ISO1212: Why we need SENSE PIN in ISO1212?

Hi Gabriel,

Thanks for reaching out and seeking clarifications.

Gabriel Xu said:

When I compare the pin out of SI8384, we have an extra pin "SENSE" in ISO1212.

I am quite confused about the purpose of this pin, why we need to add this pin in ISO1212? Why competitor's solution didn't need this pin ?

The device Si8384 monitors the input voltage and provides an isolated digital output, it doesn't control the input current. While ISO1212 not only monitors the input voltage and provides isolated digital output but also limits input current to minimize power dissipation inside and output device. This is the distinguished feature of ISO1212 that enables a much smaller overall solution size while maintaining the heating to very minimal. Please refer to the below tech note that describes how the current limit of ISO1212 benefits the overall solution and how it compares to other solutions without any current limit.

How To Simplify Isolated 24-V PLC Digital Input Module Designs (Rev. B)

Since ISO1212 both monitors and limits input current, it requires two input pins (IN & SENSE) and an additional current sense resistor (RSENSE) compared to other solutions. SENSE pin measures the voltage and IN & SENSE together measure the voltage across RSENSE to deduce the actual current flowing into the device thereby enabling the device to regulate the current to a defined value.

Gabriel Xu said:

What's the criterion of high level "INPUT"? Is it mean both IN pin and SENSE pin receive high logic?

Pins IN and SENSE together form one industrial digital input. The function table in datasheet refers to both IN and SENSE together as one INPUT.

Gabriel Xu said:

About SUB pin, in pin function description section, it recommends connecting to 2mm*2mm floating plane on the board. I am not understand which plane we are talking about?

SUB pin needs to be unconnected / floated. A floating plane is a plane that is not connected to any other circuit. The recommendation states to connect the SUB to such a floating plane of 2mm x 2mm. This is to better dissipate heat from device and improve overall board temperature. It is optional and needed only of customer needs better thermal performance than what the device is already offering.

I hope all these responses address your questions and provide clarification. Let me know if anything is not clear, thanks.

Regards,
Koteshwar Rao

Hi Gabriel,

Thanks for additional inputs.

Gabriel Xu said:

Customers also looking at competitor solutions without sense pin, however, they could also achieve the current limit without sense pins/ Some customers don't like the solution with too many pins out. Any extra advantage of ISO1212 could highlight to customers?

As explained in the tech note that I shared in my previous post, current limiting is presently done in 2 ways. One way to use a simple series resistor and the other way to use an external current limiting circuit (with about 12 discrete components), as shown in the diagram below. The first approach doesn't regulate the current and it varies as input voltage increases. This is a big limitation as it demands for high wattage external resistors and increases overall board temperature limiting the number of channels that can be used on a single board.

The second solution improves the situation significantly but requires lots of additional components per channel. The ISO1212 integrates all this circuit and offers better current regulation than these two solutions. That is the advantages of ISO1212 and many customers are able to integrate more channels on a single board because of minimal external components and good thermal performance. Please do share the tech note with customer so that they understand the benefits of ISO1212.

Gabriel Xu said:

What's the meaning of the red line?

The grey region is the acceptable region defined by the standard IEC 61131-2 while the red line is ISO1212 operating line. As you can see in the diagram, even when the input voltage increases from 5V to 30V, the device regulates the current and doesn't let it increase. In contrast, the traditional approach will let the current increase and thereby demanding higher wattage resistors and causing a significant increase in board temperature. Please refer to System I-V curve provided in Si838x datasheet to see how current increases and is not regulated.

Gabriel Xu said:

If the input voltage exceeds the turn-on threshold but the input current is less than 2mA, output side is high level or low level?

When the input voltage crosses turn-on threshold, the output goes HIGH as stated in the Function Table of datasheet. Since device has very good current regulation, it doesn't let input current increase and that favors the application helping thermal management.

Let me know if you have any further questions, thanks.

Regards,
Koteshwar Rao