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High-Performance Power ICs and Hall-Effect Sensors

ACS704 Current Sensor Frequently Asked Questions ....

Can the ACS704 family sense both dc and ac currents?

The ACS704 family uses Hall effect technology, which is capable of sensing electrical currents having both dc and ac components. The bandwidths of the ACS704ELC-015 and –005 are 50 kHz typical. There is usually a phase lag of at least 45º on the output for ac currents at frequencies exceeding these values.

What does "ratiometric" mean?

The ratiometric feature of the ACS704 means its gain and offsets are proportional to its supply voltage, VCC. This feature is particularly valuable when using the ACS704 with an analog-to-digital converter. A-to-D converters typically derive their LSB from a reference voltage input. If the reference voltage varies, then the LSB varies proportionally. If the reference voltage and the supply voltage for the ACS704 are derived from the same source, then both the ACS704 and the A-to-D converter track those variations, and such variations will not be a source of error in the analog-to-digital conversion of the ACS704’s output. Below is a plot of primary current, IP, to output voltage, VOUT, of the ACS704ELC-015 when varying VCC. The offset and sensitivity levels shift proportionally with VCC. For example, when VCC = 5.5 V, the 0 A output is 5.5 / 2 = 2.75 V nominal, and the sensitivity is 110 mV/A nominal.

ACS704 Solder Pad Layout

What external components are required?

Allegro recommends the use of a 0.1 µF bypass capacitor between the VCC pin and the GND pin. The capacitor should be located as close as practical to the ACS704 package body.

Is there any way to adjust the gain of the ACS704?

No, the ACS704’s mV/A sensitivity and 0-ampere quiescent voltage level are programmed at the factory.

How small of a current can the ACS704 resolve?

The filtering characteristics are provided in the following table:

ACS704ELC-015 Noise Filtering versus Frequency Response

Break Frequency
of R-C Filter on Output
(kHz) 		Nominal Programmed Sensitivity
(mV/A) 		Peak-to-Peak Noise
(mV) 		Current Resolution
(mA) 		Measured Rise Time for 5 A Step, Filtered
(µs) 		Bandwidth as Derived from Step Response
(kHz)
Unfiltered 100 75 ≈750 9 50
50 46 ≈460 10.5 33.3
40 43 ≈430 12 30
10 25 ≈250 35 10
7.0 17 ≈170 70 5
3.3 12 ≈120 101 3.3

What is the ESD tolerance of the ACS704?

Typical ESD tolerance is 6 kV human body model, 600 V machine model.

Can I get the Gerber files for your evaluation board?

Yes, download from:
http://www.allegromicro.com/en/Products/Part_Numbers/0704/ACS700-RevC-Gerber Files.zip.

I can't use Gerber files; is any other layout data available?

Yes, a layout drawing .PDF file can be downloaded from:
http://www.allegromicro.com/en/Products/Part_Numbers/0704/ACS700-PWB-Rev_C_Fabrication_Drawing.pdf.

How thick are the copper traces on your evaluation board?

The evaluation board uses 2-oz. copper.

What is the typical footprint for the SOIC8 package?

The typical footprint is given as follows.
Land pattern dimensions are given in the table below.

ACS704 Solder Pad Layout

(Max)
Z 		(Min)
G 		(Max)
X 		(Ref)
Y 		(Ref)
C/C 		D E
7.4 3 0.6 2.2 5.2 3.81 1.27
Dimensions in millimeters

What are the creepage and clearance distances between the current path and the sensing circuit?

The creepage and clearance distances in a ACS704 application are typically equal to each other, and are equal to the distance between the solder pads on opposite sides of the board on which the part is placed. This is dimension G in the footprint above, and is equal to 3 mm.

Are there any other design guidelines for applying the ACS704?

Care should be taken to minimize the inductance of the current path to be measured. Also, attention should be paid to minimizing the contact/connection resistance of any connections in that path.

What is the inductance of the ACS704?

Typical measured inductance versus test signal frequency:

  • 6.5 nH at  10 kHz
  • 6.5 nH at 100 kHz
  • 6.7 nH at 200 kHz

Does the ACS704 contain lead?

The lead frame of the ACS704 is plated with lead-free, 100% matte tin and hence should be processed and soldered accordingly. However the ACS704 is a flip-chip device, and the solder balls inside the package that connect the die to the leadframe are 95% lead, 5% tin. Lead-free alternatives for high-temperature flip-chip solder balls are not yet commercially available, and therefore solder balls of this composition are exempted from lead-free requirements by the WEEE (Directive 2002/96/EC of the European Parliament and of the Council of 27 January 2003 on Waste Electrical and Electronic Equipment) and RoHS (Directive 2002/95/EC of the European Parliament and of the Council of 27 January 2003 on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment).

What is the composition of the leadframe?

The leadframe is made of oxygen-free copper.

How susceptible is the ACS704 to stray magnetic fields?

Because the ACS704 does not employ a magnetic concentrator, it is more susceptible to external magnetic fields than our ACS75x sensor families. The susceptibility of the ACS704 to external fields can be mitigated via proper orientation, spacing from external field sources, and if necessary, shielding of the sensor. An application note, Managing External Magnetic Field Interference When Using the ACS704 addresses mitigation techniques in greater detail. It can be downloaded from: http://www.allegromicro.com/en/Products/Design/an/an26030.pdf.

Efect of a perpendicular field on unshielded ACS704

What safety certifications does the ACS704 have?

The ACS704 family also has been certified by TÜV America to the following standards:

  • UL 60950-1:2003
  • EN 60950-1:2001
  • CAN/CSA C22.2 No. 60950-1:2003

The mold compound is UL recognized to UL94V-0.

What happens if I try to drive more than the specified 10 nF maximum capacitance with the output of the sensor?

The output of the sensor may oscillate.

What happens if I try to drive less than the specified 4.7 kΩ minimum resistance with the output of the sensor?

The sensor may not produce an output, as its output driver will not be able to supply sufficient current.

What is the ESD tolerance of the ACS704?

Typical ESD tolerance is 5 kV human body model, 400 V machine model.

What is the overcurrent tolerance of the ACS704?

Because of its low, 1.5 mΩ internal resistance, the overcurrent capability of the ACS704 sensor is highly dependent on the characteristics of the circuit board on which it is mounted, including: trace width and thickness, the number of layers, and the presence or absence of ground and/or power planes. It is also dependent on the maximum operating temperature of your application and the frequency with which the overcurrent occurs.

By way of example, we have characterized ACS704 devices mounted on the Allegro ACS704 evaluation board. This a 2-layer board, with 2-oz. copper. (For drawings and Gerber files, see the FAQs: Can I get the Gerber files for your evaluation board? and I can't use Gerber files; is any other layout data available? on this page.) The results are provided in the following table.

Tested Maximum ACS704 Overcurrent Levels and Durations
Applicable to devices on Allegro ASEK 704 evaluation boards connected with 2 AWG cables
Ambient Temperature
(°C) 		Maximum Current
(A)
10 s, 10% Duty Cycle, 100 pulses applied
25 40
85 30
3 s, 3% Duty Cycle, 100 pulses applied
25 50
85 40
1 s, 1% Duty Cycle, 100 pulses applied
25 60
85 50
250 ms, 0.25% Duty Cycle, 100 pulses applied
25 80
85 60
100 ms, 0.1% Duty Cycle, 100 pulses applied
25 100
85 80

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