High-Voltage MOS Input Stage for Gate Oxide Protection
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional MOS differential input stages face issues with gate oxide rupture due to high operating voltages exceeding the rated breakdown voltage, particularly in comparator applications where input voltages can vary significantly, leading to damage of PMOS devices.
Innovation Solution
A high voltage MOS differential input stage is designed with a configuration that includes multiple PMOS transistors with thin gate oxides, where the inner transistors have a larger W/L ratio than the outer transistors, and short channel FETs are used to increase voltage swing and protect the input devices by maintaining both inner transistors in the active region, even under significant voltage differences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If DMOS transistors with higher operating voltages are used, then the operating voltage increases, but the gate oxide breakdown voltage becomes insufficient
Solution Approach 1:
The input stage is divided into four transistors arranged in a differential configuration with two input transistors and two feedback transistors. This segmentation allows the circuit to handle high voltage differentials by distributing the voltage stress across multiple devices rather than relying on a single high-voltage transistor with thick gate oxide.
Solution Approach 2:
The feedback transistors act as intermediary elements that sense the voltage differential and control the current distribution. They mediate between the high voltage differential at the inputs and the lower voltage requirements of the gate oxides, preventing direct voltage stress on any single gate oxide while maintaining the high voltage operating capability.
2Reliability
If external back-to-back diodes are added across the input pair, then gate oxide protection is achieved, but the design becomes complicated and requires significant design effort
Solution Approach 1:
The protection function is merged with the signal processing function. The feedback transistors simultaneously provide gate oxide protection and enable the differential signal to be processed and amplified. This integration eliminates the need for separate external protection diodes and reduces design complexity by providing a unified solution that handles both protection and amplification within the same transistor network.
Solution Approach 2:
The feedback transistors serve multiple functions: they protect the gate oxides from breakdown, amplify the differential signal, and provide current control. This multi-functionality replaces the need for separate protection components and simplifies the overall design by consolidating multiple functions into a single integrated circuit block.
3Ease of operation
If the W/L ratio of inner transistors is increased, then voltage swing at the output increases, but the transistor dimensions change
Solution Approach 1:
Different W/L ratios are assigned to different transistors based on their specific functional requirements. The inner feedback transistors have larger W/L ratios optimized for voltage swing and current control, while the outer input transistors have different ratios optimized for signal reception. This local optimization allows each transistor to be sized appropriately for its specific role rather than using uniform dimensions.
Data Source
AI summary
A differential input stage including two input branches each with a pair of transistors. A bias circuit supplies a separate bias current to each of the input branches. A first transistor of each branch has a first current terminal coupled to a source node receiving a bias current, a second current terminal coupled to an output node, and a control terminal coupled to an input node. A second transistor of each branch has a first current terminal coupled to the corresponding source node, a control terminal coupled to the corresponding input node, and a second current terminal coupled to an intermediate node. The second transistors operate as a current path in higher differential voltage conditions to keep the first transistor active to avoid violating the maximum gate-source voltage.