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About this book

This book provides a detailed description of fault tolerant design techniques for smart power drivers and their application in the design of automotive airbag ICs to ensure correct deployment. The book begins with an introduction to the nature of electrical loads in the car, then moves on to describe various current sensing circuits, featuring thermal simulations. It shows how simple design techniques can be applied to ensure appropriate functionality of the IC under any power up condition. It concludes by introducing diagnostic circuits and measurement results. This book is a useful reference for automotive IC designers and provides specifications and design guidelines not found in the current literature.

Table of Contents


Chapter 1. Introduction

Ever since the microprocessors got introduced in electronics, the electronic controls have gained immense use in household appliances to large-scale complex industrial controls. In the last two decades, the need for electronic controls along with the development of power semiconductor devicesPower semiconductor like LDMOSLaterally diffused metal oxide semiconductor (LDMOS) (laterally diffused metal oxide semiconductor) has shown aggressive growth in the integrated power devices along with their driver circuitry. For example, integrating 40 V rated, low resistance transistors (300 mΩ) for switches, regulators, current sensingCurrent sensing, etc. along with their control/driver circuitry is a state of art. This led to the miniaturization of the PCBPrinted circuit board (PCB) (printed circuit board) space due to less external components paving its way into maximizing the electronic content in automotive. Today, electronics represent around 25% to 30% of a modern automotive. The demand for electronic control is increasing and is replacing most of the hydraulics in the automotive. As shown in Fig. 1.1, between 2012 and 2017, the automotive electronic production has grown and is expected to grow at an average annual rate of 7.0% worldwide, to reach 179 billion euros by the end of the period [1]. The whole of the automotive industry rely on electronic systems and technologies with the electronic systems now contributing 90% of automotive innovations and new features, from emission levels to active-passiveSafetyactive-passive safety systems and entertainment-connectivity features.

Sri Navaneethakrishnan Easwaran

Chapter 2. State-of-the-Art Current Sensing, Biasing Schemes and Voltage References

Current regulation circuit is essential for the squib drivers to inflate airbags. Energy less than 8 mJ should be provided to the FETs to deploy the airbag. These are applicable when the driver is powered and the ACU microprocessor commands to deploy the squibs. The HS_FET is needed to provide a precise regulated current of 1.95 A (1.75 A < Ideploy_HI < 2.14 A) for 0.5/0.7 ms and 1.35 A (1.2 A < Ideploy_LO < 1.47 A) for 2 ms. The LS_FET has a current limitation of 3 A ±15%. Based on which setting is used, i.e. either Ideploy_HI or Ideploy_LO, the duration of the current limit on the LS_FET is set for a short-to-battery condition on the drain of the LS_FET. In the unpowered or partly powered (missing supplies) scenarios, the airbag misfire should not occur, and hence additional protection is needed. However, as will be discussed in this chapter, the realization of a current regulation loop and a current limitation in unpowered state, biasing scheme, diagnostic circuits exploiting voltage and current selector circuits is not currently available and is therefore the objective of the present research.

Sri Navaneethakrishnan Easwaran

Chapter 3. Current Sensing Principles with Biasing Scheme

State-of-the-art electronic circuits heavily utilize the information related to the amount of current being delivered to the load. In safety-critical automotive applications like the airbag squib drivers, it is critical to utilize this information in order to regulate the current that is delivered to the squib. To know about the current that is delivered, a current sensing technique is needed. A sense resistor is typically used to sense the amount of current being delivered to the load, or a senseFET is used in order to sense the amount of current being delivered to the load. In this chapter, the fundamentals of current sensing and how this sensing concept is used to limit the current are being reviewed.

Sri Navaneethakrishnan Easwaran

Chapter 4. High-Side Current Regulation and Energy Limitation

The current regulation loop drives squibs that are used to inflate multiple airbags in cars. Squibs are electrically R-L-C networks with the resistance ranging from 1 Ω to 8 Ω. The inductance range varies from 1 μH to 3 mH and the capacitive loads range from 22 nF to 220 nF. The high-voltage LDMOS transistors are used as the HS and LS FETs. The specified currents are 1.2 A for 2 ms, 1.75 A for 0.5 ms or 0.7 ms and an optional mode of 1.2 A for 1.5 ms. In this chapter, the advantages and disadvantages of current sense circuit with sense resistor and senseFET approaches are compared. Based on this comparison, this chapter shows how the targeted current regulation is achieved for the HS driver by using the sense resistor-based approach along with the energy limitation circuit for inadvertent deployment protection.

Sri Navaneethakrishnan Easwaran

Chapter 5. Low-Side Current Regulation and Energy Limitation

The airbag squib drivers have the LS_FET operated in Rds_on mode. However to protect the LS_FET against short-to-battery conditions, a current regulation or current limitation is necessary. In this chapter the current regulation of the LS_FET with a senseFET-based current sensing scheme is described. It describes the challenges that are needed to stabilize the LS_FET for the R-L-C loads. The same load conditions applied to the HS_FET apply to the LS_FET. The targeted current limit is 3 A for 2 ms. Similar to the HS_FET, the LS_FET needs energy limitation in the unpowered state in order to prevent inadvertent deployment of the squib.

Sri Navaneethakrishnan Easwaran

Chapter 6. Biasing Schemes and Diagnostics Circuit

The airbag squib drivers operate with multiple supply voltage (MSVMultiple supply voltage (MSV)) levels, viz. the LV, MV and HV rails. Our goal is to ensure that there is no inadvertent activation of the drivers that will lead to the activation of the HS_FET and LS_FET. In addition to this inadvertent activation, diagnostic circuits for measuring the squib resistance are required. In this chapter, a robust biasing scheme that prevents inadvertent activation of the powerFETs, high currents triggered by tri-stated circuits, too high output voltage on the output buffer with a voltage and current selector biasing scheme is presented. In addition to the biasing scheme, the design of a bidirectional fault-tolerant current limited voltage source (CLVSCurrent limited voltage source (CLVS)) that is mandatory for the squib resistance measurement is presented. Bidirectional fault-tolerant CLVS indicates both short to battery and short to ground and can be present simultaneously.

Sri Navaneethakrishnan Easwaran

Chapter 7. Conclusions and Future Work

In Chap. 1, airbag squib driversAirbag squib drivers were introduced, and the specifications of an airbag squib driver were discussed. The current sensing technique along with the regulation methods for HS and LS drivers were discussed. In this chapter, the need to limit the current when short to battery fault occurs in the unpowered state of the ASIC is discussed. It is important to ensure that inadvertent deployment during this fault is avoided.

Sri Navaneethakrishnan Easwaran


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