LDO Regulator Multiplexed Power Supply Back-Gate Control
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Solution Overview
Problem
Conventional low dropout (LDO) regulators face challenges in efficiently selecting between power supplies due to significant voltage drops and sequence requirement violations, particularly when handling large currents and external power supply configurations.
Innovation Solution
The proposed regulator design includes a supply node with back-to-back PMOS transistors and switches controlled by control signals, allowing for parallel operation and direct power supply from input terminals to the output, with the back-gates of power transistors coupled together, enabling smaller switch sizes and efficient current handling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a large current (500 mA) is provided through a switch, then the current handling capability is improved, but the switch area increases significantly to avoid voltage drop
Solution Approach 1:
The power supply selection function is segmented between two independent paths: one path uses switch 102 for direct power supply, while the other path uses capacitor C and switch 104 for regulated power supply. This segmentation allows each switch to be optimized for its specific function, reducing the area requirement for switch 104 since it only needs to handle capacitor charging/discharging rather than full load current.
Solution Approach 2:
Capacitor C is introduced as an intermediary energy storage element between the external supply and the LDO regulator. This capacitor mediates the power delivery, allowing switch 104 to operate with smaller size since the capacitor handles the bulk of current delivery during transient and steady-state operation, while the switch only needs to control charging and selection.
2Reliability
If an external supply with capacitor C is used to provide regulated voltage, then the voltage regulation is improved, but sequence requirements are violated
Solution Approach 1:
The control signals SEL1 and SEL2 are dynamically adjusted based on the operational state of the regulator. When the external supply is not yet regulated (during startup), SEL1 is asserted to enable switch 102 and provide power through the LDO. Once the external supply is regulated, SEL2 is asserted to switch to the external supply path. This dynamic control ensures sequence requirements are always satisfied while maintaining proper voltage regulation.
Solution Approach 2:
The system uses control signals SEL1 and SEL2 that respond to the operational status of the power supply paths. The feedback mechanism ensures that the switch between internal LDO and external supply occurs at the appropriate time, preventing sequence violations while maintaining continuous power supply and proper voltage regulation.
3Power
If switch 102 or 104 is made large to handle 500 mA current, then the current handling is improved, but the device complexity and area increase
Solution Approach 1:
The power delivery function is segmented between the LDO regulator path and the external supply path with capacitor. This segmentation allows switch 104 in the external supply path to be much smaller since it only needs to handle capacitor charging currents and selection switching, not the full 500 mA load current continuously. The LDO path with switch 102 handles the full current when needed.
Solution Approach 2:
Capacitor C acts as an intermediary that decouples the current handling requirements from switch 104. The capacitor absorbs and releases current as needed, allowing the switch to be small while still supporting large current delivery capability through the external supply path during transient conditions.
Data Source
AI summary
Generally, with low drop out (LDO) regulators that use multiplexed power supplies, the transistors within the regulator can use a substantial amount of area. Here, a regulator is provided that uses a multiplexer to commonly control the back-gates of multiple power transistors within the LDO. By doing this, the area overhead that would normally be present with these switches (of the multiplexer) can be dramatically reduced without sacrificing performance.