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Amplifiers forum

Part Number: LM339

Tool/software:

Greeting Ti team:


I'm using LM339DR2G Comparator to doing our over current protection. On the negative Input, I put 2.77V as stable voltage. When there is current, our detection model will generate IF voltage scale of current, which will dedicate sending to Input Voltage port. But I find out LM339 Negative Input Voltage drop when IF putting out.


 Then I realize that I should put resister on Pin 11, It did work but there are still some voltage dropping and I really don't know why (Add1K Left/ Add 20K Right)


Thank you in advance for your assistance.

Regards,

Ziming Li

  • Hi Ziming,

    That scope seems to say 20A per division, is that correct?

    There is a 100kHz 40Amp peak signal going into IF? and sounds like there wasn't R129 on the first 2 scope shots? 

    In your first scope shot channel 1 shows 1.5V, what is that monitoring?

     

    So the issue is that the negative input drops from 2.77V to -1.22V when there's ~18A peak 100kHz going through R129?

    Then going to 16k on R129 drops the negative input voltage from 2.68V to 1.8V.

    and going to 25k drops it from 2.6V to -1.96V.

     

    Please confirm so I can best assist you.

    Best,

    Michael

  • Thank you for your fast response and I’m apologize for my broken English let you misunderstanding the current and the IF things.

    Now, I’m going to show you something very weird (at least for me). Channel one is our 5V voltage source and channel two is our Primary current (I need to monitor that in order to make sure our power is safe, but you may just ignore that). Then, channel three is IF voltage. Least, channel 4 is LM339DR2G Negative Input Voltage. This chip is power by 5V voltage (VCC) and the other side connect to GND.


       1


    I think I need to mention that there is no resister to limit the positive input current for those 6 pictures (just like IF connect a voltage source), after I realized that, I put some resister on positive input (like 1K or 20K), Negative input voltage Did not dropping that hard. And I really don’t know why (Even if input out of common mode voltage, the negative input voltage shouldn’t drop)


    I really hope you may give me suggestions and help me figure out what’s wrong on this Comparator.

    Thank you again and hope you have nive day. O7

  • Hi Ziming,

    No worries, I just want to make sure I can answer any questions clearly.

    From the scope shots, IF voltage goes down to -1.5V.

    The absolute minimum input voltage (before possible permanent damage) into this comparator is -.3V, while the recommended operation condition is (Vee - 0.1, in this case (-0.1V). 

    www.ti.com/lit/ds/symlink/lm339.pdf

     

    I can't say with certainty why the positive input is pulled down since its operating outside its abs min input voltage.

    In terms of engineering guidance, the inputs are tied together with a current mirror which may be why you're seeing the drop.

       

    Let me know if you have any questions.

    Best,

    Michael