The packaging integrates the magnet and a monolithic silicon IC. The speed sensor incorporates a dual-element Hall-effect device and signal-processing circuitry that switches in response to differential magnetic signals created by rotating ferrous targets. A built-in compensation circuit eliminates magnet and system offsets immediately at start-up. Digital tracking of the analog signal is used to provide true zero-speed operation, while also setting the optimal device switchpoints. The resulting switchpoints are air-gap independent, greatly improving output accuracy and duty cycle. A special algorithm fine-tunes switchpoints in running mode, maintaining specifications even through large changes in air gap.
Packaging is via a single-step overmold process and combines a samarium-cobalt magnet and the Hall-effect IC. It eliminates the presence of air voids that can be problematic during other multistep package configurations. The close proximity of the magnet to the IC promotes heat sinking, permitting higher operating temperatures in a smaller package.
Allegro MicroSystems Inc., 115 Northeast Cutoff, Worcester, MA 01606, (508) 854-5363, allegromicro.com