ACS758 Current Sensor ICs Frequently Asked Questions

What are the differences between the various members of the ACS758 family?

The differences are the output sensitivities (in mV/A) of each version and their operating temperature ranges. Operating temperatures are also related to maximum current ranges.

How is the CB package used in the ACS758 different from the CA package used in the ACS756 family?

Externally, the packages are identical and have the same footprint. Internally, the CB package leadframe has greater cross-sectional area where the conductor passes the Hall-effect device. As a result, the CB package conductor has a resistance of only 100 µΩ, versus 130 µΩ for the CA package. In addition, the CB core is made of laminated steel and has a much higher saturation point (at higher temperature) than the ferrite core inside of the CA package.

Can the ACS758 family sense both DC and AC currents?

Yes. The ACS758 family uses Hall-effect technology, which is capable of sensing electrical currents having both DC and AC components. As the datasheet states, the bandwidth of the ACS758 is 120 kHz typical. There may be phase lag and amplitude attenuation of the output for AC currents with frequency content greater than 120 kHz. For transient current signals, the response time is ≈4 µs.

Can the ACS758 sense 0 to 400 A as well as ±200 A?

No, the maximum current in the sensing range of the ACS758 is an absolute current of 200 A. The magnetic circuit within the ACS758 package will not provide a linear output above field levels generated by 200 A.

What does "ratiometric" mean?

This feature is particularly valuable when using the ACS758 with an analog-to-digital converter. A/D converters typically derive their LSB from a reference voltage input. If the reference voltage varies, the LSB will vary proportionally. The ratiometric feature of the ACS758 means its gain and offsets are proportional to its supply voltage, VCC. If the reference voltage and the supply voltage for the ACS758 are derived from the same source, the ACS758 and the A/D converter will both track those variations, and such variations will not be a source of error in the analog-to-digital conversion of the ACS758 output. Figure 1 is a plot of primary current, IP, versus output voltage, VOUT, of the ACS758-100A 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 22 mV/A nominal.

Primary current vs. output voltage 

Figure 1. ACS758-100A VOUT versus IP at various VCC

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 ACS758 package body.

Is there any way to adjust the gain of the ACS758?

No, the ACS758 sensitivity and 0-ampere (quiescent) voltage level are programmed at the factory.

How small of a current can the ACS758 resolve?

The current resolution of the ACS758 family of current sensor ICs is limited by the noise floor of the device output signal. For example, the ACS758-050 version can resolve a change in current level of about 250 mA, at 25°C, through its primary conductor leads. The 200 A version can resolve approximately 380 mA. At these levels, the amount of magnetic field coupled into the linear Hall-effect IC is just above its noise floor. Resolution can be improved significantly by filtering the output of the ACS758 for applications requiring lower bandwidth. Table 1 lists the noise levels, and hence current resolutions, at various bandwidths. Filtering was accomplished with a simple, first order RC filter. Note the related graphs, figures 2 through 5, which provide a better understanding of the device output resolution that can be achieved through filtering.

Table 1. ACS758 Noise Level and Current Resolution versus Bandwidth

Device Bandwidth -3 dB
(kHz)
Noise
(mVp-p)
Current Resolution
(mA) (% of full scale)
ACS758-200B 120 3.84 384 0.192
10 0.92 92 0.046
1 0.55 55 0.028
0.2 0.15 15 0.008
ACS758-150B 120 4.36 328 0.219
10 1.08 81 0.046
1 0.52 39 0.026
0.2 0.16 12 0.008
ACS758-100B 120 5.69 285 0.285
10 1.49 75 0.075
1 0.67 34 0.034
0.2 0.22 11 0.011
ACS758-50B 120 10.03 251 0.502
10 2.95 74 0.148
1 1.05 26 0.053
0.2 0.43 11 0.022


noise and current resolution
Figure 2A
noise and current resolution
Figure 2B
noise and current resolution
Figure 3A
noise and current resolution
Figure 3B
noise and current resolution
Figure 4A
noise and current resolution
Figure 4B
noise and current resolution
Figure 5A
noise and current resolution
Figure 5B

What is the ESD tolerance of the ACS758?

