Measure Currents of Up to 50, 150, 400 Amps: A Galvanically Isolated Current Sensor IC

Allegro Microsystems has announced a new family of current sensors for AC or DC current-sensing applications.

Allegro Microsystems, AKA Allegro, recently released a family of current sensor ICs designed to offer economical and precise solutions for AC or DC current-sensing applications. 

The two parts in this series, the ACS772 and the ACS773, are both automotive grade qualified (AEC-Q100 Grade 1), offer a wide current-sensing range (from 50A to 400A), and have an internal conductive path with a very low resistance of 100 µΩ.

 

A typical application circuit, of the ACS773, requires minimal external components. Image courtesy of the ACS773 datasheet (PDF).

 

As Allegro calls out in the datasheets of the ACS772 and ACS773, ideal applications that warrant such high-current-sensing ICs include:

• Automotive applications—think DC-to-DC converters and EV (electric vehicle) charging stations
• Industrial applications—like motor control, load detection, and power supplies
• Overcurrent fault protection products

 

Is it just me, or does this IC scream high-current capabilities? Image taken from the ACS772 datasheet (PDF).

ACS772 vs. ACS773: What's the Difference?

If you read the ACS772 and ACS773 datasheets, you will find that both devices are very similar in nature and that their datasheets are nearly identical.

However, there are indeed some differences between the two ICs and their documents. The three primary distinctions are:

• Supply voltage specifications
• Sensitivity specifications;
• Current-sensing range.

 

Supply Voltage

While the ACS772 calls for a supply voltage of 5 V typical (4.5 V min and 5.5 V max), the ACS773 requires a slightly lower voltage of 3.3 V typical (3 V min and 3.6 V max).

It is interesting, however, to see that both parts have an absolute maximum supply voltage rating of 6.5 V.

Sensitivity

The ACS773 IC has greater sensitivity than the ACS772. The ACS772 operates at 5V and has an output range of 0-4 V, whereas the ACS773 operates at 3.3V and has a measurement range of 0-2.64 V. Therefore each additional ampere of current is represented as a greater voltage in the ACS772, leading to increased sensitivity.

 

The 5 V part (ACS772) offers larger mV/A output values than the 3.3 V part (ACS773). Tables taken from the ACS772 and ACS773 datasheets (PDFs).

Current-Sensing Range

While there is some overlap of the current-sensing capabilities of these two ICs (see the figure above), only the ACS772 device offers a current range of ±150 A.

Also, if your current-sensing needs require only DC current measurements, then you might consider using the 100A-rated ACS772 part because it provides more than three-times the sensitivity than the ±100A-rated ACS773 device.

Galvanically Isolated

As called out in both datasheets, this family of current-sensing ICs have "reinforced galvanic isolation," which means that the current path is separate from the signal path. This design allows these devices to be used "in applications requiring electrical isolation without the use of opto-isolators or other costly isolation techniques."

As seen in the figure below, the sensing IC signal leads are not electrically connected to the current-carrying leads.

 

Sensing signal leads are not electrically connected to the current-carrying leads. Image courtesy of Allegromicro.com

 

The following figure contains a table, from the datasheet, regarding detailed information on the isolation characteristics of these ICs.

 

Allegro has been generous by providing isolation testing characteristics. Taken from page 3 of the datasheet (PDF).

Minimal External Parts

The figure below illustrates that only three external parts (C_BYP, C_F, and R_F) are needed for proper operation of these current-sensing ICs. All hall effect sensors have some noise associated with them that can be filtered with a simple low-pass RC filter. R_F and C_F form a low-pass filter network for "optimal noise management" with their values being dependent upon the application. The datasheet fails to elaborate on exactly how the values of these two parts are chosen, however, they can be calculated with the equation:

 

$$F_{cutoff}=\frac{1}{2\pi R_f C_f}$$

 

In-field product testing might be required once the PCB has been designed. Perhaps in the next datasheet revision, Allegro will provide more information on exactly how these component values are chosen. But until then, if you're thinking about using these ICs, then you might kindly ask Allegro for more information regarding the selection process of these two external parts.

 

Unfortunately, the datasheets lack detail on how the values of R_F and C_F are chosen. Drawing taken from the datasheet (PDF).

Demo Boards Are Available

According to Allegro's website, an ACS772 demo board (part number ASEK772ECB-200B-T-DK) and an ACS773 demo board (part number ASEK773ECB-200B-T-DK) are both available... although on a very limited basis. And, unfortunately, datasheets and user manuals are not yet available for these demo boards.

It's likely that this is because Allegro's priorities have been focused on releasing the ICs and their respective datasheets. So, it's probably safe to assume that Allegro will have demo board datasheets and/or user manuals available soon. 

Have you had an opportunity to use one of the devices from this new family of high-current sensing ICs? If so, leave a comment and tell us about your experiences.