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Original question:

TPS61022: Paralleling Outputs

LM51551: Cannot drive 3A output load at 5V

Sadanori Kato
Guru 36540 points
Part Number: LM51551
Other Parts Discussed in Thread: LM5155, LM5155EVM-BST

Hi,

As mentioned in the attached file, the circuit coefficients were determined and evaluated, but it wasn't able to drive a 3A output load with 5V and LM51551 operated in pulse skip mode with 0.4A output load when 1.8V was supplied to VIN.
So, could you please tell me how to improve this issue ?
Additionally, I don't have EVM, so could you please double-check whether you can replicate with EVM?

Modified_LM5155_Excel_Quickstart_Calculator_for_Boost_Converter_Design.xlsx

Best regards,
Kato

  • 0
    TI__Mastermind 21720 points

    Hi Kato,

    Thanks for reaching out with your question and for using the LM51551.

    Looking at the excel calculator it looks like the current resistor is sized to large. To me this indicate that the LM51551 is running into the peak current limit. Decreasing the value of the current sense resistor to 7mOhm should help the circuit provide full load.

    I would also suggest changing the compensation network to the following value to help improve the loop stablity.

    RCOMP: 1.2kOhm

    CCOMP: 47nF

    CHF: 3.3nF

    Please let me know if you have any questions.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Guru 36540 points

    Hi Garrett-san,

    Thank you for your quick response.

    I just modified the excel calculator, so please confirm it just in case.
    Additionally, if these coefficients are applied to the LM51551, which doesn't work normally, the competitor products will be used. So, could you please double-check whether LM5155EVM-BST normally works at 5V/3A when 1.8V is supplied?

    Modified_1_LM5155_Excel_Quickstart_Calculator_for_Boost_Converter_Design.xlsx

    Best regards,
    Kato

  • 0 in reply to Sadanori Kato
    Guru 36540 points

    Hi Garrett-san,

    Our customer changed these components according to your comment, but this changing decreased ability to drive load current.
    So, could you please test whether LM51551 can really drive 3A at 5V when 1.8V is supplied to LM51551?

    Best regards,
    Kato

  • 0 in reply to Sadanori Kato
    TI__Mastermind 21720 points

    Sandanori Kato,

    Sorry to hear you are having issues getting the board to supply full output power. Based on the quick-start calculator values the device should work. I have a few questions to help figure out what is causing the issue.

    1. What is the maximum output current that is able to be supplied be for the output voltage sags?

    2. Can you provide the schematic?

    3. Waveforms: Please try to probe the SW and CS pin voltage. This will help to see if the device is running into peak current limit or not.

    4. Check the UVLO voltage. Make sure the input votlage is not falling low enough to trigger the UVLO off.

    I am not currently able to test this in the lab.

    Please let me know if you have any questions.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Guru 36540 points

    Hi Garrett-san,

    Thank you for your reply.

    Please see the following comments.

    A1. LM51551 worked in pulse skipping mode with approximately 0.4A output load by using an electronic load when 1.8V was supplied to VIN.

    A2. I will share the schematic via a private message. So, could you please accept my friendship request?

    A3. I will ask our customer to observe these pins.

    A4. It seems that UVLO isn't detected because a bulk capacitor is added to the input VIN.

    I understand that you cannot test this in the lab. So, is it possible to provide me the recommended value for the inductor, resistors and capacitors to convert 1.8V to 5.0V@3A using the excel calculator again?

    Best regards,
    Kato

  • 0 in reply to Sadanori Kato
    TI__Mastermind 21720 points

    Kato,

    I have accepted your friend request. We can continue the conversation with direct messages.

    -Garrett

  • 0 in reply to Garrett Roecker
    Guru 36540 points

    Hi Garrett-san,

    Thank you for the approval.

    I just sent the additional information via a private message, so please check it.

    Best regards,
    Kato

  • 0 in reply to Sadanori Kato
    TI__Mastermind 21720 points

    Hi Kato,

    I am going to close this case since we are communicating over direct message.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Guru 36540 points

    Hi Garrett-san,

    Thank you for your continuous support.

    I understand.

    Best regards,
    Kato