts the harmful common-mode leakage current into a measurable signal by modulating it with a pilot tone. The leakage current, which normally causes measurement errors, is now transformed into a calibrated reference signal that can be used to compensate for measurement inaccuracies. The system measures the sense voltage at the pilot tone frequency to determine common-mode leakage gain, then uses this information to correct the output current measurement.
Solution Approach 2:
The patent introduces a pilot tone as an intermediary signal to mediate between the common-mode leakage current and the measurement system. The pilot tone modulation creates a distinct frequency component that allows the system to separately measure and compensate for leakage effects without interfering with the normal audio signal path. This intermediary approach enables the system to isolate and characterize the leakage behavior.
2Measurement precision
If sense resistor is placed in the Class-D output stage to sense complete output current, then full current sensing capability is achieved, but the topology becomes highly sensitive to common-mode leakage
Solution Approach 1:
The patent transforms the sensitivity to common-mode leakage from a disadvantage into a useful characteristic. By intentionally modulating the common-mode voltage with a pilot tone, the system creates a measurable response at a specific frequency that directly reveals the leakage behavior. This allows the system to quantify and compensate for the sensitivity rather than merely suffering from it.
Solution Approach 2:
The patent changes the parameter of common-mode voltage by modulating it with a pilot tone at a specific frequency. This parameter change creates a time-varying common-mode voltage that produces a corresponding variation in the sense voltage at the pilot tone frequency. By measuring this frequency-specific response, the system can determine the common-mode leakage gain and use it to correct measurements.
3Measurement precision
If pilot tone injection is used to calibrate common-mode leakage, then measurement accuracy is improved, but device complexity increases due to additional circuitry
Solution Approach 1:
The patent implements multi-functionality by using the same sense resistor and measurement circuitry for both audio signal monitoring and pilot tone-based leakage calibration. The sense resistor serves dual purposes: sensing the audio output current and measuring the common-mode leakage response. The measurement circuit processes both audio-frequency and pilot tone-frequency signals, eliminating the need for separate calibration hardware and reducing overall system complexity.
Solution Approach 2:
The system performs self-calibration by using its own existing components (sense resistor, measurement circuitry) to characterize and compensate for its own leakage behavior. The pilot tone injection and measurement process is integrated into the normal operation of the system, allowing it to automatically determine its common-mode leakage gain without requiring external calibration equipment or separate adjustment procedures.
Data Source
AI summary
A system may include a Class-D stage comprising a first high-side switch coupled between a supply voltage and a first output terminal of the Class-D stage, a second high-side switch coupled between the supply voltage and a second output terminal of the Class-D stage, a first low-side switch coupled between a ground voltage and the first output terminal, and a second low-side switch coupled between the ground voltage and the second output terminal. The system may also include current sensing circuitry comprising a sense resistor, such that an output current through a load coupled between the first output terminal and the second output terminal causes a first sense voltage proportional to the output current across the sense resistor. The system may additionally include a modulator for generating a differential pulse-width modulation driving signal to the first high-side switch, the second high-side switch, the first low-side switch, and the second low-side switch and pilot tone injection circuitry configured to inject a periodic pilot tone into the differential pulse-width modulation driving signal at a pilot tone frequency.