Dual-Loop LDO Regulator for Low-Frequency PSRR and Gain Stability
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Solution Overview
Problem
Low dropout (LDO) voltage regulators face instability and gain errors due to high output impedance and low gain amplifiers, which affect their ability to reject power supply noise and maintain regulated voltage, especially at low frequencies.
Innovation Solution
A dual feedback loop system is implemented, where the first feedback loop adjusts the resistance of the pass element to maintain the regulated voltage and the second feedback loop adjusts the bias voltage to correct gain errors, using a high gain, low bandwidth amplifier for the second loop to counteract the low gain, high bandwidth amplifier in the first loop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a high output impedance amplifier is used in the feedback loop, then the LDO regulator achieves low dropout voltage, but power supply rejection ratio deteriorates at low frequencies
Solution Approach 1:
The feedback system is segmented into two independent loops: a first feedback loop for DC regulation and a second feedback loop for AC noise rejection. This segmentation allows each loop to be optimized for its specific function without compromising the other, resolving the contradiction between low dropout voltage and power supply rejection ratio.
Solution Approach 2:
A second feedback loop acts as an intermediary mechanism to compensate for the deficiencies of the first feedback loop. The second loop specifically targets low-frequency power supply noise that the first loop cannot reject effectively, using an intermediate amplifier stage to provide the necessary gain without affecting the dropout voltage.
2Power
If a low gain amplifier is used to achieve low dropout voltage, then the voltage regulation is maintained, but gain errors increase
Solution Approach 1:
Two feedback loops are merged into a unified regulation system where the first loop handles DC voltage regulation with a low gain amplifier to maintain low dropout voltage, while the second loop compensates for gain errors using a high gain amplifier. The combined system achieves both low dropout voltage and high gain accuracy.
Solution Approach 2:
The system changes the gain parameter dynamically by using different amplifier configurations in different feedback loops. The first loop uses a low gain configuration for DC regulation, while the second loop uses a high gain configuration for error compensation, effectively changing the overall system parameters to achieve both objectives.
3Device complexity
If a single feedback loop is used, then the device complexity is reduced, but stability across wide frequency range deteriorates
Solution Approach 1:
The feedback system transitions from a static single-loop design to a dynamic multi-loop architecture where each loop operates optimally at different frequency ranges. The first loop provides stable DC regulation while the second loop dynamically compensates for AC disturbances, achieving broad frequency stability without excessive complexity.
Data Source
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AI summary
In certain aspects, a method for voltage regulation includes adjusting, using a feedback circuit, a resistance of a first pass element in a direction that reduces a difference between a reference voltage and a feedback voltage, wherein the first pass element is coupled between an input and an output of a voltage regulator, and the feedback voltage is equal to or proportional to a voltage at the output of the voltage regulator. The method also includes adjusting a bias voltage of the feedback circuit in a direction that reduces the difference between the reference voltage and the feedback voltage.