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

Can Allegro bend the leads under the package or outward, so that I can surface-mount the ACS758?

In order to safely conduct 200 A currents, the power leadframe in the ACS758 has been constructed with a relatively heavy gauge. Because of this heavy gauge, the terminal leads are not very flexible. If the ACS758 is surface-mounted, small amounts of board flex, or the action over time of thermal expansion and contraction, could potentially break the IC off the board. Allegro does not recommend surface mount assembly of this device.

How should I solder the ACS758 onto my board?

To ensure a robust joint to the board, Allegro recommends adding a ring of through-holes in the solder pad area around each of the two broad primary conductor leads that carry the current being sensed. These holes are shown in figure 6. General soldering recommendations for the CA and CB packages have been added to the application note "Soldering Methods for Allegro's Products (SMD and Through-Hole)".

How should I connect to the straight-leaded package configurations?

Allegro recommends a tin-fusing welding technique. This method is described in our application note "Guidelines for Designing Subassemblies Using Hall-Effect Devices".

Do you have a recommended footprint for the ACS758 (CB package)?

Yes, figure 6 shows the recommended footprint for the –PFF leadform configuration. The portion (A), specifically the three small through-holes for the signal pins, also applies to the –PSF configuration (which has straight primary conductor leads).

 

 

layout
Figure 6. ACS758 PFF configuration recommended PCB layout

Can I get the Gerber files for your evaluation board?

Yes, download from: Allegro_CA_CB_EvalBoard (ZIP).

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

Yes, an AutoCAD 2004 .DXF file can be downloaded from: Allegro_CA_CB_EvalBoardDXF (ZIP).

The copper areas are defined as "regions" in these files.

How thick are the copper traces on your evaluation board?

The evaluation board uses 4–oz. copper.

How can I tell if I am heating up the part too much, and how can I measure the die temperature?

The temperature of the primary current path terminals can be used to estimate the temperature of the die inside the package. As can be seen from figure 7, the temperature on the sides of the primary current path terminals, close to the package case, will be at almost the same temperature as the primary current path bridge inside the package. The die inside the package will be approximately 1°C below this temperature. In order to measure the temperature inside the package, solder a thermocouple in either of the locations shown in figure 8. Then, subtracting 1°C gives a very good estimate of the die temperature. The die temperature should be kept within the range specified in the datasheet for the version of the ACS758 that is being used.

thermal gradient  thermal test points 

Figure 7. Temperature profile of primary current path

Figure 8. Top view of ACS758 package with thermocouple locations

Are there any other design guidelines for applying the ACS758?

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 the primary path.

What is the inductance of the ACS758 (CB package)?

Typical measured inductances versus test signal frequency are:

  • 32 nH at 10 kHz
  • 24 nH at 100 kHz
  • 21 nH at 200 kHz

Does the ACS758 contain lead (Pb)?

No, the ACS758 family is lead (Pb) free. All of the signal pins and terminals are plated with 100% matte tin, and there is no Pb inside the package.

What is the high-current leadframe made of?

The heavy gauge leadframe is made of oxygen-free copper.

How susceptible is the ACS758 to stray magnetic fields?

The ACS758 contains a concentrator core that acts not only as a concentrator of the flux lines generated by IP, but also as a shield to protect the Hall circuit IC from ambient common-mode fields (typical rejection of common mode fields is –41 dB). The results are detailed in figure 7, which compares the output voltages, VOUT, of an unshielded linear Hall-effect sensor IC and that of the ACS758. The devices have the same gain, and are exposed to the same magnetic field, applied in an air core through the top of the package.

stress magnetics 

Figure 7. ACS758 performance in stray magnetic fields

What safety certifications does the ACS758 have?

The ACS758 family has been certified by Underwriters Laboratories to the following standards:

  • IEC 60950-1:2001, First Edition

In addition, the ACS758 family 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 is the behavior of the ACS758 output during a slow ramp-up of VCC?

