LDO Startup Inrush Limiting with Pass MOSFET Current Sensing
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Solution Overview
Problem
Low dropout (LDO) regulators experience high inrush currents during startup in both LDO and Bypass modes, which are undesirable and can lead to brownout conditions when the system supply is near the lower operating limit.
Innovation Solution
A low dropout (LDO) device with a current limit control loop that includes a power source, ground source, current control signal input, current startup signal input, and a switch, which limits the startup inrush current by sensing the current flowing through the p-channel MOSFET pass transistor and adjusting the gate voltage to control the current flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the LDO regulator is designed to provide high current capability, then the regulator can meet high power demands, but large inrush current occurs during startup that exceeds operational mode limits
Solution Approach 1:
The patent applies preliminary action by detecting the startup condition before full power operation and preemptively limiting the current. The inrush current limiter circuit activates during startup to prevent excessive current before it can damage the system, then disables itself once normal operation is established. This resolves the contradiction by preparing the protective measure in advance rather than reacting to the harmful effect after it occurs.
Solution Approach 2:
The patent implements dynamics by making the current limiting capability adjustable and time-dependent. The inrush current limiter dynamically changes its operation based on startup detection, providing high current limitation during startup transient conditions while allowing full current capability during normal operation. This dynamic behavior resolves the contradiction between high power capability and inrush current limitation by adapting the current limit to the operational phase.
2Object-affected harmful factors
If the startup inrush current is limited using traditional feedback network modification, then the inrush current is reduced, but the response time of the LDO regulator increases
Solution Approach 1:
The patent applies segmentation by separating the inrush current limiting function from the main feedback control loop. The inrush current limiter operates as an independent parallel path that only activates during startup conditions, while the feedback network handles normal regulation. This segmentation resolves the contradiction by allowing aggressive inrush current limiting without interfering with the fast response of the main feedback loop during normal operation.
Solution Approach 2:
The patent uses an intermediary approach by introducing a dedicated inrush current limiter circuit that acts as a mediator between the power input and the LDO regulator. This intermediary circuit temporarily absorbs or limits the inrush current during startup, allowing the main LDO feedback loop to respond quickly once normal operation begins. The intermediary resolves the contradiction by handling the transient condition separately from the main control path.
3Object-affected harmful factors
If a soft-start mechanism is implemented to decrease resistor value incrementally, then the inrush current is controlled, but the startup time and power consumption increase
Solution Approach 1:
The patent applies preliminary anti-action by detecting the startup condition and immediately activating the inrush current limiter to counteract the harmful inrush current before it can occur. Rather than gradually increasing current as in soft-start, the patent preemptively blocks excessive current at startup, then quickly transitions to normal operation. This resolves the contradiction by preventing the harmful effect immediately rather than managing it gradually, thus minimizing startup time.
4Reliability
If the feedback network is modified with variable resistors to control startup overshoot, then the voltage regulation is improved, but the device complexity and power consumption increase
Solution Approach 1:
The patent applies the taking out principle by extracting the inrush current limiting function from the main feedback network. Instead of modifying the feedback network with variable resistors, the patent removes this complexity from the feedback path and places the current limiting function in a separate dedicated circuit. This resolves the contradiction by maintaining simple feedback network for reliable voltage regulation while handling startup control in a separate, simpler circuit designed specifically for that purpose.
Data Source
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AI summary
A startup control apparatus providing a current limit control device which comprises : - a current control signal input; - a current startup signal input; - a first current source between a power source and said current control signal input; - a second current source between a ground source and said current control signal input; - a switch whose input is said current startup signal input.