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LM3150 EVM board, Current limit

Botao Miao
Prodigy 70 points
Other Parts Discussed in Thread: LM3150

Hi,

I am using LM3150 EVM board (500kHz) to build a buck converter from 20V to 14.5V. I changed the feedback resistor, inductor, output capacitors and Ron based on my application.  Everything works fine.

But I tried to test the current limit, it didn't work. I want  the current limit is between 3A and 4A. Based on the equation in the datasheet, I pick Rlim about 300 ohm, 250 ohm, 200 ohm. None of them work. Did I miss anything?

Thanks,

BM

  • 0
    TI__Genius 16025 points
    Hi Botao,

    Thank you for the question. We will get back to you soon.

    Thanks,
    Anston
  • 0
    TI__Expert 6860 points
    Hi Botao,

    Assuming you need Current limit at 3A and you are designing your supply with 20V in 14.5V@2.5A specs (Inductance at 10uH for 30 percent ripple) . The Rlim resistance for typical application comes out to be close to 250 ohm .

    Have you tried increasing the load and see where current limit kicks in . Also please share your updated schematic too .

    Ambreesh
  • 0 in reply to Ambreesh Tripathi
    Prodigy 70 points

    Hi Ambreesh,

    Thanks for your replay. In my setup, the inductance is 10uH, the output capacitance is 20 uF. and the switching frequency is 250 kHz. Now I changed the Rlim to 100 ohm. I can observe the current limit triggered around 2.5A. 

    Why is there so much difference in Rlim between the calculated value and experimental value?

    Thanks,

    BM

     

  • 0 in reply to Botao Miao
    TI__Expert 6860 points
    Hi Botao,

    In the datasheet ,Rdson max is taken for the calculation , which results in higher value of Rlim as current limit kicks in if the voltage across
    RLIM and the voltage across the low-side FET are equal ( current limit comparator will terminate the next on-time cycle). This is done because even in higher temperature (where Rdson increases), the solution should be able to provide required current without hitting the current limit .

    With Regards
    Ambreesh