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ISO1211: Protection question

Vincent Chen64988
Expert 4480 points
Part Number: ISO1211
Other Parts Discussed in Thread: ISO1212

Hi Team,

My customer is surveying ISO1211/12 for PLC application, this project has to pass functional safety and he would like to know the device's behavior during the abnormal senario.

If Rthr, Rsense get damaged, at the meantime whole system keeps normal operation then what's the ISO1212's reaction or triggering any protection functions.

There are three cases and what happened on ISO1212.

Rthr is shorted, Rsense is good : 

Rthr is good, Rsense is shorted :

Both Rthr and Rsense are shorted : 

Thanks a lot.

Vincent Chen

  • 0
    TI__Guru 55815 points

    Hi Vincent,

    Thanks for reaching out. Could you please confirm what is the input voltage under these conditions? Is it 24V or will it be <60V?
    Assuming the input is <60V which is the maximum recommended voltage of the IN and SENSE pins, please see below my response.

    Vincent Chen64988 said:
    Rthr is shorted, Rsense is good :

    This makes the voltage at the IN/SENSE pin be >VIH and hence, OUT will go HIGH. Since the RSENSE is intact, current into the device is still limited and regulated to the set value as per RSENSE (i.e., ~2.3mA for RSENSE = 562Ω). No damage to device and device will continue to function normal.

    Vincent Chen64988 said:
    Rthr is good, Rsense is shorted :

    The device supports an input current of upto 6.5mA for which RSENSE needs to be 200Ω. I am not quite sure if the current gets clamped to 6.5mA or if it allows higher current when RSENSE < 200Ω. Please allow me to check on this and come back to you.

    Vincent Chen64988 said:
    Both Rthr and Rsense are shorted :

    The explanation to situation where RSENSE & RTHR are shorted will be similar to the situation only RSENSE is shorted. Please allow me to come back on this. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Expert 4480 points

    Hi Koteshwar,

    Thanks a lot for the response.

    1. The situation we discuss is Vin <60V.

    2. For fault condition 1, the device will continue to function normal. Does that mean the VIH is 8.25V and VIL is 7.1V based on equation 2, 3. Is it correct?

    In this condition, ISO1212 cannot recognize Rthr is fault, right?

    3. Due to this case is for functional safety application, do we have more suitable solution that is under development can be introduced to customer?

    Btw, is any update for fault condition 2 and 3.

    BR,

    Vincent Chen

  • 0 in reply to Vincent Chen64988
    TI__Guru 55815 points

    Hi Vincent,

    Sorry about the delay in response.

    Vincent Chen64988 said:
    1. The situation we discuss is Vin <60V.

    Thanks for confirming this. This means the device will not be damaged by excess voltage, as the device is rated for 60V.

    Vincent Chen64988 said:
    2. For fault condition 1, the device will continue to function normal. Does that mean the VIH is 8.25V and VIL is 7.1V based on equation 2, 3. Is it correct?
    In this condition, ISO1212 cannot recognize Rthr is fault, right?

    I meant to say that the device will not be damaged and is going to work the way it is expected to. But since RTHR is shorted, the VIL and VIH thresholds are going to change. The updated thresholds will be similar to the conditions with RTHR = 0, as given in the datasheet and also copied below.

    Vincent Chen64988 said:
    Rthr is good, Rsense is shorted :

    Since RSENSE is shorted here, the current will no longer will be regulated by device and is only limited by RTHR. Assuming RTHR = 1kΩ, the current into device will depend on the applied input voltage. i.e., it will be about 24mA for 24V inputs. Continuous operation with these conditions could lead to device reliability.

    Vincent Chen64988 said:
    Both Rthr and Rsense are shorted :

    This is similar to earlier case that current will not be regulated by ISO1211 and since RTHR is also 0Ω now, the current into device is also not limited by any resistor. Based on voltage applied, this can lead to a large input current and eventual device damage.

    To address this challenge for functional safety application, customer can follow the below approach where two ISO1211 devices are used in series. Instead of two ISO1211 devices, you can also use on ISO1212 where the two channels can be connected in series. The second channel / device acts as a fault indicator which under normal operating conditions output LOW output. But when there is RTHR and RSENSE short of first channel / device then the output of second channel / device goes HIGH indicating a fault. Since the second device is limiting the current to 6mA, neither of the device will be damaged.

    Let me know if you have any questions, thanks.

    Regards,
    Koteshwar Rao