Class-D Amplifier Current Sensing Using Sample-and-Hold Timing
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Solution Overview
Problem
Class-D amplifiers face challenges in continuous current sensing due to rail-to-rail voltage swings, which interfere with normal operation and are not adequately addressed by existing sensing circuits that require external components, degrading efficiency and causing issues with common mode signals.
Innovation Solution
A current sensing apparatus for class-D amplifiers utilizing a sample-and-hold circuit, current generator, and current-to-voltage converter, with scaled sense transistors and switches, that samples voltages on output terminals when low-side NMOS transistors are actuated, generating sense currents and converting them to a sense voltage without external components, thereby overcoming the limitations of existing solutions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external sense resistors are placed in series with the load to detect impedance, then current sensing capability is improved, but amplifier efficiency is degraded and common mode signal issues are created
Solution Approach 1:
The patent extracts the current sensing function from external components and relocates it entirely within the integrated circuit. By using internal sense transistors (Q9, Q10) and circuitry to mirror and measure the current through the H-bridge transistors, the design eliminates the need for external sense resistors, thereby maintaining amplifier efficiency while achieving accurate current sensing for impedance detection.
Solution Approach 2:
The internal current sensing circuit serves multiple functions: it detects load current for impedance monitoring, protects against overdriving the speaker, and operates within the existing H-bridge architecture without requiring separate external sensing components. This multi-functional approach integrates measurement and protection capabilities within the amplifier itself.
2Measurement precision
If continuous current sense measurements are performed in class-D amplifiers, then measurement accuracy is improved, but normal amplifier operation is interfered with due to rail-to-rail voltage swings
Solution Approach 1:
The patent implements periodic sampling of the current sense measurements at specific moments when the H-bridge transistors are switched on, rather than attempting continuous measurement during rail-to-rail voltage swings. This periodic sampling approach captures sufficient current information for impedance detection while avoiding interference with the normal switching operation of the class-D amplifier.
Solution Approach 2:
The circuit performs current sensing measurements at predetermined moments in the switching cycle when the low-side transistors are actuated and voltage conditions are favorable. By selecting specific sampling instants before the rail-to-rail swings occur, the circuit achieves accurate measurements without disrupting the amplifier's normal operation.
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 proposed solution enables accurate and efficient current sensing in class-D amplifiers, improving linearity performance and reducing total harmonic distortion, as demonstrated by improved THD measurements, without interfering with the normal operation of the amplifier.
Implementation Method 1
a sample-and-hold (S/H) circuit that samples a voltage on each of the pair of output terminals of the class-D amplifier when its associated low-side NMOS transistor is actuated
Implementation Method 2
the current generator uses the voltages on the pair of output terminals sampled by the S/H circuit to minor the drain-source voltages of the pair of low-side NMOS transistors on a pair of sense transistors
Implementation Method 3
wherein the drain-source voltages mirrored on the pair of sense transistors generates a pair of sense currents
Implementation Method 4
a current-to-voltage (I-to-V) converter that is coupled to the current generator so as to convert the sense currents to a sense voltage
Data Source
AI summary
Recently, there has been an increased desire to measure load currents of class-D amplifiers to improve performance. The traditional solution has been to include one or more discrete components in series with the load, but this degrades performance. Here, however, circuit is provided (which includes sample-and-hold circuit) that accurately measures load currents without inhibiting performance and that is not inhibited by the phase differences between the load voltage and load current.