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SiP styles for magnetic position sensors in automotive apps

Posted: 10 May 2016 ?? ?Print Version ?Bookmark and Share

Keywords:magnetic position sensor? MPOS? automotive rotary position sensor? PCB? SiP?

The body of the package includes the external passive components; the head of the package contains only the sensor IC, which is the magnetically sensitive area. In an application, a diametrically magnetized magnet is placed over the head with a specified air gap between them. The sensor's angle measurements are produced on the output pin.

Since the AS5171 SiP is made up of two moulded bodies with an inner connection, it may be mounted in a variety of ways, and the inner and outer connections may be bent in different directions (figure 8). The top pins shown in figure 6 have no electrical function, and may be used to stabilise the mounting.

Figure 8: Examples of the flexible mounting options for the AS5171.

How to guarantee reliable, safe operation
Car manufacturers set extremely high standards for the quality, reliability and functional safety of the products made by automotive suppliers. Two parameters of system performance are particularly important in the operation of a magnetic position sensor: its resistance to severe electro-magnetic events; and its immunity to stray magnetic fields.

For safety reasons, the Tier 1 supplier is required to demonstrate that the system can withstand electro-magnetic interference. The SIP IC helps the Tier 1 supplier to meet EMC and electro-static discharge (ESD) standards because of its combination of lead frame (package) design, and the separation of the external components (in the body) from the sensor IC (in the head). This is in fact one of the advantages of the SiP implementation: it is a complete circuit, including all necessary protection devices, and does not require a PCB designed by the user.

The ams SiP design, then, ensures that the device is safe from electrical distortion or damage. Many magnetic position sensors, however, are also at risk of magnetic interference from stray fields. Automotive environments, especially in the engine bay, often contain very powerful magnetic fields generated by the operation of electric motors, solenoids and other devices. Stray magnetism can interfere with the magnetic field of the sensor's two-pole paired magnet, randomly corrupting many ICs' angle measurements. Because it is random, this error cannot be compensated by the host ECU or microcontroller. System designers are therefore obliged to shield the IC. This raises BoM and assembly costs; it might also compromise the mechanical design in a space-constrained application.

Shielding, then, is a highly unsatisfactory response to the risk of stray magnetic interference, and is made even more unattractive by the application of the ISO 26262 functional safety standard, which requires vehicle manufacturers to design immunity to magnetic stray fields into motor systems.

The patented differential sensing technology in all magnetic position sensors from ams, including the AS5171, is therefore particularly well suited to automotive applications, since it inherently provides full immunity to magnetic stray fields up to a value of 25,000A/m. This technology eliminates the requirement for shielding from stray fields below this 25,000A/m threshold.

In fact, the SIP IC is fully compliant with the ISO 26262 standard, and was developed in accordance with the SEooC (Safety Element out of Context) specifications, to support the integrated safety mechanisms and ASIL levels of any safety-critical application.

Conclusion and outlook
The introduction by ams of a magnetic position sensor in a SiP means that automotive manufacturers can meet OEMs' requirements for performance, reliability and safety with the use of a cost-effective, small and robust device, without any requirement to design their own PCB.

This SiP is available first in the AS5171A and AS5171B automotive magnetic position sensors, qualified to AEC-Q100 grade 0, to support applications in chassis, throttle, pedal and fuel-level sensors. These parts are the first sensors in the '7x' family from ams. This new 7x family uses the new package style, and also benefits from a range of interfaces including Analog, SENT, PWM and PSI5.

In automotive applications that require very accurate and precise measurement of angular (rotational) or linear motion, the magnetic position sensor (MPOS) has emerged as the preferred choice of component type. The MPOS helps automotive suppliers to meet car manufacturers' demands for systems that comply with the highest standards of safety and efficiency, enabling the OEMs to conform to the specifications of industry standards such as ISO 26262 (for functional safety), as well as government-mandated fuel efficiency standards.

At the same time, the MPOS offers a very high degree of electrical, mechanical and environmental robustness, far superior to that of other component types. By using an MPOS, the automotive supplier can develop products that are smaller and more cost-effective.

Conventionally, the complete measurement solution consists of an MPOS IC and a small number of external components mounted on a PCB. Although the total number of components mounted on the PCB might be small, this is still a more complex assembly for the automotive supplier to produce than the complete system implemented by a potentiometer.

This article examines the case for using a complete MPOS system produced as a System-in-Package (SiP), thus relieving the automotive supplier of the need to produce a PCB assembly when implementing magnetic position sensing.

About the authors
Roland Einspieler and Markus Zieserl are with ams.

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