ALLEGRO'
mxcrosystems
|_
High-Precision Linear Hall-Effect-Based
Current Sensor IC With 200 µΩ Current Conductor
ACS780xLR
22
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
APPLICATION-SPECIFIC INFORMATION
Field from Nearby Current Path
TobestusetheCMRcapabilitiesofthesedevices,thecircuit
board containing the ICs should be designed to make the external
magnetic fields on both Hall plates equal. This helps to minimize
error due to external fields generated by the current-carrying
PCBtracesthemselves.Therearethreemainparametersforeach
current-carrying trace that determine the error that it will induce
onanIC:distance from the IC, width of the current-carrying
conductor, and the anglebetweenitandtheIC.Figure3shows
an example of a current-carrying conductor routed near an IC.
The distance between the device and the conductor, d, is the
distance from the device center to the center of the conductor.
The width of the current path is w. The angle between the device
and the current path, θ, is defined as the angle between a straight
line connecting the two Hall plates and a line perpendicular to the
current path.
d
H1
H2
θ
I
w
Figure 3: ACS780 with nearby current path, viewed
from the bottom of the sensor
Whenitisnotpossibletokeepθcloseto90°,thenextbest
option is to keep the distance from the current path to the current
sensor IC, d, as large as possible. Assuming that the current path
isattheworst-caseangleinrelationtotheIC,θ=0°or180°,the
equation:
Error =
2 × I
Cf
×
1
d – 2
Hspace × cosθ
┌
│
│
│
–
1
d + 2
Hspace × cosθ
┐
│
│
│
where Hspace is the distance between the two Hall plates and Cf is
the coupling factor of the IC. This coupling factor varies between
the different ICs. The ACS780 has a coupling factor of 5 to 5.5
G/A,whereasotherAllegroICscanrangefrom10to15G/A.
Other Layout Practices to Consider
When laying out a board that contains an Allegro current sensor
ICwithCMR,thedirectionandproximityofallcurrent-carrying
paths are important, but they are not the only factors to consider
whenoptimizingICperformance.Othersourcesofstrayfields
that can contribute to system error include traces that connect to
theIC’sintegratedcurrentconductor,aswellasthepositionof
nearby permanent magnets.
The way that the circuit board connects to a current sensor IC
must be planned with care. Common mistakes that can impact
performanceare:
• The angle of approach of the current path to the IP pins
• ExtendingthecurrenttracetoofarbeneaththeIC
THE ANGLE OF APPROACH
OnecommonmistakewhenusinganAllegrocurrentsensorICis
tobringthecurrentinfromanundesirableangle.Figure4shows
an example of the approach of the current traces to the IC (in this
case, the ACS780). In this figure, traces are shown for IP+ and
IP–. The light green region is the desired area of approach for the
current trace going to IP+. This region is from 0° to 85°. This rule
applies likewise for the IP– trace.
The limitation of this region is to prevent the current-carrying
trace from contributing any stray field that can cause error on
the IC output. When the current traces connected to IP are outside
thisregion,theymustbetreatedasdiscussedabove(Fieldfroma
Nearby Current Path).