Adaptive RC Feedback Network for LDO Stability
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Solution Overview
Problem
Conventional low-dropout (LDO) linear voltage regulators face instability and reduced power supply rejection (PSR) when dealing with a wide range of capacitive load conditions, particularly from no load to tens of nano-Farads, and large load currents, leading to phase margin degradation and unsuitable performance across various frequencies.
Innovation Solution
The implementation of an improved feedback network with an adaptive RC network and a supply rejection circuit, which includes a voltage-controlled variable resistor to sense node voltage and adjust resistance values, producing an adaptive zero in the transfer function to reduce phase margin degradation and enhance stability across capacitive and resistive load variations, while using transconductance amplifiers and capacitors to drive large capacitive loads and maintain high PSR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional LDO linear voltage regulators are used, then the circuit is simple, but stability and power supply rejection deteriorate when dealing with wide range of capacitive load conditions
Solution Approach 1:
The patent implements an adaptive RC network where the resistance value is dynamically adjusted based on the sensed node voltage, which varies with load conditions. This dynamic adaptation allows the circuit to maintain stability across wide ranges of capacitive loads by automatically tuning the zero location in the transfer function, resolving the contradiction between reliability and fixed circuit complexity.
Solution Approach 2:
The patent employs a voltage-controlled variable resistor that senses node voltage in the feedback network and adjusts the RC network resistance accordingly. This feedback mechanism enables the circuit to respond to changing load conditions and maintain optimal stability margins, addressing the reliability improvement while managing circuit complexity through intelligent control.
2Reliability
If conventional LDO linear voltage regulators are used, then the circuit is simple, but power supply rejection ratio deteriorates at high frequencies and under large load currents
Solution Approach 1:
The adaptive RC network dynamically adjusts its resistance value based on sensed node voltage, which changes with load current and frequency conditions. This dynamic tuning optimizes the power supply rejection ratio across wide frequency ranges and load conditions by positioning the adaptive zero appropriately, resolving the contradiction between PSR performance and feedback network complexity.
3Quantity of substance
If the capacitive load increases to tens of nano-Farads, then the load driving capability is improved, but phase margin degradation occurs leading to instability
Solution Approach 1:
The patent changes the parameter of the feedback network by introducing an adaptive RC network whose resistance value varies with load conditions. This parameter change creates an adaptive zero that compensates for phase margin degradation caused by large capacitive loads, enabling the circuit to maintain stability while supporting tens of nano-Farads of capacitive load.
4Reliability
If larger capacitors are used to maintain stability, then stability over wide capacitive load range is improved, but microchip area and manufacturing costs increase
Solution Approach 1:
Instead of using fixed large capacitors to ensure stability, the patent employs a dynamic adaptive RC network that adjusts its resistance value based on sensed node voltage. This dynamic approach allows the use of smaller capacitors while maintaining stability across wide capacitive load ranges, thereby reducing microchip area and manufacturing costs.
Data Source
AI summary
A method to maintain stability of a low drop-out linear voltage regulator (LDO) includes sensing, by a voltage controlled variable resistor, a node voltage in a feedback network of the LDO linear voltage regulator, wherein the feedback network includes an error amplifier configured to regulate an output voltage level of the LDO based on a reference voltage, wherein the node voltage has a dependency on a resistive load current of the LDO, and adjusting, by the voltage controlled variable resistor and based on the sensed node voltage, a resistance value of a RC network in the feedback network, wherein the adaptive RC network produces an adaptive zero in a transfer function of the feedback network, wherein the adaptive zero reduces phase margin degradation due to an output non-dominant pole in the transfer function, and wherein a frequency of the adaptive zero is inversely proportional to the resistance value.