Schmitt Trigger Pull-Up Topology for High-Voltage Fast Switching
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Solution Overview
Problem
Designing a Schmitt trigger that operates at high switching speeds and high supply voltages while protecting transistors from damage is challenging due to the difficulty in managing voltage thresholds and preventing transistor damage from high supply voltages.
Innovation Solution
The Schmitt trigger incorporates a feedforward connection using a pass transistor and a pull-up transistor to ensure that the input of the second inverter is pulled up to a high reference voltage, allowing for high-frequency operation without damaging the transistors, and includes a control voltage generator to manage switching in both high and low voltage modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high supply voltages are used to ensure proper operation of the Schmitt trigger, then the switching threshold is sufficiently high, but the transistors may be damaged from the high voltage
Solution Approach 1:
A pass transistor is introduced as an intermediary between the first inverter output and the second inverter input. This pass transistor couples the high supply voltage to the second inverter input while simultaneously protecting the second inverter transistors from direct exposure to the damaging high voltage levels, thus resolving the contradiction between achieving sufficient switching threshold and preventing transistor damage
Solution Approach 2:
The patent changes the voltage parameter at the second inverter input by using the pass transistor to couple a scaled version of the high supply voltage. This allows the second inverter to operate at appropriately reduced voltage levels while still responding to the high supply voltage switching thresholds, thereby protecting transistors while maintaining proper operation
2Speed
If small transistors are used to enable high-frequency operation, then the switching speed is high, but the transistors are more susceptible to damage from high supply voltages
Solution Approach 1:
The pass transistor serves as a protective intermediary that allows small, high-speed transistors to be used in the second inverter while preventing direct exposure to damaging high supply voltages. This enables the use of smaller transistors for high-frequency operation without sacrificing reliability
Solution Approach 2:
The Schmitt trigger is segmented into two separate inverter stages with different voltage requirements. The first inverter operates at the full high supply voltage, while the second inverter operates at a reduced voltage level through the pass transistor coupling. This segmentation allows each stage to be optimized independently - the first for high voltage operation and the second for high-speed operation with protected transistors
Data Source
AI summary
An integrated circuit includes an input pad and a Schmitt trigger coupled to the input pad. The Schmitt trigger includes a first inverter and a second inverter. The Schmitt trigger includes a pull-up transistor coupled to an input of the second inverter and configure to supply a high reference voltage to the input of the second inverter.