Miller-Compensated LDO With Active Feedback for Low-Load Stability
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Solution Overview
Problem
Existing Miller-compensated Low-Dropout (LDO) voltage regulators face challenges in achieving stability over a wide range of operating conditions, particularly at low loads, while also requiring a trade-off between compensation network size, quiescent current, and minimum output capacitance.
Innovation Solution
The proposed solution involves a Miller-compensated LDO voltage regulator architecture that incorporates an active feedback loop and a local feedback circuit to actively regulate the output resistance of the first amplification stage, thereby improving phase margin at low loads without increasing the Miller capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the output resistance of the first amplification stage is increased to improve phase margin at low loads, then stability is improved, but the bias current decreases causing worse load transient performance
Solution Approach 1:
The patent applies dynamics by making the output resistance of the first amplification stage variable rather than fixed. A control circuit dynamically adjusts the output resistance based on load conditions: at low loads, the output resistance is increased to improve phase margin and stability; at high loads, the output resistance is decreased to maintain sufficient bias current for good load transient performance. This dynamic adjustment resolves the contradiction between stability and load transient performance.
Solution Approach 2:
The patent changes the parameter of output resistance based on operating conditions. By using a control circuit that monitors load current and adjusts the output resistance accordingly, the system optimizes phase margin at low loads while maintaining adequate bias current at high loads. This parameter change approach allows the system to achieve both improved stability and maintained productivity across different operating ranges.
2Reliability
If a Miller compensation capacitor is used to improve stability, then phase margin is improved, but the compensation network size increases
Solution Approach 1:
The patent changes the effective capacitance value dynamically by using a variable resistance in parallel with the Miller compensation capacitor. At low frequencies where stability is critical, the resistance is set to a high value, allowing the full capacitance value to contribute to phase margin. At high frequencies, the resistance is reduced to decrease the effective capacitance, thereby reducing the compensation network size and improving bandwidth without sacrificing low-frequency stability.
3Reliability
If the Miller capacitance is increased to fulfill phase margin requirements, then stability is improved, but the quiescent current increases
Solution Approach 1:
The patent changes the effective capacitance parameter dynamically using a voltage-controlled resistor in parallel with the Miller capacitor. When stability is needed (low load conditions), the resistance is increased to allow the full capacitance effect, improving phase margin. When energy efficiency is prioritized (high load conditions), the resistance is decreased to reduce the effective capacitance, thereby reducing the current required to charge and discharge the capacitor, thus lowering quiescent current while maintaining adequate stability.
Data Source
AI summary
A linear voltage regulator includes a first amplification stage configured to produce an error signal at an intermediate node as a function of a difference between a first reference voltage and a regulated output voltage. An intermediate amplification stage amplifies the error signal to produce an amplified error signal. A driver stage produces a drive signal as a function of the amplified error signal. A pass device is controlled by the drive signal to produce the regulated output voltage. A feedback circuit produces a feedback current as a function of a difference between the drive signal and a second reference voltage. The feedback current is the sourced to the intermediate node.