Airbag Squib Current Driver Using Metal Resistors
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Solution Overview
Problem
Conventional deploy circuits for airbag squib drivers suffer from inaccurate current regulation and systematic offset between matched LDMOS transistors, which can lead to uncontrolled firing events.
Innovation Solution
A metal resistor is used as a sense resistor to regulate current, with a reference voltage generated by a series of Metal 3 resistors, and a bandgap circuit on chip to trim the reference current, making it independent of process, supply, and temperature variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a poly resistor is used as a sense resistor to regulate current, then current regulation can be achieved, but it consumes too much die area
Solution Approach 1:
The patent changes the material parameter of the sense resistor from poly to metal, which fundamentally alters the resistance characteristics and temperature coefficient. This material substitution enables achieving the same current regulation function with significantly reduced die area while maintaining or improving regulation accuracy.
2Power
If conventional LDMOS transistors are used in deploy circuits, then high voltage driving capability is achieved, but systematic offset between matched transistors occurs
Solution Approach 1:
The patent implements a feedback mechanism using the sense resistor to monitor the actual current flowing through the LDMOS transistor. The amplifier compares this sensed current with a reference current and adjusts the gate voltage accordingly, creating a closed-loop control system that compensates for systematic offsets and achieves precise current regulation despite transistor mismatches.
Solution Approach 2:
The patent replaces reliance on precise mechanical matching of LDMOS transistors with an electrical feedback control system. Instead of depending on transistor parameter matching, the system uses electronic sensing and amplification to actively regulate current, substituting passive component matching with active electronic control.
3Area of stationary object
If metal resistors are used as sense resistors, then die area is reduced, but temperature coefficient matching becomes critical
Solution Approach 1:
The patent applies local quality by using metal resistors with specifically engineered temperature coefficients in critical positions within the circuit. The sense resistor is designed with particular attention to its temperature characteristics, and the reference voltage generator uses metal resistors with matched temperature coefficients to ensure stability across temperature variations.
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 solution provides precise current regulation, minimizing errors and maintaining accuracy across various conditions, ensuring a well-controlled firing event by using metal resistors with identical temperature coefficients and matching resistors to cancel out amplifier offset contributions.
Implementation Method 1
a metal resistor (Metal 3) in LBC5 process has been used to create the voltage drop
Implementation Method 2
The reference voltage is generated by using the series of Metal 3 resistors over which a constant current flows through. The reference current is trimmed in order to make this current independent of process, supply and temperature variations. This reference current is generated from the Bandgap circuit on chip.
Data Source
AI summary
An integrated regulated current drive circuit for driving a squib of an inflatable airbag has a current sense resistor connected in series with a load, and a reference resistor connected in series with a reference current source. Both resistors are matched to define a precise ratio of resistance values which determines the amount of current fed to the squib. Both resistors are implemented by combining a number of identical on-chip resistor elements.