LDO Regulator Dropout Detection for Quiescent Current Limiting
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Solution Overview
Problem
Low dropout (LDO) voltage regulators experience significant increases in quiescent current consumption during dropout mode, which can quickly deplete battery charge and compromise system operation, especially in battery-powered systems.
Innovation Solution
A circuit is introduced that monitors the operational status of the regulator and intervenes by limiting the quiescent current in the dropout mode through a dropout detector and quiescent current limiter circuit, which senses voltage differences and modifies the impedance of the variable impedance circuit to control the quiescent current consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the LDO voltage regulator operates in dropout mode without intervention, then the output voltage can still be supplied, but the quiescent current consumption increases significantly
Solution Approach 1:
The dropout detector circuit proactively monitors the voltage difference between input and output terminals before the regulator fully enters dropout mode. When the differential voltage exceeds a threshold indicating approaching dropout condition, the circuit preemptively activates the quiescent current limiter, which increases the impedance of the variable impedance circuit. This preliminary action prevents the excessive current draw before it occurs, maintaining low quiescent current while still allowing dropout mode operation when necessary
Solution Approach 2:
The circuit continuously monitors the differential voltage between VIN and VOUT through the dropout detector. This feedback mechanism compares the actual voltage difference against a reference threshold and dynamically adjusts the quiescent current limit accordingly. When dropout is detected, the feedback loop activates the current limitation; when regulation is restored, the limitation is released, creating a closed-loop control system that adapts to operating conditions
2Reliability
If the VGS of the power MOSFET increases in dropout mode, then the output voltage can be maintained, but the bias current of the power MOSFET increases
Solution Approach 1:
The invention segments the control of the power MOSFET into two independent pathways: the main drive signal from the differential amplifier that controls output voltage regulation, and a separate quiescent current limit signal from the dropout detector that specifically controls the bias current. This segmentation allows the regulator to maintain output voltage while independently limiting the harmful bias current increase in dropout mode
Solution Approach 2:
The variable impedance circuit acts as an intermediary element between the differential amplifier output and the power MOSFET gate. This intermediary component (implemented as a series combination of resistor and diode-connected MOSFET) provides a voltage drop that limits the maximum VGS voltage reaching the power MOSFET during dropout conditions, thereby controlling the bias current without completely disabling the output drive capability
3Use of energy by moving object
If a quiescent current limiter circuit is added to control current in dropout mode, then current consumption is reduced, but the device complexity increases
Solution Approach 1:
The dropout detector and quiescent current limiter functions are merged into a single integrated circuit block that shares common components with the main regulator. The dropout detector utilizes the existing differential amplifier output and voltage reference infrastructure, while the variable impedance circuit is integrated into the existing driver stage. This merging approach minimizes additional complexity by reusing existing circuit elements rather than adding completely separate subsystems
Solution Approach 2:
The variable impedance circuit serves multiple functions simultaneously: it acts as part of the driver stage for the power MOSFET, provides quiescent current limitation during dropout mode, and functions as an impedance element for stability compensation. The dropout detector also serves dual purposes by monitoring both the regulation status and triggering the current limitation. This multi-functionality reduces overall circuit complexity by making existing components work harder rather than adding dedicated single-function elements
Data Source
AI summary
An amplifier stage of an LDO regulator circuit includes an amplifier stage that generates a drive signal in response to a first voltage difference an output voltage of the LDO regulator circuit and a reference voltage. A drive stage having a quiescent current consumption is configured to generate a control signal in response to the drive signal. The control signal is applied to the control terminal of a power transistor. A dropout detector senses whether the LDO regulator circuit is operating in closed loop regulation mode or in open loop dropout mode by sensing a second difference in voltage between the drive signal and the control signal. A quiescent current limiter circuit responds to the sensed second difference by controlling the quiescent current consumption of the drive stage, and in particular limiting current consumption when the LDO regulator circuit is operating in the open loop dropout mode.