Schmitt Trigger Circuit Topology for Near Rail-to-Rail Hysteresis
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Solution Overview
Problem
Conventional Schmitt trigger circuits are sensitive to process-voltage-temperature (PVT) variations when operating near the supply rails, leading to improper operation due to differences in physical characteristics of PMOS and NMOS devices.
Innovation Solution
The Schmitt trigger circuit employs NMOS devices for both threshold and input voltage comparisons, coupled through a PMOS-based current mirror for rising trips and PMOS devices for falling trips, coupled through an NMOS-based current mirror, to maintain high and low threshold values near the supply and ground rails, reducing PVT sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional Schmitt trigger uses PMOS and NMOS devices for voltage and current comparisons, then the circuit can operate with dual threshold action, but the circuit becomes sensitive to PVT variations when operating near the rails
Solution Approach 1:
The patent applies homogeneity by using devices of the same doping type for both threshold and input voltage comparisons. Specifically, NMOS devices are used for rising trip comparisons and PMOS devices are used for falling trip comparisons, ensuring that comparisons are performed by devices with matching physical characteristics, thereby eliminating PVT sensitivity issues that arise from mixing different device types
Solution Approach 2:
The Schmitt trigger circuit is segmented into two independent operational paths: a rising trip path using NMOS devices and a PMOS-based current mirror, and a falling trip path using PMOS devices and an NMOS-based current mirror. This segmentation allows each path to handle one direction of threshold comparison using homogeneous device types, resolving the PVT variation sensitivity problem
2Adaptability or versatility
If threshold levels are set close to the supply rails, then the circuit achieves near rail-to-rail hysteresis, but the circuit operates improperly due to PVT variations
Solution Approach 1:
By using devices of the same doping type for both threshold and input voltage comparisons in each trip direction, the patent ensures that the physical characteristics of comparing devices match, making the threshold levels stable even when set close to the supply rails. This homogeneity enables reliable near rail-to-rail hysteresis operation
Solution Approach 2:
The patent introduces current mirrors as intermediary elements that couple the threshold voltage comparisons. The PMOS-based current mirror mediates the rising trip comparison and the NMOS-based current mirror mediates the falling trip comparison, providing a stable reference mechanism that maintains reliability near the rails
Data Source
AI summary
Schmitt trigger with rail-to-rail or near rail-to-rail hysteresis. In some embodiments, a method includes switching an output of a Schmitt trigger from a first logic state to a second state in response to an input meeting a threshold, where the threshold is applied to a first transistor of a first doping type and the input is applied to a second transistor of the first doping type, the first and second transistors operably coupled to each other through a current mirror of a second doping type. The first doping type may be an n-type, the second doping type may be a p-type, and the threshold may be a rising threshold having a value within 10% of a supply voltage. Alternatively, the first doping type may be a p-type, the second doping type may be an n-type, and the threshold may be a falling threshold having a value within 10% of ground.