LDO Soft-Start Reference Current Control for Supply Ramp Variations
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Solution Overview
Problem
Existing low-dropout (LDO) voltage regulators face challenges in reducing staircase behavior on the reference voltage or output during soft-starting, particularly when the input supply voltage has varying ramp rates.
Innovation Solution
The proposed solution involves a dropout detection circuit that dynamically adjusts the fast soft-start current based on the ramp rate of the input supply voltage, using a transistor's region of operation to assess the ramp rate and modulate the resistance of an input resistor accordingly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a fixed fast soft-start current is used during dropout mode recovery, then the regulator can quickly recover from dropout, but it causes overshoot and staircase behavior on the reference voltage and output when the input supply voltage has varying ramp rates
Solution Approach 1:
The patent implements a dynamic soft-start current adjustment mechanism that adapts the reference current magnitude based on the detected input voltage ramp rate. The dropout detection circuit continuously monitors the input voltage transition characteristics and dynamically modifies the soft-start current profile, switching between different current magnitudes to match the actual ramp rate conditions, thereby preventing both overshoot and excessive slowness
Solution Approach 2:
The patent changes the electrical parameter (reference current magnitude) based on the detected input voltage ramp rate characteristics. By detecting whether the input voltage is rising quickly or slowly during dropout recovery, the system adjusts the reference current parameter accordingly - using higher current for fast ramps and lower current for slow ramps - to optimize both recovery speed and output stability
2Manufacturing precision
If a slow soft-start current is used to prevent overshoot, then the output voltage remains stable, but the regulator takes too long to recover from dropout mode
Solution Approach 1:
The system dynamically adjusts the soft-start current magnitude based on real-time detection of input voltage ramp rate. When a slow ramp rate is detected, the system uses a lower reference current to prevent overshoot, while when a fast ramp rate is detected, it switches to a higher reference current to accelerate recovery, thus adapting the speed characteristic to match the actual operating conditions
Solution Approach 2:
The patent modifies the reference current parameter according to the detected ramp rate conditions. By changing the current magnitude parameter from low to high based on the input voltage transition characteristics, the system optimizes the trade-off between stability and recovery speed for each specific operating scenario
3Speed
If a high reference current is used during soft-start, then the regulator recovers quickly from dropout, but it increases capacitive in-rush current stress on the input power supply
Solution Approach 1:
The patent adjusts the reference current parameter dynamically based on the detected input voltage ramp rate. By using higher current only when the input voltage is rising quickly (fast ramp rate) and lower current when the ramp rate is slow, the system achieves fast recovery when necessary while minimizing in-rush current stress on the power supply during slower transition conditions
Data Source
AI summary
Techniques for controlling a low-dropout (LDO) voltage regulator. In an example, an LDO voltage regulator circuit includes an amplifier having an output coupled to a transistor. First and second inputs of the amplifier are coupled to a power supply node via first and second resistors, respectively. The transistor gate is coupled to the amplifier output, the transistor source is coupled to the second input of the amplifier, and the transistor drain is coupled to a reference voltage node. The second resistor is variable based on the amplifier output and a reference voltage from the reference voltage node. In an example, the reference voltage node is connectable to ground via a reference resistor connected in parallel with a noise-filtering capacitor, which causes a reference current to flow through the transistor. The reference current is adjusted based on the drain-to-source voltage of the transistor.