IntroductionThe ACS71x families of Hall effect-based electrical current sensor ICs measure current by sensing the magnetic field it generates as it passes adjacent to the Hall element (see figure 1). They measure this field directly, without the use of a magnetic concentrator, which is a common feature in other magnetic devices (for example, in the Allegro® MicroSystems CA and CB packages, used for the ACS75x families of current sensor ICs).
AlignmentFlux lines form circles around a conductor in planes that are orthogonal to the direction of current flow through a conductor. The Hall element only responds to the flux component that is perpendicular to its surface, and is only susceptible to magnetic fields in this direction. As shown in figure 2, although the path of the primary current, IP, is in the same plane as the Hall element, the magnetic flux vectors generated by current passing through the U-loop are perpendicular to the Hall element plane. Those that intersect the Hall element induce a voltage across it, which is then amplified and used to generate the output voltage.
Spacing and Layout GuidelinesWhen laying out a PCB where the ACS71x device will be mounted, it is good practice to keep as much space as possible between the device and other traces carrying significant amounts of current. Figure 4 shows the measured effect of an adjacent current trace on the same side of the PCB as the device. Although this is a consideration for optimizing designs, the effect of the neighboring trace is small and much less than the signal generated by the current being measured within the device.
If greater protection against external fields is needed, a surface mount magnetic alloy shield covering the device package is recommended. A shield such as the one depicted in figure 5 shunts external magnetic flux away from the SOIC8 package, and results in no significant effect on the magnetic field generated inside the package around the primary current path. Typical magnetic field results are depicted in figure 6.
In cases where a high level of shielding is required, a second shield of a similar type can be attached to the underside of the PCB, protecting the Hall element from the perpendicular flux passing through the PCB from below.
Experimental ResultsAn experiment was performed with ±240 G applied perpendicularly to the Hall element of an 8-pin device package, through the use of an air core field source. The attenuation levels achieved by shields constructed of various ferrous alloys and material thicknesses were recorded. The results are provided below. Two types of ferrous alloys were used, silicon steel (SiFe), and HyMu alloy, which is a generic class of alloys that have a high magnetic permeability level, µ. It should be noted that in most applications the Hall element will not be exposed to field levels as great as 60 G. For example, up to 60 G of field would pass through the Hall transducer only if the ACS71x were placed within 6 mm of an adjacent busbar carrying 500 A.
SummaryThe alignment, spacing, and shielding techniques described in this note may be used as increasingly aggressive steps to mitigate the effects of external magnetic fields when applying ACS71x current sensor ICs.
If you need further assistance in dealing with the effects of external fields, please contact your local Allegro sales office to be put in touch with one of our applications engineers.