Floating Substrate CMOS Reference Voltage Circuit
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Solution Overview
Problem
Existing reference voltage generation circuits in CMOS technology are susceptible to temperature and substrate voltage variations, leading to instability in LCD display synchronization and noise issues due to stray currents.
Innovation Solution
A circuit design that generates a reference voltage independent of temperature and substrate voltage by using two stages: one stage producing a current proportional to temperature and another inversely proportional, with a current mirror and summing circuit to combine these currents at a voltage distinct from the substrate, ensuring stability and noise reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a reference voltage generation circuit is built on a CMOS substrate using bipolar transistors, then the circuit can generate a reference voltage, but the reference voltage becomes susceptible to substrate voltage variations and low-frequency stray currents
Solution Approach 1:
The patent introduces a floating substrate voltage Vsub' as an intermediary reference level that is electrically isolated from the actual substrate. By connecting the bipolar transistor collectors to this floating substrate through high-value resistors (1MΩ to 10MΩ) rather than directly to the substrate, the circuit mediates the interaction between the substrate environment and the reference voltage generation, thereby blocking harmful substrate variations while allowing necessary signal flow.
Solution Approach 2:
The patent extracts the reference voltage generation function from the noisy substrate environment by creating a separate, electrically isolated circuit domain. The floating substrate concept effectively separates the sensitive reference voltage circuitry from the noisy digital substrate, allowing the reference voltage to be generated in a clean, isolated electrical domain that is not directly affected by substrate fluctuations.
2Temperature
If diodes with different active areas are used to generate temperature-independent reference voltage, then temperature compensation is achieved, but the circuit remains affected by substrate noise in CMOS technology
Solution Approach 1:
The floating substrate Vsub' acts as an intermediary reference that preserves the temperature compensation mechanism while isolating the circuit from substrate noise. The bipolar transistors continue to utilize their base-emitter voltage temperature characteristics for reference generation, but the floating substrate connection mediates the interaction with the noisy CMOS substrate, allowing temperature-independent operation without substrate noise contamination.
3Device complexity
If the reference voltage is defined with respect to the substrate voltage, then the circuit is simple to implement, but any substrate voltage variation directly affects the reference voltage level
Solution Approach 1:
The patent maintains circuit simplicity by using the same basic bandgap reference topology, but introduces a floating substrate Vsub' as a mediating reference level. This intermediary allows the reference voltage to be defined relative to a stable, isolated reference rather than the noisy substrate, preserving implementation simplicity while dramatically improving reference voltage stability against substrate variations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides a stable and temperature-independent reference voltage, reducing noise and ensuring reliable synchronization in LCD displays by decoupling the reference voltage from substrate voltage variations, thus enhancing the operational stability and accuracy of analog-digital conversion circuits.
Implementation Method 1
a first stage for generating a first current proportional to temperature
Implementation Method 2
a second stage for generating a second current inversely proportional to temperature
Implementation Method 3
The branches respectively comprise a first and a second MOS transistor that are identical, one being connected between the current mirror and the first diode and the other between the current mirror and the resistor
Data Source
AI summary
A circuit generates a reference voltage that is independent of temperature. The circuit is built on a substrate according to a CMOS technology, and includes a first stage for generating a first current proportional to temperature and a second stage for generating a second current inversely proportional to temperature. These first and second currents are summed in a resistor connected to a voltage distinct from the ground of the first and second stages and formed by the voltage of the substrate on which the circuit is built.