PMOS Charge Pump Inversion for Threshold Voltage Stability
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Solution Overview
Problem
As supply voltage decreases with advanced technologies, the efficiency of charge pump circuits, such as the Dickson charge pump, is reduced due to increased threshold voltage of NMOS devices, limiting the number of cascaded stages and requiring thick oxide, high voltage transistors, which prevents the use of thin oxide, low voltage standard devices.
Innovation Solution
The use of p-type metal-oxide-semiconductor field-effect transistors with control gate phase inputs to enhance switching performance by reducing threshold voltage degradation, allowing for the construction of efficient, low voltage charge pumps with standard components, including two half pump stages and phase inputs to manage voltage drops and maintain transistor performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If NMOS transistors are used in charge pump stages, then the circuit can be constructed with standard components, but the threshold voltage increases due to body effect, limiting the number of cascaded stages and reducing pumping efficiency
Solution Approach 1:
The patent inverts the conventional approach by using PMOS transistors instead of NMOS transistors in the charge pump stages. This inversion allows the source to be connected to the bulk (substrate), eliminating the body effect that causes threshold voltage increases in NMOS devices. The PMOS transistors maintain stable threshold voltage even when cascaded stages create voltage drops, while still being compatible with standard low-voltage fabrication processes.
Solution Approach 2:
The patent changes the transistor type parameter from NMOS to PMOS, which fundamentally alters the electrical characteristics. PMOS transistors have opposite polarity and different threshold voltage behavior compared to NMOS, allowing the charge pump to operate with stable threshold voltage under voltage drops caused by cascaded stages, thereby improving pumping efficiency without requiring thick oxide high-voltage devices.
2Strength
If the number of cascaded stages is increased to achieve higher voltage multiplication, then the output voltage increases, but the threshold voltage degradation accumulates, reducing the pumping efficiency
Solution Approach 1:
By inverting to PMOS transistors, the patent eliminates the body effect that causes threshold voltage degradation in cascaded NMOS stages. Each PMOS stage maintains stable threshold voltage regardless of the voltage drop from previous stages, allowing multiple stages to be cascaded without accumulating threshold voltage degradation, thus maintaining high pumping efficiency at higher output voltages.
3Reliability
If thick oxide high voltage transistors are used to sustain large voltage drops, then the circuit reliability improves, but the use of thin oxide low voltage standard devices is prevented, increasing manufacturing cost
Solution Approach 1:
The patent inverts the transistor configuration to PMOS with source connected to bulk, which intrinsically protects against threshold voltage degradation under voltage drops. This eliminates the need for thick oxide high-voltage transistors, allowing the use of thin oxide low-voltage standard devices that are cheaper to manufacture while still reliably sustaining the necessary voltage drops in cascaded stages.
Data Source
AI summary
An improved charge pump design useful in low power applications derives an alternative voltage from a supply voltage. The design can be constructed using PMOS manufactured according to standard processes such that triple well manufacturing processes are not required. The design can incorporate control gate circuitry to increase efficiency and decrease degradation due to the threshold voltage of the transistors used.