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TINA/Spice/ISO1212: Sinking / Sourcing input configurations and Rthr resistor placement

Ronald Rehfinger
Intellectual 290 points
Part Number: ISO1212
Other Parts Discussed in Thread: TINA-TI, , ISO1211

Tool/software: TINA-TI or Spice Models

Hi,

I want to replace a typical optocoupler based 24 V input with a ISO1211 or ISO1212. The optocoupler doesn't care if it is connected to a high- or low side switch.

Since I want to stay compatible, the ISO121x circuit has to support high and low side switches. But in the datasheet it seems that I have to connect Rthr to FGND instead of SENSE when connecting a low side switch (called "Sourcing Inputs" in the datasheet).

1. What is the reason for this?
2. What happens when the ISO121x is configured as "Sinking Input", but is used as "Sourcing Input"?
3. Would it be useful to split Rthr into two resistors, with Rthr/2 connecting to SENSE and Rthr/2 connecting to FGND when I want to support both configurations?

A followup question:

I tried to fiddle around with the TINA-TI reference design, but was not able to get a working simulation for a sourcing input configuration. Is the simulation model limited in this regard?

Regards

  • 0
    TI__Expert 6955 points

    Ronald,

    If I understand your use case correctly, you want to be able to drive the IS0121x from either a high-side or low-side switch. The diagram below shows the ISO1211 being modulated by a sensor/switch on the high-side. The same effect can be achieved if the sensor/switch were between the FGND and the system ground. This assuming that the 24V is DC. Does this help? Also, could you elaborate on how the simulation wasn't working for you?

    Respectfully,

    Lucas Schulte

  • 0 in reply to Lucas Schulte9
    Intellectual 290 points

    Hi Lucas,

    Lucas Schulte9 said:

    If I understand your use case correctly, you want to be able to drive the IS0121x from either a high-side or low-side switch.

    Exactly.

    Lucas Schulte9 said:

    The same effect can be achieved if the sensor/switch were between the FGND and the system ground. This assuming that the 24V is DC. Does this help? 

    The datasheet seems to demand that Rthr must be placed on the terminal that is connected to the switch, at least this is what I read out of Figure 28 "Typical Application Circuit With Sourcing Inputs". The datasheet gives no further explanation for this, though.

    So I want to be sure that it is OK to use a circuit like in your picture with either high- or low-side switches.

    Lucas Schulte9 said:

    Also, could you elaborate on how the simulation wasn't working for you?

    I took sllm391.tsc, replaced the voltage generators with time-controlled switches and a 24 V voltage source wired one switch as high-side (switching the 24 V source), the other as low-side switch (switching GND). The high-side switch works, the low-side switch does not. I attached my circuit.sllm391_low_side.TSC

    Regards

  • 0 in reply to Ronald Rehfinger
    TI__Expert 6955 points

    Ronald,

    Rthr doubles as part of an RC filter when paired with a capacitor like in the "Typical Application Circuit With Sourcing Inputs" from the datasheet. This helps reduce incoming noise. You can split Rthr into 2 resistors and have one on both the "SENSE" and "FGND" pins if you are looking to have both high-side and low-side switches. Do you know yest if you want to use the single channel or multi-channel device? Also, I posted on the WEBENCH forum about your simulation problem here. They should be able to help you with that.

    Respectfully,

    Lucas Schulte

  • 0 in reply to Lucas Schulte9
    Intellectual 290 points

    Lucas,

    I plan to use the ISO1212 two-channel device. Would that be of any concern? I'm aware of the functional Isolation between the channels.

    Thanks for forwarding the simulation model problem to the appropriate forum.

    Regards

  • 0 in reply to Ronald Rehfinger
    TI__Expert 6955 points

    Ronald,

    There shouldn't be any concerns but if you run into problems please feel free to seek help from me. 

    Respectfully,

    Lucas Schulte

  • 0 in reply to Lucas Schulte9
    Intellectual 290 points

    Lucas,

    OK, thanks!

    Regards