Voltage Regulator Soft-Start Circuit for Controlled Reference Ramp-Up
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Solution Overview
Problem
Voltage regulators experience excessive current flow during start-up due to rapid snap-up of output voltage, which can damage the regulator or prevent output, and existing solutions either increase cost and space with external capacitors or occupy die area with integrated capacitors.
Innovation Solution
A soft-start circuit that uses a transistor with low threshold voltage, an inverter, and resistors to generate a reference voltage that ramps up at the same rate as the supply voltage, avoiding the need for capacitors and preventing false triggering of overvoltage protection during start-up.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a charging capacitor is provided as an external component, then the regulator can achieve slow ramp-up to prevent excessive current, but the cost and space requirements are increased
Solution Approach 1:
The patent merges the soft-start charging function with the existing reference current generator circuitry. The reference current generator that already exists in the voltage regulator is used to charge the soft-start capacitor, eliminating the need for separate external charging components. This integration achieves slow ramp-up protection while reducing external component requirements
Solution Approach 2:
The reference current generator serves dual purposes: it provides the reference voltage for the voltage regulator and simultaneously charges the soft-start capacitor during startup. This multi-functionality eliminates dedicated charging components and reduces overall circuit complexity and external component count
2Reliability
If a charging capacitor is provided as an integrated circuit component, then the regulator can achieve slow ramp-up to prevent excessive current, but the die area is increased
Solution Approach 1:
The soft-start capacitor is integrated within the voltage regulator die, merging the soft-start function with the main regulator circuitry. This integration eliminates the need for external capacitors while keeping the die area minimal by sharing circuit resources
Solution Approach 2:
The soft-start capacitor is temporarily used during startup to limit inrush current, then discarded after serving its purpose. The capacitor discharges its stored energy once the regulator reaches normal operating voltage, allowing the circuit to transition to standard operation without the capacitor interfering
3Speed
If the regulator output voltage quickly ramps up, then the regulator responds faster to power-on, but excessive current flows that can damage the regulator or prevent output
Solution Approach 1:
The soft-start capacitor is charged in advance during the startup phase before the regulator begins normal operation. This preliminary charging action limits the initial current surge by controlling the rate at which the reference voltage rises, preventing damage while allowing subsequent fast response
Solution Approach 2:
The soft-start capacitor acts as a cushion during startup, absorbing the initial energy surge and providing a controlled ramp-up profile. This beforehand cushioning protects the regulator from excessive inrush current while allowing the system to reach operational voltage
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for a slow and controlled ramp-up of the regulator voltage, achieving a sufficient time constant without capacitors, thereby reducing the risk of damage and false triggering while minimizing circuit area.
Implementation Method 1
a transistor having a low threshold voltage that turns on initially during a start-up period when a supply voltage starts to ramp up
Implementation Method 2
if the regulator has a resistor-capacitor (RC) triggered electrostatic discharge (ESD) clamp on its load
Implementation Method 3
if the regulator snaps up faster than the RC time constant of the ESD clamp
Implementation Method 4
an inverter coupled between the reference current generator and the transistor; an output signal from the inverter is provided to a gate of the transistor
Data Source
AI summary
Techniques are disclosed for providing an improved start-up (soft-start) circuit for use with voltage regulators, and an improved regulator start-up methodology. For example, an apparatus comprises a voltage regulator circuit and a start-up circuit operatively coupled to the voltage regulator circuit. The start-up circuit is configured to provide a current signal, during a start-up period, that generates a reference voltage at a reference input of the voltage regulator circuit such that the reference voltage ramps up at a rate substantially equal to a ramp-up rate of a supply voltage coupled to the start-up circuit and the voltage regulator circuit.