Current Mirror Frequency Compensation for Cap-Less LDO Stability
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Solution Overview
Problem
Current frequency compensation methods for Low-Drop Out (LDO) regulators, especially those without external capacitors, face challenges in achieving flexible pole-zero placement and stability without increasing power consumption or semiconductor area.
Innovation Solution
The proposed solution involves modifying current mirror circuits by splitting transistors and resistors into symmetrical units, allowing for independent control of pole and zero positions through RC low-pass networks, enabling various compensation scenarios such as pole-zero doublets, simple zeros, or zeros-pole doublets without additional power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If external capacitor and resistor are added to create pole-zero doublet, then frequency compensation is improved, but device area and cost increase
Solution Approach 1:
The patent extracts the frequency compensation function from external components (capacitor and resistor) and implements it using only internal transistor units and their inherent parasitic capacitances. The compensation network is taken out of the external domain and integrated into the core current mirror structure, eliminating the need for additional external components while maintaining compensation effectiveness.
Solution Approach 2:
The patent merges the compensation function with the current mirror function by using the same transistor units (M1, M1', M2) for both purposes. The parasitic capacitances of these transistors are utilized to create the pole-zero doublet, combining the compensation network with the current mirror structure rather than keeping them separate.
2Area of stationary object
If cap-less LDO internal solutions are used, then external component area is reduced, but internal circuit complexity and power consumption increase
Solution Approach 1:
The patent makes the current mirror circuit self-sufficient by using its own transistor parasitic capacitances to provide frequency compensation. The circuit serves its own compensation needs without requiring external components or additional dedicated compensation circuits, reducing both external area and internal complexity.
Solution Approach 2:
The transistor units M1, M1', and M2 serve multiple functions: they provide current mirroring action and simultaneously generate the pole-zero doublet for frequency compensation through their parasitic capacitances. This multi-functionality eliminates the need for separate compensation components while maintaining simple circuit structure.
3Reliability
If traditional frequency compensation is used, then stability is achieved, but flexibility in pole-zero placement is limited
Solution Approach 1:
The patent introduces dynamic control of the compensation network by adding control terminals to transistor M2 and making the capacitance values variable. This allows the pole and zero positions to be dynamically adjusted based on operating conditions, providing flexibility in pole-zero placement while maintaining stability through adaptive compensation.
Data Source
AI summary
A current mirror includes first and second transistors having current paths coupled to an input current line. The current paths for the first and second transistors are referenced to ground via respective first and second resistors having resistance values twice a first resistance value. The first transistor is diode connected. A third transistor has a current path coupled to an output current line and referenced to ground via a third resistor having a second resistance value equal to the first resistance value divided by a mirror factor. Control terminals of the first and third transistors are coupled together, and further coupled to a control terminal of the second transistor through a coupling resistor. A first capacitor is coupled between ground and the control terminal of the second transistor unit. A second capacitor is coupled between ground and the current path through the third transistor.