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LM3481: LM3481MM/NOPB

Steven Lo
Prodigy 10 points
Part Number: LM3481

Hello,

We have a design using the LM3481 as SEPIC to provide 12V@4A with 12V input from a battery.

We wanted to measure the power efficiency with electronic load to drain the current from output. However, the output current became so unstable that the system shut down when the loader hits 2.8A...

We have tried to improve the compensation by replacing Rc = R98 with 2.18k and adding another 22nF cap Cc2 that connects COMP pin to GND directly.

But this little experiment didn't help too much..

Please let me know if you have any ideas to solve this issue.

Thanks!

Steven

  • 0
    TI__Mastermind 25200 points

    Hi Steven,

    Sorry to hear that you are having trouble, and thank you for using the LM3481 in your design. I'm afraid that the schematic you provided is drawn a little bit messily. Can you redraw the schematic in the format shown by Figure 6.1 in the attached application note: https://www.ti.com/lit/ug/snvu628/snvu628.pdf?ts=1639600876631&ref_url=https%253A%252F%252Fwww.google.com%252F?

    Secondly, can you let me know if your input supply is current limited?

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 10 points

    Hi Richard,

    Thank you for looking into this.

    I've redrawn the schematic to make it a bit clearer as below image.

    Yes, there is current limited circuit in input supply but it gets noisy when raising the output current.

  • 0 in reply to Steven Lo
    TI__Mastermind 25200 points

    Hi Steven,

    Thank you for redrawing the schematic. In general, it looks much cleaner.

    The reason I asked about the current limit of the power supply is that I want to check that you have provided enough input current for the 2.8A. Can you confirm that you have allowed enough input current?

    Secondly, can you let me know the purpose of the R99_P and R99_N jumper? The area of the schematic concerning R99, R97, C50 is quite messy. For the current sense network, Q26 is connected in series with R99 to GND. Then, R97 and C50 form a low pass filter so that ISEN pin receives a cleaner current signal. I'm afraid that the connection does not look correct. Can you confirm the above connection?

    Thanks,

    Richard

  • 0
    Prodigy 10 points

    Hi Richard,

    Thank you for getting back to me.

    We ran more tests with the following scenarios:

    1. Use DC Power Supply as power source. Set up its maximum output voltage and current to 12V and 10A respectively.

    Similar situation to the one using a battery, the system shut down before reaching 4A.

    2. Use Battery and setup the electronic load to request a pulse current Duty Cycle (DC) varying from 10% to 90%.

        IpeakMax means maximum peak current that can be requested to the SEPIC without getting shut down.

    • Duty Cycle IpeakMax
      10%  6.9A
      20%  4.3A
      30%  3.6A
      40%  3.3A
      50%  3A
      60%  2.7A
      70%  2.5A
      80%  2.6A
      90%  2.8A

      Also, we found there is a chance that Q26 might get broken in this test.

    Please let me know why you think there is some mistake in our current sense network.

    The resistors R99_P and R99_N are Kelvin Current-sense resistors to reduce measurement error for very low value resistors.

    Additionally, the current sense network comes from the User's Guide of LM3481, document name: LM3481 SEPIC Evaluation Board - AN2094.

    As you can see, the Rsen = R97 = 1k is not exactly the same ( in evaluation board it is R1 = 100 ohm).

    We did replace R97 with 100 ohm but it didn't help too much as well.

    You can find the schematic in the following.

    Thanks,

    Steven