LDO Voltage Regulator Current Source Not Working As Expected.

Discussion in 'General Electronics Chat' started by MattIsHere, Dec 23, 2015.

  1. MattIsHere

    Thread Starter New Member

    Dec 23, 2015
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    So everyone know famous LM317 can be configured as Current Regulator like this:

    [​IMG]

    However the dropout voltage of LM317 is pretty high and so I am looking for some LDO chip to do the same thing. Here I got some RT9018 chip to do this:

    http://www.richtek.com/Products/Linear Regulator/Single Output Linear Regulator/RT9018ART9018B.aspx

    I have successfully configured RT9018 as Voltage Regulator like what it supposed to be. However I can't get it working as a current regulator. I connect the load exactly like what I do for LM317 but it just don't working. Is it not all the voltage regulator can regulate current or is there something I am missing? Thanks for helping.
     
  2. dl324

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    Mar 30, 2015
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    Your first post. Welcome to AAC!

    Post a schematic for the circuit that isn't working and give current and load requirements.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Are you connecting Vin to the EN pin? The EN pin is "enable" so the regulator can be turned on/off. It myt have a voltage applied to make the chip work.
     
  4. MattIsHere

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    Dec 23, 2015
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    Thanks Here is How I connect the LDO:
    1.jpg
     
  5. MattIsHere

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    Dec 23, 2015
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    According to the datesheet:
    "For RT9018A, the EN pin function pulls high level internally. So the regulator will be turn on when EN pin is floating."

    I have successfully configure it as a voltage regulator so the problem is changing it from voltage regulator to current regulator. I am not sure if all the voltage regulator can be configured as current regulator.
     
  6. dl324

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    Show connections for all pins and give current and load requirements.
     
  7. GopherT

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    It is possible that the adj needs a ground reference (directly or through a resistor), so it would not be able to work as a current source.
     
  8. MattIsHere

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    Dec 23, 2015
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    You maybe right. I included the block diagram for you to determine the problem better:

    block.JPG

    If you are right. I think I have to look at another chip to do the job. How could I know whether the adj need a ground reference? Can it be determined by looking at the block diagram?

    Thanks
     
  9. dl324

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    The regulator you're using doesn't work like LM317. What are your current and load requirements?
     
  10. MattIsHere

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    Dec 23, 2015
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    Hi Dennis,

    I am using this LDO as a LED drive (LED Vf about 3.3v). I am planning to use single li-ion cell and power source (3.7-4.2v), I want to get the maximum current at about 1A. Thanks
     
  11. dl324

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    What is the amp hour rating of the battery?
     
  12. MattIsHere

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    Dec 23, 2015
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    It is a 3400mAh 18650 cell.
     
  13. dl324

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    You don't have enough headroom for a current source @ 3.7V.
     
  14. MattIsHere

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    Dec 23, 2015
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    Not the problem. I am using the 5V power supply when I test the circuit. I think it it like what GopherT say.
     
  15. dl324

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    You don't have enough headroom for even a simple current source. Even at full battery voltage of 4.2V, a medium power transistor in a current source would be near saturation @ 1A. Not enough voltage to consider a MOSFET.
     
  16. MattIsHere

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    Dec 23, 2015
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    Hi Dennis,

    As I said before. I am using a 5V power supply which provide enough headroom when I test the circuit. I don't really care about the input voltage as I can use 2 cell. I just want to get the LDO working as a current regulator now. Thanks
     
  17. dl324

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    It wasn't clear to me that this was just an exercise to use the LDO as a current source.
    It won't work. The regulator you've chosen just uses the resistor divider to feedback a portion of the output voltage to the control logic. It doesn't work like an LM317.

    There are ways to build a current source that don't require a voltage regulator. That's where I'm trying to guide you. But you don't have a high enough voltage for any I can think of.
     
  18. MattIsHere

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    Dec 23, 2015
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    How can I know whether the regulator I choose can limit the current like LM317?

    I don't understand . LDO only have around 0.2-0.3v drop at 1A. With a fully charged cell (4.2v) why it cannot provide enough headroom for 3.3Vf LED? (4.2v-0.3v=3.9v which > 3.3v)
     
  19. dl324

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    Read the datasheet.
    The LDO you selected doesn't have a constant current mode like LM317, so the fact that it only drops 0.3V@1A isn't significant. It regulates voltage, not current.
     
  20. crutschow

    Expert

    Mar 14, 2008
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    I think a basic 3-terminal LDO regulator such as the LT1086 with a 1V drop should operate similarly to the LM317 as a current regulator.
    But note that the minimum operating voltage drop would still be about 1V plus the 1.25V reference voltage or 2.25V.
    If you want a lower dropout than that, you may have to go to an op amp/transistor type of constant-current circuit which can reduce the dropout to well less than a volt.
     
    RichardO likes this.
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