PTAT Low-Voltage Detector Using Self-Cascode MOSFETs
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Solution Overview
Problem
Existing low voltage detectors consume high power, occupy large areas, and are sensitive to temperature variations due to reliance on threshold voltage of MOSFETs, requiring additional circuitry for compensation and often necessitating trimming to counteract part-to-part variations.
Innovation Solution
A low-power, low-voltage detector circuit utilizing a PTAT voltage generator and a power supply voltage monitor with self-cascode MOSFET structures, which self-compensates for temperature changes and reduces area occupancy by eliminating resistors, ensuring reliable operation across a wide temperature range without the need for trimming.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If voltage dividers and bandgap reference voltage are used to operate the low voltage detector, then reliable voltage detection is achieved, but power consumption increases to hundreds of microamperes
Solution Approach 1:
The patent changes the operating parameters by using a PTAT voltage generator instead of traditional bandgap reference, and employs self-cascode MOSFET structures to operate at nanowatt power levels while maintaining detection reliability through temperature-proportional voltage generation
Solution Approach 2:
The self-cascode MOSFET structures generate their own bias currents and voltages internally, eliminating the need for external power-consuming reference circuits. The circuit serves itself by using the PTAT voltage to automatically establish appropriate operating points for detection
2Reliability
If ladder resistors are used to generate voltage taps for detecting trip point, then voltage detection is achieved, but the detector occupies a large area
Solution Approach 1:
The patent replaces the physical ladder resistor structure with a voltage generator using self-cascode MOSFETs. This substitution eliminates the need for large physical resistors while maintaining the voltage tap generation function through active device operation
Solution Approach 2:
The patent changes from passive resistor-based voltage division to active MOSFET-based voltage generation, where the voltage taps are derived from the PTAT voltage generator's internal node voltages rather than external resistor dividers
3Reliability
If the trip-point depends on threshold voltage Vth of MOSFET, then voltage detection is achieved, but part-to-part variation is greatly increased due to temperature dependence
Solution Approach 1:
The patent changes the reference from threshold voltage (which has high temperature coefficient) to PTAT voltage (which has predictable linear temperature dependence). This parameter change allows the trip point to track temperature changes systematically rather than varying randomly with process and temperature
Solution Approach 2:
The PTAT voltage generator inherently provides temperature compensation feedback by generating a voltage that increases with temperature, automatically counteracting the threshold voltage shifts and maintaining consistent trip points across temperature and process 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 achieves reliable low-voltage detection in the nanowatt power range with minimal area usage, reducing sensitivity to temperature variations and eliminating the need for additional power-consuming compensation circuits, thus providing a compact and efficient voltage detection system.
Implementation Method 1
The thermal voltage ΦT is a function of temperature and increases directly proportionately with increasing temperature. The thermal voltage is ΦT=kT/q, where T is the temperature measured in kelvins
Data Source
AI summary
A low voltage detector (100) includes a power supply voltage monitor circuit (110) that produces a voltage VSP related to a first a power supply voltage, and a voltage generator (105), which includes a plurality of self-cascode MOSFET (SCM) structures (101-103) in a cascade configuration, that generates a reference voltage Vxm. A voltage comparator (140) outputs an output signal in response to a differential between Vxm and VSP, wherein Vxm and VSP have proportional to absolute temperature behavior (PTAT) over temperature with respect to a second power supply voltage. The output signal changes state when the first power supply voltage equals a trip point of the comparator. Each SCM structure is sized to provide a rate of change with temperature of the PTAT behavior of Vxm that matches a rate of change with temperature of the PTAT behavior of VSP.