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Original question:

ISO1212: Current through RSENSE

ISO1212: More precise current formula

Miroslav Mrdjan
Prodigy 175 points

Part Number: ISO1212

Hello,

We are using the formula for the current calculation from this link https://www.ti.com/lit/zip/sllc457

However, we noticed that there is a difference compared to the datasheet. In the excel sheet you can insert a voltage value that actually has little or no effect on current values.
In the datasheet, we can see that current is highly dependable on voltage:

Do you have some better formula as we are working with a voltage around 18V and our assumption is that current can't go all the way to 2.75 which is (I suppose) 30V current?

Best regards,
Miroslav

  • 0
    TI__Guru 55965 points

    Hi Miroslav,

    Thanks for reaching out and for your interest in ISO1212. Please allow me clarify and address some of your concerns.

    Please note that ISO1212 regulates the input current and tries to minimize any variation in the current so that the power dissipation in external components is kept to minimal, which, otherwise, is not possible when optocouplers are used in place of ISO1212. This is one of the key features of ISO1212.

    The formula provided in the spreadsheet is to determine the value of RSENSE that is needed to achieve a given typical regulation current. Please note that the current is primarily set by the RSENSE and variation is due to temperature and sample-to-sample variation. The input voltage doesn't directly determine this current.

    Miroslav Mrdjan said:

    These are actual tested values on a small sample set during device characterisation. The input voltage does shift the current range marginally as the input voltage increases, it doesn't directly determine the actual current observed.

    The typical regulated current is 2.5mA for RSENSE of 560Ω and when you are testing device at room temperature, you should typically see values close to 2.5mA for most samples. The variation provided in datasheet are worst-case values for worst-case temperature and samples.

    I hope this helps you understand current regulation of ISO1212 better. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Guru 55965 points

    Hi Miroslav,

    I would like to elaborate on what I have shared in my previous post and also share additional information.

    Since your application needs to support 18V inputs, I am assuming you would be designing VIL to be around 7V or higher by using RTHR = 0Ω or higher. With this assumption, the input current is going to be in the range of 2.05mA to 2.75mA for VSENSE voltage range of 7V to 30V.

    I would like to clarify that this definition doesn't mean that 2.05mA corresponds to 7V and 2.75mA to 30V, this is not correct. It instead means that, 2.05mA can occur for worst-case lower temperature on a worst-case sample and 2.75mA can occur for a worst-case higher temperature on a worst-case sample.

    The above explanation is also evidently visible in the plot in figure 5 of ISO1212 datasheet (also copied below) where change in input voltage doesn't change the current much while change in temperature changes the current noticeably. It still doesn't hit the entire current range because this testing is done on small set of typical samples.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 175 points

    Hello Koteshwar,

    I would not agree on this as test at higher voltages are providing higher current values.
    Anyhow, I'm perfectly clear that more info can't be provided duo to way of measurement.

    Thanx.

    Best regards,
    Miroslav