Voltage Expander for High Voltage Current Source in Low Voltage CMOS
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Solution Overview
Problem
Low voltage integrated circuits, such as those produced by advanced CMOS processes, face challenges in interfacing with higher voltage circuits, particularly in applications like implantable medical devices for functional electrical stimulation, where processing high voltage signals is necessary without exceeding transistor breakdown voltages.
Innovation Solution
A voltage expander and high voltage current source are implemented in a low voltage semiconductor process, using a diode and voltage divider to extend the operating voltage range of a stack of transistors up to k times the supply voltage without exceeding breakdown voltages, and a high voltage wide swing current source is utilized to maintain constant output current across the extended voltage range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a stack of low voltage MOSFET transistors is used to provide high voltage operation, then the circuit can operate at elevated voltage levels, but the output voltage swing is limited and cannot exceed the breakdown voltage of individual transistors
Solution Approach 1:
A voltage expander circuit is introduced as an intermediary between the low voltage CMOS core and the high voltage output stage. This expander uses a stack of transistors with dynamically controlled gate voltages to amplify the voltage swing from 1xVdd to kxVdd while keeping individual transistor voltages within safe operating limits. The gate voltage control circuit acts as a mediator that distributes the voltage swing across multiple transistor gates proportionally.
Solution Approach 2:
The high voltage output stage is segmented into multiple transistor stages stacked in series. Each transistor handles a portion of the total voltage swing, specifically Vout/k, rather than requiring a single transistor to withstand the full kxVdd swing. This segmentation allows the use of standard low voltage transistors to achieve high voltage operation by dividing the voltage stress across multiple devices.
2Adaptability or versatility
If special processes are used to produce low voltage and high voltage devices in the same IC, then high voltage interfacing capability is achieved, but performance capabilities are limited
Solution Approach 1:
The invention uses standard low voltage CMOS transistors to perform multiple functions: they serve as both the core logic devices and as the high voltage output stage transistors. By incorporating a voltage expander circuit that dynamically controls gate voltages, the same transistor technology achieves both low voltage logic operation and high voltage signal output, eliminating the need for separate high voltage device fabrication processes.
Solution Approach 2:
The gate voltage parameters of the transistor stack are dynamically changed and controlled based on the output voltage level. The voltage expander circuit adjusts each transistor's gate voltage proportionally to the output swing, allowing the transistors to operate safely within their breakdown limits while achieving kxVdd output swing. This parameter control enables standard transistors to function in high voltage applications.
3Temperature
If the output voltage swing is extended to k times Vdd, then high voltage signal processing capability is achieved, but the risk of exceeding transistor breakdown voltage increases
Solution Approach 1:
The voltage expander circuit incorporates feedback mechanisms where the output voltage is sensed and used to control the gate voltages of the transistor stack. This feedback ensures that each transistor's voltage stress remains proportional to the total output swing and never exceeds its breakdown limit. The gate control circuit continuously adjusts gate voltages based on the instantaneous output voltage level, preventing breakdown conditions.
Solution Approach 2:
The gate voltages of the transistor stack are preliminarily controlled and set before the full output voltage swing occurs. The voltage expander circuit establishes appropriate gate voltage levels in advance, ensuring that as the output voltage reaches kxVdd, each transistor is already biased to handle its share of the voltage stress safely. This preliminary gate control prevents breakdown by preparing the transistor operating points before high voltage conditions arise.
Data Source
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AI summary
A high voltage current source and a voltage expander (250) implemented in a low voltage semiconductor process. The voltage expander extends the operating voltage range of a stack of transistors to k times the supply voltage Vdd at the output node of the stack without exceeding the breakdown voltage of any of the transistors in the stack. The voltage expander uses a diode (220) and a voltage divider (215) to detect the output node voltage (Vout) changes and generates a plurality of voltages (Vgk-1) that control the gate voltages for the stack of transistors (213-1,213-k-1). A high voltage wide swing current source utilizes a transistor to set the output current and the voltage expander to extend the output voltage range of the current setting transistor. An additional transistor and another current source ensure that the output current is constant throughout the entire output voltage range between about 0V and k x Vdd.