Pass Transistor Current Sensing With Auto-Zero Offset Compensation
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Solution Overview
Problem
Current sensing circuits face challenges in accurately indicating the current through a pass transistor due to mismatches in voltage drops across the sense and pass transistors, which affects the accuracy of current regulation.
Innovation Solution
A current sense circuit with a differential amplifier and sub-amplifiers operating in alternating phases, coupled with a differential difference amplifier, ensures precise offset compensation and matching of voltage drops across the sense and pass transistors, enabling accurate current measurement and regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sense transistor is used as a scaled down copy of the pass transistor for current sensing, then the current indication is obtained, but the voltage drop mismatch between sense and pass transistors causes measurement inaccuracy
Solution Approach 1:
A differential amplifier is introduced as an intermediary component between the sense transistor and the control circuit. The amplifier receives voltage inputs from both the sense transistor and pass transistor, amplifies the difference between them, and outputs a corrected control signal. This mediator compensates for voltage drop mismatches, allowing accurate current indication even when the sense transistor is significantly scaled down.
Solution Approach 2:
The differential amplifier creates a feedback mechanism where the voltage difference between the sense transistor and pass transistor is continuously monitored and corrected. The amplifier output feeds back to adjust the gate voltage of the pass transistor, ensuring that the voltage drops are matched and the current indication remains accurate throughout operation.
2Adaptability or versatility
If the size of the sense transistor is significantly smaller than the pass transistor (2-3 orders of magnitude), then the current sensing range is improved, but the voltage drop matching becomes more difficult to maintain
Solution Approach 1:
The differential amplifier serves as a mediator that bridges the size discrepancy between the sense transistor and pass transistor. By amplifying the voltage difference and providing corrective feedback, it enables the sense transistor to be much smaller (2-3 orders of magnitude) while still maintaining accurate voltage drop matching and current sensing across the full current range.
Solution Approach 2:
The system dynamically adjusts the gate voltage parameter of the pass transistor through the differential amplifier's output. This parameter change compensates for the inherent voltage drop differences caused by the large size ratio between transistors, maintaining precise matching despite the significant size difference.
3Device complexity
If a single differential amplifier is used for voltage difference amplification, then the circuit complexity is reduced, but the offset voltage causes measurement errors
Solution Approach 1:
The differential amplifier is segmented into multiple sub-amplifiers (first sub-amplifier and second sub-amplifier) that operate in alternating phases. This segmentation allows one sub-amplifier to be in the auto-zero phase while the other is in the amplification phase, eliminating the need for complex offset compensation circuits while maintaining high measurement precision.
Solution Approach 2:
The sub-amplifiers operate in periodic alternating phases (auto-zero phase and amplification phase). During the auto-zero phase, offset voltages are measured and stored; during the amplification phase, the stored offset is subtracted from the signal. This periodic action effectively eliminates offset errors without requiring complex continuous compensation circuits.
4Measurement precision
If auto-zero phase and amplification phase are implemented in a single amplifier, then the offset compensation is achieved, but the sensing continuity is interrupted
Solution Approach 1:
The amplifier is divided into multiple sub-amplifiers that can operate independently in different phases. This segmentation allows one sub-amplifier to perform auto-zero compensation while another simultaneously performs signal amplification, maintaining continuous sensing without interruption.
Solution Approach 2:
By using multiple sub-amplifiers operating in alternating phases, the system ensures that at least one sub-amplifier is always in the amplification phase, providing continuous current sensing. The transition between phases is seamless, maintaining the continuity of the useful sensing action without interruption.
Data Source
AI summary
A current sense circuit for a pass transistor is described. The circuit comprises a sense transistor having input and control ports that are coupled to input and control ports respectively of the pass transistor. The circuit comprises a differential amplifier comprising a differential input and output. An output port of the pass transistor is coupled to a first port of the differential input and an output port of the sense transistor is coupled to a second port of the differential input. The differential amplifier comprises a first sub-amplifier and a second sub-amplifier that are arranged in parallel and which are operated in an auto-zero phase and in an amplification phase in an alternating manner, and which are operated in the auto-zero phase in a mutually exclusive manner. The output of the differential amplifier is used to control voltage drops across the sense transistor and the pass transistor.