Compensation VCCS for Low Dropout Voltage Regulator Stability
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Solution Overview
Problem
Conventional low dropout (LDO) voltage regulators face challenges in stabilizing output voltage with 1 uF low ESR ceramic capacitors under large output currents, particularly due to the body effect of NMOS transistors, which limits their ability to operate with low input/output voltages required by modern system-level chips.
Innovation Solution
The design incorporates a compensation voltage controlled current source (VCCS) with four NMOS field effect transistors, a current mirror, and a compensation capacitor, optimized to minimize direct current injection and enhance stability by adjusting transistor sizes and adding a resistor to create additional zeros in the feedback loop, thereby reducing the minimum output voltage and operating supply voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional VCCS with single NMOS transistor is used, then the circuit is simple, but the minimum output voltage is high due to body effect
Solution Approach 1:
The VCCS is divided into two independent parallel branches: one branch contains NMOS transistor MN1 for generating the main compensation current, and the other branch contains NMOS transistor MN2 for providing a bias current. This segmentation allows each transistor to operate independently, eliminating the body effect that would otherwise require a high minimum output voltage in a single-transistor configuration.
Solution Approach 2:
The invention changes the operational parameters of the VCCS by using two NMOS transistors with different gate voltage configurations. MN1 has its gate connected to the output voltage node, while MN2 has its gate connected to a bias voltage node. This parameter change enables the circuit to achieve low minimum output voltage by ensuring both transistors remain in saturation region without requiring high voltage headroom.
2Speed
If transistor sizes are optimized in the VCCS, then frequency response improves, but device complexity increases
Solution Approach 1:
The invention optimizes the width-to-length ratios of the NMOS transistors as key parameters. By carefully selecting the transistor dimensions, the circuit achieves desired frequency response characteristics and stability margins without requiring additional complex circuitry. The parameter optimization is performed within the existing two-transistor architecture.
3Stability of the object's composition
If dropout voltage is reduced, then operating supply voltage decreases, but stability under large output current deteriorates
Solution Approach 1:
The VCCS is integrated into the feedback loop of the LDO regulator, with its output connected to the error amplifier. The compensation current generated by the VCCS provides frequency compensation that ensures stability under large output current conditions. This feedback mechanism allows the circuit to maintain stability while operating at low supply voltages.
Solution Approach 2:
The segmented VCCS structure with two parallel NMOS transistor branches enables independent optimization of each transistor's operating point. This segmentation allows the circuit to maintain proper current mirroring and compensation function even when operating at low voltage drops, ensuring stability under large output current while keeping the operating supply voltage low.
Data Source
AI summary
Techniques pertaining to designs of a compensation voltage controlled current source (VCCS) used in low dropout voltage regulators are disclosed. According to one aspect of the present invention, a compensation voltage controlled current source (VCCS) is so designed to meet the low input/output voltage requirements. Various features of the VCCS are demonstrated through several embodiments.