LDO Inrush Current Limiting via Miller Capacitor Pre-Charge

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Solution Overview

Problem

Low drop-out (LDO) voltage regulators face challenges in minimizing inrush current during start-up, especially with large external capacitors, where Process, Voltage, and Temperature (PVT) variations exacerbate the issue, leading to potential system lock or reset and increased board area requirements.

Innovation Solution

The implementation of a pre-charge circuit that clamps the Miller capacitor voltage close to normal operating conditions during start-up, using a constant current source to generate bias current for the input stage, and disabling the pre-charge after a defined time to ensure the input stage is biased correctly, thereby reducing inrush current independently of PVT variations.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If large decoupling capacitors are used in parallel to the supply, then the effect of inrush current is limited, but the area on printed boards is increased

Engineering Contradiction:

Improveinrush current effectVSAvoidprinted board area

Core Design Contradiction:

Object-affected harmful factorsVSArea of stationary object

Solution Approach 1:

The patent applies preliminary action by pre-charging the Miller capacitor before the main power switch is activated. The pre-charge circuit charges the Miller capacitor to a predetermined voltage level during a startup phase, so that when the main switch turns on, the inrush current is significantly reduced. This resolves the contradiction by eliminating the need for large decoupling capacitors while still protecting against inrush current effects.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If inrush current is minimized to avoid voltage drops and system lock, then the system stability is improved, but the circuit complexity increases

Engineering Contradiction:

Improvesystem stabilityVSAvoidcircuit complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent introduces an intermediary pre-charge circuit that mediates between the power supply and the main circuit during startup. This pre-charge circuit acts as a buffer, charging the Miller capacitor before the main power switch is activated. The intermediary circuit simplifies the overall design compared to using large decoupling capacitors while maintaining system stability and preventing voltage drops.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Speed

If the Miller capacitor is charged quickly during start-up, then the startup speed is improved, but the inrush current increases

Engineering Contradiction:

Improvestartup speedVSAvoidinrush current

Core Design Contradiction:

SpeedVSObject-generated harmful factors

Solution Approach 1:

The patent applies preliminary action by pre-charging the Miller capacitor to a predetermined voltage level before the main power switch is activated. This preliminary charging action enables fast startup without generating harmful inrush current, as the capacitor is already charged when the main switch turns on, eliminating the need for rapid charging during the main power phase.

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentEP2778823B1Method to limit the inrush current in large output capacitance LDOs

Publication Date: 2018.10.10 DIALOG SEMICON GMBH

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AI summary

The present document relates to a pre-charge circuit of electronic circuits having Miller compensation and significant output capacitance such as LDOs or multistage amplifiers. The pre-charge circuit limits an inrush current right after enabling of the electronic circuit. The pre-charge circuit limits and clamps the fast charging of the Miller capacitor. A delay circuit disables the pre-charge circuit when the bias conditions of the Miller capacitor are close to normal bias conditions.