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Original question:

ISO1212: ISO1212 not working as expected

ISO1212: Unable to get OUT1 high with Rthr at 1k Ohm at 24V

Daz Bahri
Prodigy 31 points
Part Number: ISO1212

Tool/software:

Hi all,

We're using ISO1212 in a design, and followed this reference:

Rthr 1k Ohm and Rsense is 562 Ohm, and we're using a SMF26CA TVS. Below is our schematic:

When Rthr for IN1 is 1k Ohm, when I connect +SUPPLY(24V) to IN1+ and GND to IN1-, I do not see OUT1 go high on the ISO1212.

However when Rthr for IN2 is 0 Ohm, and I do the same as above, I do see OUT2 go high.

I found this forum post with the same issue, however with no satisfying answer aside from it might be broken:

https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/1402702/iso1212-iso1212-not-working-as-expected

With a 24V supply, in relation to GND, I measure:

- SENSE1 = Approx 4.8V

- IN1 = Approx 3.7V

- Approx 1V across Rsense

My understanding from the datasheet is that a Rthr of 1k Ohm means that a voltage above 11V on IN1 should mean OUT1 goes high?

Any assistance in understanding would be greatly appreciated. I can provide more information as needed.

Cheers,

Daz

  • 0
    TI__Mastermind 22205 points

    Hello Daz, 

    Thank you for reaching out and linking a related post! The above schematic should work which leads me to believe that the issue could be layout related. Please share layout files or a capture that shows the solution (including RSENSE and RTHR resistors. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 31 points

    Hi Andrew,

    As I was preparing to send the layout files and a picture of my board, I just wanted to double check the Rsense and Rthr resistor values. Wouldn't ya know, I misread my Rsense resistance value Face palm It was 56 Ohm on the board, not 562 Ohm. I replaced, and now it seems everything works as expected. 

    Rookie mistake! But happy it seems to be working.

    Many thanks for the reply and for providing the impetus for me to just double check.

    Cheers,

    Daz

  • 0 in reply to Daz Bahri
    TI__Mastermind 22205 points

    Hello Daz, 

    No worries, and thank you for providing the thread with a complete solution too! 

    Best,
    Andrew