Sri Navaneethakrishnan Easwaran

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.

Biasing Schemes and Diagnostics Circuit

The airbag squib drivers operate with multiple 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 (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.

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.

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 < I deploy_HI < 2.14 A) for 0.5/0.7 ms and 1.35 A (1.2 A < I deploy_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 I deploy_HI or I deploy_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.