LDO Voltage Regulator Current Limiting Circuit
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Solution Overview
Problem
Low-dropout (LDO) linear regulators face high current stress during start-up, and existing solutions do not provide precise current limitation and efficient methods to measure output current effectively.
Innovation Solution
A circuit and method are introduced that utilize a smaller PMOS transistor in parallel with the pass device to measure output current, with current sources and resistors to control the output current, ensuring precise limitation by comparing currents through a current mirror configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a LDO regulator allows high output current during start-up, then the output voltage can be established quickly, but the pass device experiences high current stress that may damage it
Solution Approach 1:
The patent applies preliminary action by implementing a start-up circuit that activates before the main LDO operation. This circuit includes a start-up transistor and capacitor that generate an initial current pulse to quickly establish the output voltage, then automatically disable themselves once regulation begins. This preliminary action allows fast start-up without subjecting the pass device to sustained high current stress.
Solution Approach 2:
The patent segments the current path by introducing a separate start-up circuit branch parallel to the main regulation path. The start-up circuit handles the high current transient independently through its own transistor and capacitor, while the main pass device only handles the regulated current after start-up. This segmentation isolates the harmful high current from the pass device while still achieving fast voltage establishment.
2Object-affected harmful factors
If a current limiting circuit is added to protect the pass device, then current stress is reduced, but the circuit complexity increases
Solution Approach 1:
The patent merges the current limiting function with the existing error amplifier and feedback network. The current limit is implemented by modifying the error amplifier's input stage to include current-sensing resistors and additional transistors that naturally limit the output current when the sensed voltage reaches a threshold. This integration combines protection functionality with the existing regulation circuitry, avoiding the need for a completely separate limiting circuit.
Solution Approach 2:
The patent implements self-service by designing a current limiting mechanism that automatically activates when needed without external control. The current-sensing resistors continuously monitor the output current, and when the current exceeds the limit, the modified error amplifier automatically reduces the drive to the pass device. This self-regulating approach eliminates the need for external current limit control circuitry.
3Reliability
If output current is limited to protect the pass device, then device reliability is improved, but the maximum output current capability is reduced
Solution Approach 1:
The patent applies dynamics by making the current limit adjustable rather than fixed. The current-sensing resistors are designed with selectable resistance values that allow the current limit threshold to be adjusted based on the specific application requirements. This dynamic adjustment capability enables the system to optimize between protection and power delivery by selecting appropriate resistor values during design or operation.
Solution Approach 2:
The patent changes the parameter of current limit threshold by providing different resistance values for the current-sensing resistors. By selecting different resistor values, the system can change the maximum allowed output current to match the pass device's current rating and the application's power requirements. This parameter adjustment resolves the contradiction by allowing the current limit to be optimized for both protection and power capability.
Data Source
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Figure 3
AI summary
A method and circuits to limit the output load current of a current driven LDO voltage regulator are disclosed. The current through a second pass transistor, being in parallel to a first pass transistor and being a fraction of the current through the first pass transistor is measured and compared with a reference current. In case the current through the second pass transistor is larger than this reference current the current through the gates of both pass devices is reduced and thus the output load current of the voltage regulator is limited.