NMOS Linear Regulator Over-Current Protection Circuit
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Solution Overview
Problem
There is a lack of suitable over-current protection circuitry for linear voltage regulators using an NMOS driving stage, which is essential for preventing damage from excessive currents during startup and short circuits, as existing solutions are designed for PMOS drivers and not compatible with NMOS devices.
Innovation Solution
A linear voltage regulator with an improved over-current protection circuit that includes a PMOS current mirror, current sensing NMOS, current sinking device, and feedback circuit to control the output current, ensuring the circuit operates effectively in both open and closed loop configurations with an NMOS driving stage, reducing the area required for protection circuitry and providing a soft start mechanism.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If PMOS driver transistor is used in voltage regulator, then low drop out capability is achieved, but over-current protection circuitry becomes complex and consumes more silicon area
Solution Approach 1:
The patent uses a current mirror circuit that copies the current through the NMOS driver transistor to a separate sensing path. The PMOS transistors Q3 and Q4 form a current mirror that replicates the driver current, allowing over-current detection without directly interfering with the driver transistor operation. This copying approach enables protection functionality while maintaining compact area usage.
Solution Approach 2:
The patent introduces an intermediary sensing transistor (Q2) and current mirror circuit that mediates between the NMOS driver transistor and the protection logic. Instead of directly monitoring the driver transistor current, the circuit uses an intermediary current mirror to sense and compare currents, providing a buffer that reduces the area burden on the critical driver path.
2Reliability
If over-current protection circuitry is added to voltage regulator, then current limiting capability is improved, but device complexity increases
Solution Approach 1:
The patent merges the over-current protection functionality with the existing voltage regulation loop by sharing common components. The error amplifier serves dual purposes: regulating output voltage and detecting over-current conditions through its feedback inputs. The PMOS transistors Q3-Q6 are integrated into the existing regulation architecture, combining multiple functions in a unified circuit structure rather than adding separate protection circuitry.
Solution Approach 2:
The error amplifier and feedback network are designed to perform multiple functions: standard voltage regulation through the primary feedback loop, and over-current protection through alternative feedback paths involving transistors Q3-Q6. This multi-functionality reduces overall device complexity by making existing components serve dual purposes rather than requiring dedicated protection circuitry.
3Area of stationary object
If NMOS driver transistor is used instead of PMOS, then silicon area is reduced, but over-current protection becomes difficult to implement
Solution Approach 1:
The patent inverts the conventional approach by using PMOS transistors for current sensing and mirroring rather than for the driver function. Instead of using PMOS as the driver (as in conventional LDOs), the invention uses NMOS as the driver and employs PMOS transistors Q3-Q6 in the protection circuit to sense and limit current. This inversion allows area-efficient NMOS driving while maintaining robust protection capability through PMOS sensing elements.
Solution Approach 2:
The patent changes the operating parameters and roles of different transistor types. The NMOS driver transistor operates with gate voltage controlled by the error amplifier, while PMOS transistors Q3-Q6 are biased to operate in specific regions (linear or saturation) depending on the protection condition. The circuit dynamically adjusts current paths and transistor operating states to provide protection without requiring the driver transistor itself to change type or fundamental operation mode.
Data Source
AI summary
A linear regulator with an N-type pass transistor includes an over-current protection circuit. A current sink is used as an indicator for an over-current condition and is coupled to the output of the linear regulator. The indicator is coupled to a feedback logic circuit that controls the current through the output load. The over-current protection circuit extensively uses N-type devices for various components including the output driver stage in the circuit. This results in reduced area for the over-current protection circuit.