LF Driver Current Sense Topology With Split Source Transistor
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Solution Overview
Problem
Existing current sensing approaches in switching amplifiers, such as using external sense resistors or current copy circuitry, are complex, costly, and inefficient for accurately measuring current flow, particularly in low-frequency applications like passive keyless automotive entry systems.
Innovation Solution
An embedded split gate-source transistor switch is used with a current sense resistor placed only on the source of the small sense transistor to efficiently and accurately measure driver current, employing separate power and sense transistors to minimize die area and cost while maintaining measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external sense resistor is used in series with the LF antenna, then current measurement can be achieved, but circuit complexity and cost increase
Solution Approach 1:
The patent merges the current sense transistor with the power transistor into a single integrated device structure. The sense transistor is formed within the same semiconductor substrate as the power transistor, sharing common fabrication processes and reducing the need for separate external components. This integration eliminates the need for external sense resistors while maintaining accurate current measurement capability through the sense transistor's source terminal voltage measurement.
Solution Approach 2:
The integrated transistor structure serves multiple functions simultaneously: the main transistor channel handles power switching and current conduction, while the sense transistor portion provides current measurement capability through its source terminal. This multi-functional design eliminates the need for separate dedicated sense resistors and reduces overall circuit complexity while maintaining measurement accuracy.
2Measurement precision
If current copy circuitry is used to generate a small copy of the actual LF driver output current, then current measurement is enabled, but device complexity and cost increase
Solution Approach 1:
The patent combines the current sensing function directly into the power transistor structure by incorporating a sense transistor that shares the same physical substrate and fabrication process. This integration eliminates the need for separate current copy circuitry while providing accurate current measurement through the sense transistor's electrical characteristics.
Solution Approach 2:
The sense transistor within the integrated structure automatically provides current measurement information through its source terminal voltage, which naturally reflects the current flow through the transistor channel. This self-service mechanism eliminates the need for additional active copy circuitry, as the transistor itself generates the measurement signal through its normal operation.
3Measurement precision
If digitizing the output current with a 12-bit analog-to-digital converter is implemented, then current measurement precision is improved, but cost and size increase
Solution Approach 1:
The patent extracts the current measurement function from the main power switching operation by utilizing the sense transistor's source terminal voltage, which naturally provides a proportional representation of the channel current. This extracted measurement signal can be directly processed by simple analog-to-digital conversion circuitry, eliminating the need for complex 12-bit ADC systems while maintaining sufficient measurement precision for the application.
Solution Approach 2:
The sense transistor's physical dimensions (width-to-length ratio) are specifically designed to scale the current measurement signal to an optimal voltage range for subsequent processing. By adjusting the transistor's geometric parameters, the circuit achieves precise current measurement with simpler and smaller conversion circuitry, reducing overall device size while maintaining measurement accuracy.
4Measurement precision
If existing current sensing approaches are used, then current measurement is achieved, but power control needs and complexity increase
Solution Approach 1:
The patent merges the current sensing operation with the power switching operation by using the same transistor structure for both functions. The sense transistor operates simultaneously with the power transistor during normal switching cycles, eliminating the need for separate power-consuming measurement circuits and reducing overall power control complexity.
Solution Approach 2:
The sense transistor automatically generates the measurement signal through its normal current conduction operation, without requiring additional power or control resources. The source terminal voltage naturally reflects the channel current, providing measurement capability as a byproduct of the transistor's primary power switching function, thereby reducing power control complexity.
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 solution provides a low-cost, area-efficient current sense circuit that accurately measures antenna driver current with minimal impact on switch resistance, enabling precise current measurement in low-frequency applications without increasing the complexity or size of the circuit.
Implementation Method 1
an embedded sense resistor in the source of the small sense transistor(s) may be used to efficiently and accurately measure the driver current
Data Source
AI summary
A switching amplifier circuit (200) connected to drive an impedance-based output load (230) includes high side and low side switches (201-204) configured and connected to connect first and second supply voltage lines to first and second output nodes (ANTP, ANTN) in response to gating control signals, and also includes an output current sensing circuit for measuring a current through the output load with a current sensing resistor (Rs) connected between the second supply voltage line and a source of one or more split gate-source switching transistors (203C) in the low side gate-source switching transistor, where a voltage sense circuit connected across the current sensing resistor is configured to sample a voltage across the current sensing resistor for measuring a sense current at the current sensing resistor.