Class-D Speaker Current Sensing via Low-Side Drive Transistors
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Solution Overview
Problem
Conventional smart amplifiers face limitations in current measurement accuracy and circuit area due to the addition of sense transistors and sense resistors, which cannot sense negative currents effectively and require additional power and components, leading to increased circuit complexity and reduced accuracy.
Innovation Solution
A class-D amplification system utilizing a sigma-delta analog-to-digital converter (ΣΔ ADC) with two digital-to-analog converters (DACs) and sense resistors inserted between low-side drive transistors and the reference voltage, allowing for accurate current measurement without the need for additional sense transistors and reducing circuit area by eliminating DAC settling time issues and accommodating common mode variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sense transistors and sense resistors are added to conventional smart amplifiers for current measurement, then current sensing capability is improved, but circuit area and complexity increase
Solution Approach 1:
The patent combines the current sensing function with the existing low-side drive transistors by inserting sense resistors in series with their source terminals. This merging approach allows the amplifier to measure current through its existing power delivery path without adding separate sensing circuits, thereby improving measurement capability while minimizing circuit complexity increase
Solution Approach 2:
The low-side drive transistors serve dual purposes: they continue to function as power switching elements while simultaneously acting as current sensing elements through the inserted sense resistors. This multi-functionality eliminates the need for dedicated sensing transistors, reducing overall circuit complexity while maintaining accurate current measurement capability
2Measurement precision
If sense transistors are added to sense negative currents, then current sensing capability is improved, but additional power and components are required
Solution Approach 1:
The amplifier's existing low-side drive transistors and power delivery path serve the additional function of current sensing, including negative current sensing. The sense resistors inserted in series with these transistors enable bidirectional current measurement without requiring separate sensing circuits, allowing the system to self-monitor all current flows including reverse currents without additional power expenditure on dedicated sensing components
3Reliability
If conventional current measurement methods are used, then current monitoring is achieved, but circuit area increases and accuracy is reduced
Solution Approach 1:
The patent segments the current measurement function into discrete sense resistors that are inserted at specific locations within the existing amplifier circuitry - specifically in series with the source terminals of the low-side drive transistors. This segmentation allows precise current sampling at the point of interest without requiring large dedicated sensing circuit areas, as each sense resistor is a compact element integrated into the existing power path
Solution Approach 2:
The sense resistors are nested within the existing amplifier circuit structure, specifically embedded in the current path through the low-side drive transistors. This nesting approach allows the sensing function to be contained within the existing circuit footprint without requiring additional external sensing components, thereby maintaining compact circuit area while achieving reliable current monitoring
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 system achieves improved current measurement accuracy with reduced circuit area, enabling real-time diagnostics and optimized power delivery to the speaker while eliminating the need for floating supplies and analog level shifters, thus enhancing sound pressure level and system efficiency.
Implementation Method 1
The sigma delta analog to digital converter is coupled to the bridge to measure voltage across the first sense resistor and the second sense resistor
Implementation Method 2
The first sense resistor is connected between a first of the low-side transistors and a low-side reference voltage. The second sense resistor is connected between a second of the low-side transistors and the low-side reference voltage
Data Source
AI summary
A class-D amplifier includes measurement of speaker current via the low-side drive transistors of the amplifier. In one embodiment, a class-D amplifier includes two high-side transistors, two low-side transistors, a first sense resistor, a second sense resistor, and a sigma delta analog to digital converter (ΣΔ ADC). The two high-side transistors and two low-side transistors are connected as a bridge to drive a bridge tied speaker. The first sense resistor is connected between a first of the low-side transistors and a low-side reference voltage. The second sense resistor is connected between a second of the low-side transistors and the low-side reference voltage. The ΣΔ ADC is coupled to the bridge to measure voltage across the first sense resistor and the second sense resistor.