The number of sensors in modern vehicles has grown rapidly in recent years as manufacturers look to improve the monitoring and feedback of key automotive power train, stability, and safety applications. Many of these sensorsand in particular those in under-hood areasmust provide robust and reliable operation at very high operating temperatures. To meet these challenges, the latest mixed-signal semiconductor processes enable integrated sensor interface ICs that significantly simplify the implementation of sensor applications for elevated automotive electronics temperatures.
Proliferation of automotive sensors is spurred by the growth in automotive electronics designed to address consumer demand for higher levels of safety, security, and comfort. In addition, sensors are necessary for the functionality needed to comply with new government regulations such as those regarding air quality or passive safety. Furthermore, according to a recent study, the number of sensors in an automobile is anticipated to increase by 45% between now and 2010.
This growth of in-vehicle sensors is fuelling requirements to adapt the "surrounding" automotive electronics. The main issue here is that, by their nature, sensors measure analog signalsfor example temperature, pressure, speed, acceleration, gas concentrationusing variations in electrical characteristics such as resistance and capacitance. The host microcontroller (MCU) that interprets and reacts to such information, however, is likely to require digital signal inputs. As a result, signal-conditioning circuitry is required between the sensor and the MCU. This circuitry captures the sensor analog signal, filters it, amplifies it, and then converts it to a digital signal that can be interpreted by the microcontroller. In recent years this circuitry has been increasingly based around a sensor interface IC, or SI2C.
It follows that the increase in the number of in-vehicle sensors demands a corresponding increase in SI2C. However, the microcontrollers typically deployed in these applications are limited in their ability to support the full management of all of the signals emanating from the various sensors. After the wave of multiplying the number of controllers in cars, therefore, comes the era of distributed intelligence, in which some of the processing and management overhead is delegated to the sensor interface IC.
At the same time, space constraints are limiting the physical size of the SI2C element of the circuit. Now, however, functionality, performance and space issues can be addressed through the application of smart, mixed-signal semiconductor technologies that allow the functionality of a full automotive module to be integrated into a single piece of silicon.
Into the pressure cooker
The integration of application functionality onto one chip allows the chips to be located in the direct neighborhood of the sensor that they manage. Being closer means fewer cables in the caronly the in-vehicle network bus cable is needed to carry the information to and from the module. Reducing cables, and therefore weight, is one of the advantages of the standardization of vehicle networks, leading as it does to lower fuel consumption and fewer harmful emissions. A drawback is that the proximity of the SI2C to the sensor often means the interface IC is exposed to a much harsher environment than the traditional, larger modular solutions developed using discrete components.
For instance, from a temperature standpoint, the IC could be exposed to a wider range than the usual –40 to 125C ambient temperature automotive standard. Here it is important to remember that one of the key considerations for a semiconductor is the need to handle junction temperature rather than ambient temperatures. The difference, between the silicon and the ambient temperature is known as the dissipated temperature and typically varies from 15 to 25C. This means that operation at a maximum automotive temperature rating of 140C requires the device to operate with a junction temperature of 165C.
Furthermore, recent studies from automotive Tier One suppliers reported ambient temperatures above 175C in the neighborhood of the exhaust system, corresponding to an even higher junction temperature for the silicon deployed in this area (see figure below). As a result, designers of automotive SI2C semiconductors must now deal with a more complex development approach as the heat dissipation of the circuit needs to be monitored carefully to avoid overheating the silicon during peak temperature operation.
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