Differential Load Current Sensing for Multi-Phase Class D Outputs
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Solution Overview
Problem
Current sensing in electronic circuitry, particularly in Class D speaker drivers, is complicated by multi-phase driving voltages, making it challenging to accurately measure current flow through loads like miniature speakers, which can fail due to temperature issues outside a specified range.
Innovation Solution
The solution involves placing sense resistors in series with the sources of transistors in the driving branches and using a differential low-pass filter to integrate the voltage differences across these resistors, eliminating the need for phase timing knowledge by leveraging the equality of current 'lost' during certain phases, thus accurately sensing current flow through the load.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sense resistors are placed in series with the load to measure current, then current measurement capability is improved, but measurement precision deteriorates due to multi-phase driving voltages causing current mismatch between sense resistors and load
Solution Approach 1:
The patent introduces an intermediary calculation process that uses the known relationship between phase voltages and currents to compute the actual load current from sense resistor measurements. Instead of directly measuring load current, the system measures sense resistor voltages during specific phases and calculates the load current using the known multi-phase driving pattern, effectively using the phase information as an intermediary to resolve the measurement mismatch.
Solution Approach 2:
The patent applies preliminary action by pre-establishing the relationship between sense resistor currents and load currents for each phase of the multi-phase driving scheme. The system is designed to know in advance which sense resistor measurements correspond to which load current components, allowing accurate reconstruction of total load current through predetermined calculation steps rather than requiring real-time complex measurements.
2Power
If multi-phase driving voltages are used to drive the load, then power delivery capability is improved, but measurement precision deteriorates because current through sense resistors does not correspond to current through load
Solution Approach 1:
The patent segments the current measurement problem by measuring sense resistor voltages during different phases separately and then combining these segmented measurements to reconstruct the total load current. Instead of attempting to measure the complex multi-phase current directly, the system divides the measurement into discrete phase-specific measurements that are then mathematically combined, allowing accurate power delivery while maintaining measurement precision.
3Temperature
If external sense resistors are used to monitor operating current, then temperature monitoring capability is improved, but device complexity increases due to the need to account for multiple phases when measuring sense resistor voltages
Solution Approach 1:
The patent applies self-service by using the existing multi-phase driving signal itself to enable accurate current measurement. The system leverages the known phase relationships and timing of the driving voltages to automatically determine when to sample each sense resistor and how to combine the measurements, eliminating the need for external complex phase-detection circuitry. The driving signal provides its own measurement framework.
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
This approach simplifies current sensing by providing a robust and accurate method to monitor load current across all phases, even with multi-phase driving voltages, effectively preventing speaker failure due to overheating.
Implementation Method 1
one or more external sense resistors may be placed in series with the load, and the voltage across such sense resistors may in turn be detected
Implementation Method 2
using a differential low-pass filter to integrate the voltage differences across these resistors
Data Source
AI summary
Techniques for sensing current delivered to a load by a differential output stage, e.g., in a Class D amplifier. In one aspect, voltages across sense resistors coupled in series with first and second branches of the differential output stage are low-passed filtered and digitized. The sense resistors may be coupled in series with the sources of transistors of the first and second branches, wherein the transistors are selectively switchable on and off by input voltage driving voltages. The input driving voltages may correspond to a ternary voltage waveform such that during a given phase, the two transistors coupled in series with the sense resistors may be turned off. Further aspects provide for the first and second branches having cascoded NMOS and/or PMOS transistors, and the sense resistors being provided between a pair of cascoded transistors.