Class AB LDO Regulator for Instant Load-Step Response
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Solution Overview
Problem
Existing LDO voltage regulators struggle to provide fast and stable voltage regulation for sudden increases in load current without increasing power consumption or requiring large external bypass capacitors, which are impractical for integrated circuits.
Innovation Solution
The design incorporates a class AB output stage with multiple current gain boost paths and additional feedback loops, including a current mirror and amplifying transistor, to rapidly respond to step changes in load current without increasing quiescent current, allowing for fast and stable regulation within an integrated circuit chip.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If existing LDO voltage regulator topologies are used, then the circuit can operate with standard components, but the response time to load current steps is too slow without large external bypass capacitors
Solution Approach 1:
The voltage regulator is divided into two independent control paths: a fast response path using source follower transistor MP4 for immediate load current step response, and a precision regulation path using the differential amplifier and error amplifier for accurate voltage regulation. This segmentation allows each path to be optimized for its specific function, achieving fast response without compromising regulation precision.
Solution Approach 2:
Source follower transistor MP4 acts as an intermediary element between the differential amplifier output and the pass transistor gate. It provides a low-impedance drive capability that enables fast charging of the pass transistor gate capacitance during load current steps, while the differential amplifier maintains precision voltage control. This intermediary structure resolves the conflict between speed and precision.
2Speed
If large external load bypass capacitors are used, then fast response to load current steps is achieved, but the cost and area increase significantly
Solution Approach 1:
The function of large external bypass capacitors is extracted and replaced by an internal active circuit consisting of source follower transistor MP4 and associated current sources. This internal circuit provides the necessary charge delivery capability without requiring external passive components, eliminating the need for large bonding pads and reducing overall device area and cost.
Solution Approach 2:
The voltage regulator circuit provides its own fast response capability through integrated active components rather than relying on external passive components. The source follower transistor MP4 and current sources generate the necessary drive current internally to respond to load transitions, making the system self-sufficient and eliminating external capacitor requirements.
3Speed
If external load bypass capacitors are used, then fast response is achieved, but parasitic inductance causes voltage drops that exceed regulation requirements
Solution Approach 1:
The fast response function and precision regulation function are merged into a single integrated circuit structure. The source follower transistor MP4 and differential amplifier work together in close proximity, eliminating the need for external connections and associated parasitic inductances. This integration ensures that the full drive current reaches the load without voltage drops, maintaining both fast response and regulation accuracy.
Solution Approach 2:
Source follower transistor MP4 serves as an intermediary that buffers the differential amplifier output from the high-current pass transistor gate. This intermediary provides low-impedance drive capability, ensuring that the gate capacitance can be charged rapidly without significant voltage drops, thereby maintaining regulation accuracy during fast transient responses.
4Speed
If the quiescent current is increased to speed up the response, then faster response is achieved, but power consumption increases
Solution Approach 1:
The circuit uses dynamic current control where current sources I1 and I2 can be rapidly adjusted in response to load transitions. During normal operation, the currents are at quiescent levels for low power consumption. During load current steps, the currents are dynamically increased to provide fast response, then returned to quiescent levels afterward. This dynamic operation achieves fast response only when needed, minimizing overall power consumption.
Solution Approach 2:
The current sources operate in a periodic manner, switching between quiescent current levels and elevated current levels during load transients. The source follower transistor MP4 and associated current sources are activated only during load current steps to provide fast response, and remain at low quiescent current levels during steady-state operation, thereby achieving fast response without continuous high power consumption.
Data Source
AI summary
An LDO regulator (10) produces an output voltage (Vout) by applying the output voltage to a feedback input (6) of a differential input stage (10A) and applying an output (3) of the differential input stage to a gate of a first follower transistor (MP4) having a source coupled to an input (8) of a class AB output stage (10C) which generates the output voltage. Demanded load current is supplied by the output voltage during a dip in its value to a gate of a second follower transistor (MP5) having a gate coupled to the output of the input stage to decrease current in a current mirror (MN5,6) having an output coupled to a current source (I1) and a gate of an amplifying transistor (MN7). This causes the current source to rapidly turn on the amplifying transistor to cause it to rapidly turn on a cascode transistor (MN3), causing it to turn on a pass transistor (MP3) of the output stage.