Class D Amplifier Current Sensing With Replica Loops and Ripple Filtering
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Solution Overview
Problem
Current solutions for sensing the output current of switching amplifiers, such as Class D amplifiers, face challenges including the need to avoid external components for efficiency, address voltage levels above the supply or below ground, and achieve high precision and accuracy while filtering ripple in the bridge current.
Innovation Solution
The solution involves a replica loop circuit integrated within the amplifier to sense scaled currents from output inverters, using a current-to-voltage converter and auto-zero offset technique, along with a sample-and-hold mechanism to cancel ripple and buffer the signal for measurement, thereby eliminating the need for external components and enhancing precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external sense resistance is used for current sensing, then current measurement is achieved, but efficiency loss occurs and additional external components are required
Solution Approach 1:
The sensing function is extracted from external components and integrated into the amplifier circuit itself through replica loop circuits that sense currents internally, eliminating the need for external sense resistance and associated efficiency losses
Solution Approach 2:
The output inverter stages serve dual purposes: driving the load and providing current sensing through integrated replica loops, combining amplification and sensing functions within the same circuit blocks
2Measurement precision
If external sense resistance is used for current sensing, then current measurement is achieved, but additional external components are required
Solution Approach 1:
The sensing circuitry is merged with the amplifier circuit by implementing replica loop circuits within the amplifier itself, combining the amplification and sensing functions into a single integrated device without external components
3Productivity
If ripple is present on bridge current, then switching operation is achieved, but measurement accuracy deteriorates
Solution Approach 1:
A sample-and-hold circuit is employed that samples the sensed current at specific periodic intervals synchronized with the switching cycle, capturing the mean value while rejecting the periodic ripple components
Solution Approach 2:
An offset cancellation procedure uses feedback to measure and compensate for ripple components, subtracting them from the total sensed current to isolate the mean load current value
4Measurement precision
If offset cancellation is implemented for high accuracy, then measurement precision is improved, but circuit complexity increases
Solution Approach 1:
Offset cancellation is performed preliminarily through auto-zeroing techniques that pre-measure and store offset values before actual current sensing, eliminating the need for complex real-time offset compensation circuits
Data Source
AI summary
A switching amplifier, such as a Class D amplifier, includes a current sensing circuit. The current sensing circuit is formed by replica loop circuits that are selectively coupled to corresponding output inverter stages of the switching amplifier. The replica loop circuits operated to produce respective replica currents of the output currents generated by the output inverter stages. A sensing circuitry is coupled to receive the replica currents from the replica loop circuits and operates to produce an output sensing signal as a function of the respective replica currents.