Subthreshold Switch Circuit for Wideband PSRR in Regulator References
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Solution Overview
Problem
Existing regulator and reference circuits face challenges in achieving high Power Supply Rejection Ratio (PSRR) across a wide range of frequencies due to power supply noise injection through parasitic capacitance, requiring large chip area and increased cost for low pass filtering.
Innovation Solution
A switch circuit with a supply transistor biased in the subthreshold region, combined with a current mirror circuit and a low pass filter, effectively filters power noise across a wide frequency range, reducing chip size and cost by utilizing a PMOSFET and filtering capacitor configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a low pass filter with large resistor and capacitor is used to filter power supply noise, then the PSRR is improved, but the chip area and cost increase
Solution Approach 1:
The patent changes the operating parameters of the switch transistor by biasing it in the subthreshold region, which dramatically increases its on-resistance without requiring a physically larger component. This parameter change allows achieving the same filtering effect with much smaller chip area
Solution Approach 2:
The patent introduces a supply transistor as an intermediary element between the power supply and the driver circuit. This transistor acts as a controlled impedance element that blocks power supply noise while allowing the drive signal to pass through, thereby improving PSRR without needing large external filtering components
2Reliability
If a low pass filter with large resistor and capacitor is used to filter power supply noise, then the PSRR is improved, but the cost increases
Solution Approach 1:
The patent replaces expensive large-value passive components (10 MΩ resistor and 10 pF capacitor) with a standard-sized switch transistor operated in subthreshold mode. This substitution uses a common, inexpensive transistor to achieve the same noise filtering function, significantly reducing component cost
3Reliability
If a switch is made large to negate the effect of on resistance, then the switching performance is improved, but the parasitic capacitance increases causing power supply noise injection
Solution Approach 1:
The patent changes the operating state of the switch from linear/ohmic region to subthreshold region. In subthreshold operation, the transistor exhibits extremely high on-resistance and reduced parasitic capacitance effects, allowing the switch to maintain adequate performance while minimizing noise injection through the parasitic capacitance
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 higher PSRR across a wider frequency range with reduced chip size and cost, filtering out noise from 100 Hz onwards, and occupies less space compared to conventional RC filters.
Implementation Method 1
a supply transistor, configured to operably generate the supply current, wherein the supply transistor is biased in a subthreshold region, such that the PSRR of the main circuit is higher than a predetermined level within a predetermined frequency range
Implementation Method 2
a filtering capacitor coupled to an output terminal of the first driver
Data Source
AI summary
A switch circuit includes: a switch coupled to control an electrical parameter of a main circuit. A. PSRR of the main circuit is determined by the switch; a first driver configured to operably drive the switch, wherein the first driver is powered by a supply current; and a supply transistor configured to operably generate the supply current, wherein the supply transistor is biased in a subthreshold region, such that the PSRR of the main circuit is higher than a predetermined level within a predetermined frequency range.