LDO Current Limiting Loop for In-Rush and Short-Circuit Control
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Solution Overview
Problem
Low dropout linear voltage regulators face inefficiencies due to high in-rush currents during startup and potential damage from excessive current demands, particularly in applications with changing load demands and external capacitors, where existing solutions like soft-start circuits are difficult to optimize and can lead to instability.
Innovation Solution
Incorporating a current limiting loop within the LDO regulator with a higher bandwidth than the main loop, allowing the current limiting loop to bypass the error amplifier and control the pass element to manage in-rush currents and provide short-circuit protection, using a sense transistor and current source to mirror the pass transistor current and limit the output current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a soft start circuit is employed to limit in-rush current, then current limiting is achieved, but the system startup speed decreases and optimization becomes difficult
Solution Approach 1:
The patent divides the control function into two separate loops: a main control loop for voltage regulation and a fast current limiting loop for current protection. The current limiting loop operates independently with higher bandwidth, allowing it to respond quickly to current transients without affecting the overall system startup speed controlled by the main loop.
Solution Approach 2:
The patent introduces a current sensing mechanism that acts as an intermediary between the pass element and the control system. The sense transistor mirrors the pass transistor current, providing real-time current information to the current limiting loop, which then adjusts the pass element gate voltage to limit current without requiring a slow soft-start circuit.
2Object-affected harmful factors
If a current limiting circuit is added to a fast regulating LDO, then current protection is improved, but stability of multiple feedback loops becomes difficult to maintain
Solution Approach 1:
The patent segments the feedback control into two distinct loops with different priorities: the main voltage regulation loop and the fast current limiting loop. By giving the current limiting loop higher bandwidth and making it the dominant loop during transient conditions, the patent ensures current protection while maintaining stability through clear loop hierarchy.
Solution Approach 2:
The patent changes the operational parameters of the two loops differently: the main loop operates with lower bandwidth for stability, while the current limiting loop operates with higher bandwidth for fast response. The sense transistor current mirror ratio is carefully selected to ensure the current limiting loop dominates during transient conditions without causing instability.
3Stability of the object's composition
If the LDO includes an external capacitor, then output filtering is improved, but in-rush current limitation becomes more critical
Solution Approach 1:
The patent implements preliminary current limiting action through the fast current limiting loop before the external capacitor can charge up and create in-rush current problems. The current sensing and limiting mechanism is active from the moment the LDO starts up, preventing excessive current from reaching the external capacitor while still allowing normal charging current to flow.
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 current limiting loop effectively reduces in-rush currents, enhances stability by actively managing transient loads, and prevents damage from excessive current, ensuring stable voltage regulation and protecting against short circuits and brownout conditions.
Implementation Method 1
using a sense transistor and current source to mirror the pass transistor current and limit the output current
Data Source
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Figure 3A
AI summary
In described examples of limiting current in a low dropout ("LDO") linear voltage regulator (300), a pass element (330) generates an output voltage (VOUT) that is less than an input voltage. The pass element (330) may be enabled by an error amplifier (3 10) that compares a feedback signal (VFB) from an output (335) of the pass element (330) with a voltage reference (VREF). Also, the pass element (330) may be enabled by a current limiting circuit (353, 354, 355) that bypasses the error amplifier (3 10) to limit current generated at the output (335) of the pass element (330).