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WEBENCH® Tools/LM5118-Q1: I have designed a Buck-Boost regulator for 12V/3A output from an input of 5V to 40V. Please support for verifying the attached design document.

Muhammed Thaha
Prodigy 50 points
Part Number: LM5118-Q1
Other Parts Discussed in Thread: LM5118

Tool/software: WEBENCH® Design Tools

Buck-Boost Regulator_LM5118_12V-3A.xlsx

  • 0
    TI__Mastermind 21720 points

    Muhammed,

    Thank you for reaching out to TI with your question. Please note that you can use the quick-start calculator to check your design as well. This will check component values and loop stability. I would suggest inputting your design parameters and seeing how the design looks. This design is similar to the LM5118-EVM which can also be used as a reference.

    Looking through your design I would recommend the following.

    1. Select and inductor with a higher saturation current than 11A. Looking through your calculations the efficiency of the converter is assumed to be 100% where in real life efficiency will be lower resulting in an increased peak current of the inductor. Increasing the saturation rating will also give more margin for load transient conditions.

    2. Note that the current limit of the device is going to be around 15A. The inductor current should be increased or the sense resistor should be changed accordingly.

    3. Please lower R46 to ~7kOhm. This will increase the gain margin of the control loop when VIN =5V.

    Please let me know if you have any questions.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 50 points

    Thank you for your comments.

    I have little confusion from your reply.

    I think you have taken the efficiency as 100%, but If you see in the design calculation, i have considered only 80% efficiency.And as per my calculation the Peak inductor current or Maximum switching current for boost mode operation is coming 7.35A.By considering 30% margin, i have selected an inductor with a saturation current rating of 11A. Can you please check about this and help me to verify the selected inductor part is sufficient for this design. Since i am having PCB size concern, i have to select a small size inductor for this design.Also let me know how this inductor current rating will effect the over all operation if i am going ahead with the selected inductor(I sat = 11A).3515.Buck-Boost Regulator_LM5118_12V-3A.xlsx

  • 0 in reply to Muhammed Thaha
    TI__Mastermind 21720 points
    Muhammed,

    I found the issue. The equation that you are using the boost operation is not accurate. The LM5118 operates as a two switch buck-boost and not a true boost controller. When VIN <= VOUT the equation for that duty cycle follows that of a buck-boost converter and not a boost converter. See equation 8 of the datasheet and cell D54 of your excel sheet.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 50 points

    Buck-Boost Regulator_12V-3A.xlsxThank you very much for helping me to find out the mistake. As per your recommendation i have changed the equation.

    Now i am  getting the Peak inductor current or Maximum switching current for boost mode operation is coming 10.38A,

    I have selected an inductor of the part number (SRP1770TA-330M).

    Please let me know this will fit for my requirement. Also please go through the attached design sheer for seeing the calculation as per the new inductor.

  • 0 in reply to Muhammed Thaha
    TI__Mastermind 21720 points

    Muhammed,

    It looks like that inductor will not have enough current rating. The saturation current of the inductor is 15A but the current limit is going to be around 15.88A (cell D114). It could be possible to select a lower inductance value (22uH, 15uH) and keep the design relatively the same while increasing the saturation current of the inductor. Please make sure that the selected inductor has a high saturation current rating then selected current limit.

    Thanks,

    Garrett