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ISO1212: If we didn't care to be compliant with IEC 61131-2 type inputs, can the IC function with a higher Rsense to limit INx+SENSEx current?

Joey V.
Prodigy 70 points
Part Number: ISO1212

As the titles asks, If we didn't care to be compliant with IEC 61131-2 type inputs, can the IC function with a higher Rsense to limit INx+SENSEx current?

I have an application were it would be desirable to limit the input current to 0.5ma-1ma. Could I just use a higher Rsense to achieve this if I didnt care about having a IEC 61131-2 compliant input.
Any other gotchas to keep in mind if this was possible?

Thanks.

  • 0
    TI__Mastermind 26695 points

    Hello Joey,

    Yes, you can use different Resistor values to change the current if complying to EC 61131-2 type inputs is not important. You can use the equations listed in 9.2.1.2 Detailed Design Procedure to calculate your custom values.

    There should not be an issue with the device as long as you are operating withing the recommended operating conditions. However, since the device is designed with IEC 61131-2 in mind, I would suggest thoroughly testing the device in your specialized application to validate. 

    Let me know if you have further question. 

    Best,
    Andrew Jackiw

  • 0 in reply to Andrew J
    Prodigy 70 points

    Hi Andrew, 

    Thanks for the reply. 

    Is there an upper limit for Rsense?
    For instance can INx be floating or tied to GND?

    Is INx actually used as part of the input logic level sensing, or is the input logic only assessed with SENSEx? 

    Thanks,
    Joey

  • 0 in reply to Joey V.
    TI__Mastermind 26695 points

    Hi Joey,

    INx is necessary and used to set the current limit and input threshold as designated in 9.2.1.2 Detailed Design Procedure. 

    Without the Rsense resistor the equations would be invalid, and the device would not operate as expected.

     

    Although it is not directly stated, the minimum R_sense value is shown in the datasheet as 200 ohms test the maximum power dissipation (the device's worst-case condition) shown in 6.5 Power Ratings.

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 70 points

    Hi Andrew,

    Makes sense that the minimum R_sense is 200ohms. Is there a maximum R_sense where the chip would continue to operate as expected?

    Thanks,
    Joey

  • 0 in reply to Joey V.
    TI__Mastermind 26695 points

    Hi Joey, 

    Sorry for the confusion. The R_sense has a range of 200ohms - 562ohms. Rthr has a larger range. 

    Best,
    Andrew 

  • 0 in reply to Andrew J
    Prodigy 70 points

    Hi Andrew,

    So just to clarify, back to my original questions - If we didn't care to be compliant with IEC 61131-2 type inputs, can the IC function with a higher Rsense to limit INx+SENSEx current?

    I aim limit the input current to 0.5ma, so I would need a R_sense > 562ohms. 

    Is it possible to use a R_sense > 562ohms?

    Thanks,
    Joey

  • +1 in reply to Joey V.
    TI__Mastermind 26695 points

    Hi Joey, 

    As stated previously, the provided the range for RSENSE is 200Ω to 560Ω. An RSENSE greater than 560Ω (or current limit <2mA) cannot be used.

    Since the device does not have a field side VCC pin, the minimum 2mA current is needed for the device to be powered up. As a result, currents less than 2mA will not power the device and therefore cannot be used.

    Best,
    Andrew