The typical output behavior of the ACS758xCB-050 during a 500 ms ramp-up of VCC is shown for both 0 A and 50 A in figure 8:

stray magnetics 

Figure 8A. ACS758 power-on performance. VCC ramp-up with IP = 0 A
5 V/500 ms slew rate; C1: VCC = 2 V/div., C2: VOUT = 2 V/div., time = 50.0 ms/div.



stray magnetics 

Figure 8B. ACS758 power-on performance. VCC ramp-up with IP = 50 A
5 V/500 ms slew rate; C1: VCC = 2 V/div., C2: VOUT = 2 V/div., time = 50.0 ms/div.

How soon after the application of power will a valid signal be available from the ACS758?

The typical time to valid output is given in table 2 and figure 9. However, we recommend a 3x to 5x safety margin to account for power-on time variation over process and temperature ranges.

Table 2. ACS758 Time to Valid Output
   (Measured at VCC = 5 V)

IP (A) 0 50
Power-On Time (µs) 8 10


stray magnetics 

Figure 9A. Startup of ACS758-50A with 0 A applied, then a VCC step from 0 to 5 V
5 V/500 ms slew rate; C1: VCC = 2 V/div., C2: VOUT = 2 V/div., time = 10 µs/div.



stray magnetics 

Figure 9B. Startup of ACS758-50A with 50 A applied, then a VCC step from 0 to 5 V
5 V/500 ms slew rate; C1: VCC = 2 V/div., C2: VOUT = 2 V/div., time = 10 µs/div.

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

The output of the sensor IC may oscillate.

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

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

What is the overcurrent tolerance of the ACS758 devices?

Because of its low, 100 µΩ internal resistance, the overcurrent capability of the CB package of the ACS758 sensor IC is highly dependent on the characteristics of the bus bar or printed circuit board on which it is mounted. In the case of a PCB mounting, trace width and thickness, the number of layers, the presence or absence of ground and/or power planes, and the gauge of the cables that carry the current on and off the board are all significant factors. It is also dependent on the maximum operating temperature of your application and the duration, duty cycle and number of current pulses during the overcurrent event. By way of example, we have characterized the ACS758 devices on the Allegro ACS758 evaluation board, connected to the current source with 2 AWG cables. This a 2-layer board with 4-oz. copper. (For drawings and Gerber files, see the related FAQs) on this page.

The results are provided in table 3.

Table 3. Tested Maximum ACS758 Overcurrent Levels and Durations
   (Applicable to devices on Allegro ASEK 758 evaluation boards connected with 2 AWG cables)

Ambient Temperature
(°C)
Maximum Current
(A)
10 s, 10% duty cycle, 100 pulses applied
25 350
85 350
150 260
3 s, 3% duty cycle, 100 pulses applied
25 450
85 425
150 375
1 s, 1% duty cycle, 100 pulses applied
25 1200
85 900
150 600


How will capacitive coupling between the current path in the ACS758 and the Hall element effect the sensor IC output?

The lead frame noise rejection test is conducted by injecting a high-frequency sinusoidal frequency onto the high-current leads. The signal coupling onto the output of the Hall-effect device is then measured. The ACS758 family devices exhibit a high level of leadframe noise rejection as table 4 reveals. In addition, the figure 10 charts performance as a function of frequency.

Table 4. Typical Capacitive Coupling of a 20 V Peak-to-Peak Signal on the Current Path

f (MHz) 1.0 2.0 3.0 4.0 5.0 7.5 8.5 10.0 15.0 18.0 20.0
VOUT(p-p) (mV) 15.0 50.0 100.0 200.0 250.0 700.0 750.0 1000.0 1020.0 1050.0 600.0
Noise Rejection (dB) -62.5 -52.0 -46.0 -40.0 -38.1 -29.1 -28.5 -26.0 -25.8 -25.6 -30.5


stray magnetics 

Figure 10. ACS758-50A Noise Rejection versus Frequency

Can I get a .STP model for the CA/CB Package?

Yes, download from: CA/CB Package .stp model

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