Bootstrap MOS Analog Switch for Low-Voltage On-Resistance
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Solution Overview
Problem
Existing analog signal switches, particularly in low-voltage applications, face challenges due to high threshold voltages of diodes and transistor sensitivity to fatigue, leading to inefficient voltage control and increased risk of transistor degradation.
Innovation Solution
A low-voltage analog switch design incorporating a main MOS transistor with single gate oxide thickness and auxiliary transistors with double gate oxide thickness, utilizing a high-pass filter and complementary MOS transistors to manage voltage levels and reduce transistor stress, thereby optimizing on-resistance and minimizing fatigue.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the bootstrap technique is used to apply high gate-source voltage to reduce on-resistance, then the on-resistance is improved, but the transistor becomes sensitive to fatigue and stress phenomena
Solution Approach 1:
The patent uses two types of transistors with different gate oxide thicknesses (GO1 with single thickness and GO2 with double thickness) to operate at different voltage levels. The GO2 transistor is subjected to higher voltage stress during bootstrap operation, while the GO1 transistor handles the analog signal at lower voltage, thus protecting it from fatigue and stress phenomena.
Solution Approach 2:
The patent segments the voltage handling functions by using two distinct transistor types with different voltage ratings. The GO2 transistor is dedicated to handling the high voltage bootstrap signal, while the GO1 transistor is dedicated to the analog signal path, isolating the stress-induced degradation to the GO2 device only.
2Reliability
If GO2 transistors with double gate oxide thickness are used to handle high voltage, then voltage stress is reduced, but the on-resistance and cutoff frequency deteriorate
Solution Approach 1:
The patent divides the circuit into two functional segments: the GO2 transistor handles the high-voltage bootstrap control signal, while the GO1 transistor handles the analog signal switching. This segmentation allows each transistor type to operate in its optimal voltage range, with GO2 experiencing voltage stress and GO1 maintaining low on-resistance.
Solution Approach 2:
The patent changes the gate oxide thickness parameter to match the voltage requirements of each circuit function. GO2 transistors with double gate oxide thickness are used where high voltage is present, while GO1 transistors with single gate oxide thickness are used where low on-resistance is critical.
3Ease of operation
If diode threshold voltage is used in bootstrap structure, then voltage control is simplified, but the voltage reached across the capacitor is insufficient in low voltage applications
Solution Approach 1:
The patent introduces an intermediate N-channel MOS transistor as a mediator between the bootstrap capacitor and the analog signal path. This transistor allows the bootstrap capacitor to charge to a higher voltage than the diode threshold would normally permit, while still providing controlled voltage to the gate of the switching transistor.
Solution Approach 2:
The patent changes the control mechanism from direct diode threshold-based charging to MOS transistor-controlled charging, allowing the capacitor voltage to exceed the diode threshold and reach levels sufficient for low-voltage applications while maintaining controllable gate drive.
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 simple and efficient analog switch with improved on-resistance and reduced transistor fatigue, suitable for low-voltage applications by maintaining optimal voltage levels and minimizing stress on transistors.
Implementation Method 1
a capacitor Cb, having a low-side terminal connected to the source of the transistor MNH and a high-side terminal connected to the line Vdd by a diode D
Implementation Method 2
The diode D is connected to be conductive when the output V1 is pulled toward the line Vss by the transistor MNH. The capacitor Cb thus charges to a voltage Vdd−Vt, where Vt is the threshold voltage of the diode D
Implementation Method 3
An analog signal switch is generally formed by a metal oxide semiconductor (MOS) transistor, the source forming the input terminal and the drain forming the output terminal. To close the switch, i.e. render the transistor conductive, a gate-source voltage Vgs exceeding the threshold voltage Vt of the transistor is normally applied
Data Source
AI summary
A switch for an analog signal may include a main MOS transistor whose source forms an input terminal of the switch and whose drain forms an output terminal of the switch, a capacitor having a first terminal permanently connected to the source of the main transistor, a circuit for charging the capacitor, and a first auxiliary transistor configured to connect the second terminal of the capacitor to the gate of the main transistor in response to a control signal. The charge circuit may include a resistor permanently connecting the second terminal of the capacitor to a power supply line. The capacitor and the resistor may form a high-pass filter having a cutoff frequency lower than the frequency of the analog signal.