Linear Voltage Regulator Open Load Detection Circuit
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Solution Overview
Problem
Existing linear voltage regulators face challenges in accurately detecting an open load condition due to the small voltage drop across the current sense resistor, requiring high precision comparators or suffering from accuracy issues caused by transistor mismatches in separate current sense paths.
Innovation Solution
A linear voltage regulator design featuring a pair of complementary power transistors connected 'back to back' with a current sense circuit in parallel across one transistor, utilizing a comparator to compare voltage drops across the current sense and reference resistors for accurate open load detection without needing high precision comparators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current sense resistor is placed in the regulator's output path to detect open load condition, then the open load condition can be detected, but the voltage drop across the current sense resistor is very small requiring high precision comparators
Solution Approach 1:
The patent divides the power transistor function into two separate transistors (Q1 and Q2) connected back-to-back. One transistor (Q1) handles the main power regulation while the other (Q2) provides a dedicated path for current sensing. This segmentation allows the current sense resistor (R3) to operate independently with optimized values, generating sufficient voltage drop for accurate detection without requiring high precision comparators.
Solution Approach 2:
The patent introduces a replication transistor (Q2) as an intermediary element that mirrors the characteristics of the main power transistor (Q1). This replication transistor serves as a mediator that allows accurate current sensing by providing a known reference path, eliminating the need for high precision comparators while maintaining detection accuracy.
2Measurement precision
If a current sense resistor is placed in a separate current sense path with a replicated power transistor, then the voltage drop can be increased, but accuracy is affected by mismatch between the power transistor and its replication
Solution Approach 1:
The patent employs asymmetric connection of complementary transistors (Q1 and Q2) in a back-to-back configuration. This asymmetric arrangement ensures that the transistors operate in complementary modes, reducing the impact of parameter mismatches. The bulk diode connection of Q2 to ground further asymmetrically stabilizes the operating point, minimizing mismatch effects and improving detection reliability.
Solution Approach 2:
The patent changes the operating parameters of the transistors by connecting their bulk diodes to different potentials (Q1's bulk diode to VOUT, Q2's bulk diode to ground). This parameter change stabilizes the transistors' operating characteristics and reduces sensitivity to manufacturing variations, thereby improving detection accuracy despite transistor mismatches.
3Measurement precision
If the current sense resistor is dimensioned to generate a large voltage drop, then detection accuracy improves, but the voltage drop may exceed the transistor's bulk diode threshold causing current diversion
Solution Approach 1:
The patent connects the bulk diode of the replication transistor (Q2) to ground, creating a stable reference potential. This equipotential connection ensures that the voltage drop across the current sense resistor can be accurately measured without exceeding the bulk diode threshold, as the bulk diode remains reverse-biased during normal operation. This allows the current sense resistor to be dimensioned for optimal detection accuracy.
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 design achieves high accuracy in open load detection with a normal comparator, allowing the current sense resistor to generate a significant voltage drop, thereby enhancing detection reliability and avoiding transistor mismatch errors.
Implementation Method 1
A comparator has a first input connected to a terminal of the current sense resistor and a second input connected to a node between the reference resistor and the current sink. The comparator compares the voltage drop across the current sense resistor with the constant voltage drop across the reference resistor
Implementation Method 2
The current sense resistor can be dimensioned to generate a relatively high voltage drop of e.g. 100 mV
Implementation Method 3
For higher output currents the voltage drop across the current sense circuit is limited by the parallel bulk diode of the power transistor
Data Source
AI summary
A linear voltage regulator is provided which has a pair of complementary power transistors connected “back to back” in series between a voltage input and a voltage output. A current sense circuit including a current sense resistor is connected in parallel across one of the power transistors, such as the one connected to the voltage input. As long as the voltage drop in the current sense circuit remains small, i.e. less than app. 0.7V, the current flowing through the bulk diode of the power transistor remains negligible and the entire output current flows through the current sense circuit. For higher output currents the voltage drop across the current sense circuit is limited by the parallel bulk diode of the power transistor. The current sense resistor can be dimensioned to generate a relatively high voltage drop of e.g. 100 mV, and a high accuracy of open load detection is achieved without the requirement for a high precision comparator.