LDO Driver Circuit Reduces Output Capacitor Size
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Solution Overview
Problem
Existing LDO regulator designs require large external decoupling capacitors, leading to increased PCB footprint, longer charge and discharge times, high startup charge requirements, and significant charge loss when disabled, which are detrimental in handheld applications and power management integrated circuits.
Innovation Solution
A driver circuit for LDO regulators that utilizes a current mirror configuration with a supplemental transistor to reduce the size of the output capacitor while maintaining stability across a wide range of load currents, achieved by nonlinear mirroring of the load current and adaptive biasing to compensate for increased capacitance at the pass transistor gate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a large external decoupling capacitor is used at the LDO output, then stability and pole separation are improved, but PCB footprint area increases
Solution Approach 1:
The patent introduces a driver stage as an intermediary between the error amplifier and the pass transistor. This driver stage includes a first current mirror transistor and a first supplemental transistor that work together to modify the impedance characteristics at the pass transistor gate, enabling stable operation with reduced output capacitor size. The driver stage acts as a mediator that transforms the signal characteristics to achieve both stability and reduced capacitance requirements.
Solution Approach 2:
The patent changes the impedance parameter at the pass transistor gate by introducing the driver stage with current mirror and supplemental transistor configurations. By adjusting the small-signal impedance through these additional transistors, the system achieves improved pole separation and stability without requiring large output capacitance, thus reducing the PCB footprint.
2Stability of the object's composition
If a large external decoupling capacitor is used, then stability is improved, but charge and discharge time increases
Solution Approach 1:
The driver stage serves as an intermediary that actively manages the gate voltage of the pass transistor through current mirror and supplemental transistor action. This active control mechanism compensates for reduced capacitive energy storage, enabling faster charge and discharge cycles while maintaining stability through the impedance-modifying effect of the driver transistors.
Solution Approach 2:
The patent introduces dynamic control elements (driver stage transistors) that actively adjust the gate voltage in response to load changes. This dynamic control compensates for the reduced static energy storage capacity of smaller output capacitors, enabling faster transient response and reduced charge/discharge times while maintaining stability.
3Stability of the object's composition
If a large external decoupling capacitor is used, then stability is improved, but startup charge requirement increases
Solution Approach 1:
The driver stage acts as an intermediary that provides active voltage control at the pass transistor gate during startup. The current mirror and supplemental transistor configurations enable the driver stage to efficiently charge the reduced output capacitor and establish proper operating conditions with lower total charge requirements compared to conventional direct-drive configurations.
4Stability of the object's composition
If a large external decoupling capacitor is used, then stability is improved, but phase margin decreases
Solution Approach 1:
The driver stage introduces additional poles and zeros through its transistor configurations that compensate for the phase lag introduced by reduced output capacitance. The current mirror and supplemental transistor create frequency compensation effects that maintain adequate phase margin even with smaller output capacitors, thus preserving stability and reliability.
Data Source
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AI summary
Circuits and methods to reduce the size of output capacitors of LDOs or amplifiers are disclosed. Nonlinear mirroring of the load current allows scaling of gain or adapting small signal impedance of a pass transistor depending on other inputs, in case of a preferred embodiment, allows to reduce small signal impedance at the gate of the pass transistor as the load current increases, hence allowing to reduce the size of an output capacitor without compromising stability of the system.