LDO Regulator Charge Pump Offset for Low Voltage Operation
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Solution Overview
Problem
Low-dropout regulators face challenges in maintaining necessary gate-source headroom at input voltages below 0.5V, particularly in analog LDOs, which leads to increased pass transistor size and power inefficiency, making them unsuitable for near-threshold voltage operations.
Innovation Solution
A negative voltage offset is introduced between the error amplifier and the pass transistor, which can be fixed or adaptive, using a charge pump with four switched capacitors operating in non-overlapping phases to generate an offset error voltage that provides the necessary gate-source headroom, reducing the required pass transistor size and improving power efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a conventional LDO regulator is used without negative voltage offset, then the circuit structure is simple, but the gate-source headroom is insufficient at low input voltages below 0.5V
Solution Approach 1:
A charge pump circuit is introduced as an intermediary component between the error amplifier and the pass transistor. This charge pump generates a negative voltage offset that is added to the error amplifier output, providing the necessary gate-source headroom for the pass transistor to operate properly at low input voltages below 0.5V without requiring a completely redesigned conventional LDO structure.
Solution Approach 2:
The invention changes the voltage parameter by generating a negative voltage offset through the charge pump circuit. This offset voltage is dynamically added to the error amplifier output signal, effectively shifting the operating point of the pass transistor gate to ensure adequate headroom is maintained even when the input voltage drops below the traditional 0.5V threshold.
2Ease of operation
If the pass transistor size is increased to maintain headroom at low voltages, then the gate-source headroom is sufficient, but the power efficiency decreases and device area increases
Solution Approach 1:
The charge pump acts as a mediator that provides the necessary voltage boost to the pass transistor gate, eliminating the need to increase transistor size. By injecting the negative voltage offset, the charge pump enables the pass transistor to maintain proper headroom operation with its original, smaller size, thereby preserving power efficiency and minimizing device area occupation.
Solution Approach 2:
The charge pump operates using periodic switching of its capacitive elements during non-overlapping clock phases. This periodic action allows the charge pump to generate the required negative voltage offset efficiently without continuous power dissipation, maintaining high power efficiency while providing sufficient gate-source headroom for the pass transistor.
3Ease of operation
If the pass transistor size is increased to maintain headroom at low voltages, then the gate-source headroom is sufficient, but the device area increases by up to 10×
Solution Approach 1:
The charge pump circuit serves as an intermediary voltage generation device that provides the necessary negative offset to the pass transistor gate. This approach allows the pass transistor to maintain adequate headroom at low input voltages without requiring any increase in its physical dimensions, thereby avoiding the 10× area increase that would otherwise be necessary.
Solution Approach 2:
Instead of solving the headroom problem by increasing the transistor size in the spatial dimension, the invention introduces a temporal dimension through the charge pump's periodic operation. The charge pump generates voltage offsets during specific clock phases, effectively providing headroom through time-based voltage manipulation rather than spatial expansion of the transistor.
4Ease of operation
If a fixed negative voltage offset is used, then the gate-source headroom is provided at low input voltages, but the offset cannot adapt to varying supply voltages
Solution Approach 1:
The charge pump circuit is designed with dynamic operation that adapts to varying supply voltages. The capacitive elements and switching network of the charge pump automatically adjust their charging and discharging characteristics based on the input voltage level, providing an adaptive negative voltage offset that maintains proper gate-source headroom across different operating conditions and supply voltage variations.
Solution Approach 2:
The charge pump circuit incorporates feedback mechanisms through its connection to the error amplifier and the pass transistor operation. The error amplifier monitors the output voltage and adjusts the charge pump's operation accordingly, ensuring that the negative voltage offset provided is always appropriate for the current supply voltage level, thereby achieving adaptability to supply voltage variations.
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 negative voltage offset allows LDOs to operate at lower input voltages, reduces pass transistor size by up to 10×, and achieves higher current efficiency and reduced ripple, enabling operation from 0.3V to 1.0V with minimal quiescent current and low output ripple.
Implementation Method 1
A charge pump with four switched capacitors operating in non-overlapping phases to generate an offset error voltage
Data Source
AI summary
A low-dropout (LDO) regulator. The LDO regulator includes a pass transistor, a charge pump connected to the pass transistor, and an error amplifier connected through the charge pump to the pass transistor, wherein the error amplifier receives a voltage VO from the pass transistor and generates a voltage VE based on the voltage VO, wherein the charge pump receives the voltage VE from the error amplifier, generates a voltage VE* that is lower than VE by an offset and supplies the voltage VE* as a gate voltage to the pass transistor.