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Unipolar Switches Enhance Transient Protection For Auto Apps

Feb. 17, 2011
Two-wire, unipolar Hall-effect switches developed by Allegro MicroSystems Europe will ultimately find homes in automotive sensing applications for seat-belt buckle, seat position, and steering lock designs

Two-wire, unipolar Hall-effect switches developed by Allegro MicroSystems Europe will ultimately find homes in automotive sensing applications for seat-belt buckle, seat position, and steering lock designs. The A1152/3/5 devices, produced using the company’s biCMOS wafer-fabrication process, include improved high-voltage transient protection—a critical factor for auto applications that can’t locate the sensor near external protection circuitry.  

These Hall-effect switches are factory-trimmed to optimise the accuracy of the magnetic switch points. They employ a patented high-frequency, four-phase, chopper-stabilisation technique to achieve magnetic stability over temperature as well as eliminate the offset inherent in single-element devices when exposed to harsh automotive environments. The advanced chopping technique also significantly reduces jitter and power-on times.  

Two-wire unipolar switches, which require one less wire for operation than the more traditional open-collector output switches, offer diagnostic capability since current is always present at one of two specified levels. Any output current outside these two narrow ranges is a fault condition.  

In addition to the on-chip transient protection, a Zener clamp protects against overvoltage conditions on the supply line. The enhanced high-voltage transient protection allows these devices to survive ISO 7637-2 and 40V load dump for up to 0.5 seconds with minimal, and sometimes no, protection from discrete components. Electrostatic discharge protection was also enhanced so that the devices can handle a transient of 8.0kV Human Body Model (HBM).  

The A1152/3/5/6 switches, used to sense seat track position, seat-belt buckle presence, bonnet and boot latching, and steering-lock position, can also be applied in gear-shift selectors.  Two package styles are available: the SOT-23W miniature low-profile package for surface-mount applications (suffix LH); and the TO-92-style, three-lead, ultra-mini single inline package (SIP) for through-hole mounting (suffix UA).

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