Apple Inc.
Dynamic Current Control of a Transmitter for Magnetic Proximity Sensing

Abstract:

Magnetic sensing technology can be used to detect changes, or disturbances (e.g., changes in magnetic field strength), in magnetic fields and can be used to measure the precise location/positioning of an electronic device in proximity to a magnetic source. In order to avoid interference by earth's static magnetic field, a modulated magnetic field can be used for magnetic based proximity sensing. Received modulated magnetic field signals can be demodulated to determine a received magnetic field strength. A drive current of a magnetic transmitter coil can be varied to maintain the detected magnetic field strength at a target value or within a desirable range. The drive current can also be varied to remain below a burnout current level that can cause damage to the transmitter coil.