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LP2985A-33: what is IGND in LP2985-33(slvs522o)？how to test this parameter?

john Ma
Intellectual 280 points

Part Number: LP2985A-33

IGND is listed In LP2985 datasheet. I red some meterials say that IGND=Ivin-Iout, I want to know why? Can anyone help to tell me?Can you draw a draft to illustrate this question? Thank you very much.

  • 0
    TI__Prodigy 280 points

    Hi John,

    I have drawn a simple diagram, hope this will answer your questions.

    Q: Ignd=Ivin-Iout Why?

    A:  Ignd measures how much current does a LDO need in order to regulate the output voltage. As you can see from the diagram below, according to Kirchhoff laws, Iin=Iout+Ignd. So we can get the function Ignd=Iin-Iout.

    Q: How to measure Ignd?

    A: According to the explanation above, the easiest way would be connect a DMM at the GND pin of the LDO. However this way may influence the reference ground voltage for LDO, which may cause instability issue for the LDO. So the best way to do this would be using a source measure unit(SMU) to force the current back to the input. See the below diagram, in this way Iin itself is Ignd.

    Thanks,

    Song

  • 0 in reply to Andonis Wang
    TI__Prodigy 280 points

    Hi John,

     

    Sorry. It seems the diagrams are missing. I repost it again.

     

    Q: Ignd=Ivin-Iout Why?

    A:  Ignd measures how much current does a LDO need in order to regulate the output voltage. As you can see from the draft below, according to Kirchhoff laws, Iin=Iout+Ignd. So we can get the function Ignd=Iin-Iout.

     

     

    Q: How to measure Ignd?

    A: According to the explanation above, the easiest way would be connect a DMM at the GND pin of the LDO. However this way may influence the reference ground voltage for LDO, which may cause instability issue for the LDO. So the best way to do this would be use a source measure unit(SMU) to force the current back to the input. See the below draft, in this way Iin itself is the Ignd.

    Thanks,

    Song

  • 0
    TI__Mastermind 22610 points

    For simple three terminal regulator, Ignd is any current that goes in, but does not make it to the load. This is considered 'lost current'.

    For regulators with more functions (like LP2985) it gets a bit more complicated, but the concept is still the same: lost current.