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ISO1211: ISO1211 Circuit Configuration and Rsense Value confirmatio

Hyunny Choi
Prodigy 10 points
Part Number: ISO1211


 

Hello TI Experts,
I am currently using the ISO1211 in an application configured as shown in the attached schematic. 

unnamed (2).png

The input signals are connected from an outside of connector.

I initially used R671 and R672 Rsense with a value of 562ohm, but the circuit did not function as expected. 
After changing the value to 1Kohm, the circuit started working correctly.

I have two questions regarding this setup
1. Is the provided circuit connection/configuration unsuitable for use with the ISO1211 in the first place?
 (The circuit uses a standard sinking input configuration with an external connector for A_IN/B_IN and their respective grounds A_GND/B_GND
2. Is 1Kohm an acceptable and safe value for the Rsense resistors  R671 and R672 i?

 

  • 0
    TI__Mastermind 32285 points

     Hello Hyunny, 

    Thank you for reaching out. I understand that "after changing the value to 1Kohm, the circuit started working correctly". However, Rsense must be lower than 562ohms according to the datasheet.  

    I think the connections at the connector may be causing the difference here. The INx terminals are designed to be connected to sensors that are either 24V or open in sink mode. And A_GND should have firm connection to ground other than to the input pin. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 10 points

    Hello Andrew
    Thank you for your answer.
    You said the Rsense should be lower than 562 ohms.
    But the datasheet,
    "9.2.1.2.1 Setting Current Limit and Voltage Thresholds," only has recommended values ​​and doesn't seem to mention that high resistance (low current) should not be used. Where should I refer to this?

    You mentioned connection issues.
    The connector and external connections were implemented identically using two photocouplers before ISO1211 was adopted, and there was a history of mass production, so there shouldn't be any connection issues.

    To investigate this issue, I changed the board resistors on three units to 1 kΩ and tested them. This confirmed normal operation. I plan to conduct additional testing on the 25 units I currently have.
    Could you provide more precise specifications for the Rsense?

  • 0 in reply to Hyunny Choi
    TI__Mastermind 18875 points

    Hi Hyunny,

    Thanks for your response,

    Please give Andrew an extra day to respond.

    Regards,
    Aaditya

  • 0 in reply to Aaditya V.
    Prodigy 10 points

    Hello Aaditya
    Thank you for your reply.
    I'll be waiting then.

    Best Regards,

    Hyunny

  • 0 in reply to Hyunny Choi
    TI__Mastermind 32285 points

    Hello Hyunny,

    Thank you for your patience. I would like to point out that ISO1211 is not the same as an optocoupler with not internal thresholds. The ISO1211 has internal circuitry that allows for the current limit to be set using Rsense and only switches when input voltage is above the voltage and current thresholds. 

    The previous post shows that the connections are as follows:

    1. 24V connects to Ain
    2. Agnd connects to Bin 
    3. Bgnd connects to 24V_gnd 

    If this is true, then the 24V input signal's 2mA is split below the input threshold when using 560ohm resistors. Changing to 1kohm resistors effectively drops the applied current which allows for operation. However, the test conditions for the current limit feature do not specify this operation. This means that the performance with 1kohm Rsense is not guaranteed for all devices across temperature.  

    To confirm the correct operation of the device:

    • Please remove Connection #2
    • Connect Agnd to the same node as Connection #3 (Bgnd and 24V_gnd) 
    • Change Rsense back to 560ohms
    • Apply 24V signal to confirm operation. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    TI__Mastermind 32285 points

    Can you share the original optocoupler circuit and its purpose? This would help us find out the best way to use ISO1211 to match performance.