ACS780 datasheet(21/24 Pages) ALLEGRO

21 / 24 page

High-Precision Linear Hall-Effect-Based

Current Sensor IC With 200 µΩ Current Conductor

ACS780xLR

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; 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.

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

d –

Hspace × cosθ

┌

│

└

–

d +

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).

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