Comparator Circuit for Accurate Trip Points Above Supply Voltage
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Solution Overview
Problem
Comparator circuits face accuracy issues when detecting voltage thresholds above the supply rail, particularly due to variations in reference voltage and the need to avoid current draw from the input source, which can cause damage and yield problems during IC production and testing.
Innovation Solution
A comparator circuit design featuring approximately equal-sized FETs forming a differential pair with cross-coupled offset voltage circuits and a current source connected to ground, allowing for accurate trip point setting above the supply voltage without current drain from the input source, using diode-connected FETs and a precision reference voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a resistive voltage divider is used to detect voltage above the supply rail, then the voltage can be divided down for comparison, but an undesirable amount of current is sourced from the input voltage source
Solution Approach 1:
The patent introduces a current source circuit as an intermediary element that actively maintains the input voltage without drawing current from the source. This current source acts as a mediator between the voltage divider and the input source, providing the necessary bias current while preventing current draw from the external voltage source being measured.
2Measurement precision
If the input FET is made much smaller than the reference FET to achieve trip point above reference voltage, then the trip point can be set high, but the trip point varies significantly over fabrication processes and temperature
Solution Approach 1:
The patent employs feedback mechanisms through the current source circuit that actively compensates for variations in FET characteristics. The current source adjusts its output to maintain a stable trip point despite process variations and temperature changes, effectively canceling out the instability introduced by FET size mismatches.
Solution Approach 2:
The patent changes the operating parameters by using a current source to actively control the bias conditions rather than relying on fixed FET size ratios. This allows the trip point to be determined by controllable electrical parameters rather than fixed geometric parameters, making it less sensitive to fabrication variations.
3Device complexity
If supply voltage is used as reference voltage for comparison, then the circuit can be simplified, but the trip point accuracy is degraded due to supply voltage variations
Solution Approach 1:
The current source circuit serves as an intermediary that decouples the trip point determination from supply voltage variations. It provides a stable reference current that is insensitive to supply voltage changes, allowing the use of supply voltage as reference while maintaining accuracy through current-based stabilization.
Data Source
AI summary
A comparator circuit (300) has a first field effect transistor (FET) (307) with a supply voltage (301) connection and a diode connected FET (303) connected in series to form the first circuit leg of the comparator (300). A second diode connected FET (309) and a second FET (305) in series form the second circuit leg. The first FET (307) and said second FET (305) are approximately equal sized FETs. Another embodiment is an integrated circuit (401) with two n-channel FETs. A first diode connected FET (303) is connected to the first n-channel FET (307) in series to form the first circuit leg of a comparator (300) and a second diode connected FET (309) is connected to a second n-channel FET (305) in series to form the second circuit leg of the comparator. The two n-channel FETs that form the differential pair are approximately equal in size. The trip point is high with respect to the supply voltage.