When designing a digital-switch magnetic sensing system, three variables should always be considered: the magnet, sensing distance, and threshold of the sensor.

The TMAG5123 device has a detection threshold specified by parameter B_OP, which is the amount of magnetic flux required to pass through the Hall sensor mounted inside the TMAG5123. To reliably activate the sensor, the magnet must apply a flux greater than the maximum specified B_OP. In such a system, the sensor typically detects the magnet before it has moved to the closest position, but designing to the maximum parameter ensures robust turn-on for all possible values of B_OP. When the magnet moves away from the sensor, it must apply less than the minimum specified B_RP to reliably release the sensor.

Magnets are made from various ferromagnetic materials that have tradeoffs in cost, drift with temperature, absolute maximum temperature ratings, remanence or residual induction (B_r), and coercivity (H_c). The B_r and the dimensions of a magnet determine the magnetic flux density (B) it produces in 3-dimensional space. For simple magnet shapes, such as rectangular blocks and cylinders, there are simple equations that solve B at a given distance centered with the magnet.

Figure 9-2 Rectangular Block and Cylinder Magnets

Use Equation 1 for the rectangular block shown in Figure 9-2:

Equation 1.

Use Equation 2 for the cylinder shown in Figure 9-2:

Equation 2.

where

• W is width.
• L is length.
• T is thickness (the direction of magnetization).
• D is distance.
• C is diameter.

An online tool, the Hall Effect Switch Magnetic Field Calculator, that uses these formulas is located at http://www.ti.com/product/tmag5123.

All magnetic materials generally have a lower B_r at higher temperatures. Systems should have margin to account for this, as well as for mechanical tolerances.

For the TMAG5123B, the maximum BOP is 4.5 mT. Choosing a 1-cm cube NdFeB N45 magnet, Equation 1 shows that this point occurs at 3.05 cm. This means that, provided the design places the magnet within 3.05 cm from the sensor during a "turn-on" event, the magnet will activate the sensor. The removal of the magnet away from the device will ensure a crossing of the minimum BRP point and will return the device to its initial state.