Low-Voltage Reference Current Generator Circuit with Resistance Compensation
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Solution Overview
Problem
Conventional reference current generator circuits for low-voltage applications require a large chip area for loop stability due to reliance on capacitor compensation, which is inefficient and costly.
Innovation Solution
A reference current generator circuit that incorporates an operational amplifier, a capacitor, and a compensation circuit with current mirrors to provide resistance compensation, reducing the need for bulk capacitance and minimizing chip area, while maintaining phase margin and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If capacitor compensation is used for loop stability in low-voltage applications, then phase margin is improved, but chip area increases significantly
Solution Approach 1:
The patent introduces a compensation circuit that includes a compensation capacitor and a compensation transistor as intermediary elements. The compensation capacitor (Ccomp) is coupled between the output terminal and the second input terminal of the operational amplifier, while the compensation transistor is connected in parallel with the compensation capacitor. This intermediary compensation mechanism provides the necessary phase margin improvement without requiring large bulk capacitors, thus resolving the contradiction between loop stability and chip area.
Solution Approach 2:
The patent changes the parameters of the compensation network by using a small compensation capacitor in combination with a compensation transistor rather than relying on large bulk capacitors. The compensation transistor dynamically adjusts the effective capacitance and provides additional phase boost, allowing the system to achieve adequate phase margin with much smaller capacitor values, thereby reducing chip area while maintaining loop stability.
2Ease of operation
If NMOS transistor is used as output stage, then loop stability control is simplified, but headroom becomes insufficient at low operation voltages
Solution Approach 1:
The patent employs a compensation transistor that can dynamically adjust its operating state to provide the necessary phase compensation. This dynamic compensation mechanism allows the circuit to maintain adequate phase margin and stability without requiring excessive headroom, enabling the use of NMOS output stage at low operation voltages where static compensation would fail.
Solution Approach 2:
The compensation transistor acts as an intermediary element that provides the necessary phase compensation without requiring large voltage headroom. By placing the compensation transistor in parallel with the compensation capacitor, the circuit achieves stability control with minimal additional voltage requirement, resolving the contradiction between ease of stability control and headroom availability at low voltages.
3Use of energy by moving object
If PMOS transistor is used as output stage to mitigate headroom issue, then headroom is improved, but chip area increases due to additional transistor layers
Solution Approach 1:
The compensation transistor serves multiple functions: it provides phase compensation, boosts the phase margin, and enables the circuit to operate at low voltages without requiring additional PMOS output stage layers. This multi-functional compensation approach achieves headroom improvement without the area penalty of adding entire transistor layers, as the compensation transistor can be integrated within the existing circuit footprint.
Data Source
AI summary
A reference current generator circuit suitable for low-voltage applications is provided. The generator circuit is fabricated in a chip for generating a precise reference current based on a precise reference voltage and a precise external resistor. The generator circuit provides an equivalent resistance coupled in parallel with the external resistor to provide resistance compensation and reduce the impedance of seeing into the chip from a chip pad. In addition to the resistance compensation, only moderate capacitance compensation is required to enhance the phase margin of the generator circuit, so as to achieve a stable loop. Therefore, chip area and cost can be reduced in low-voltage applications. In addition, the generator circuit reproduces the reference current generated by the external resistor by utilizing current mirrors, so as to eliminate the effect on currents caused by parallel coupling of the equivalent resistance and the external resistor.