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LM25085A: Request design review

Yongki Jeong1
Intellectual 515 points
Part Number: LM25085A

Hi Ti team

A Quick Start Guide was prepared before applying the LM25085A.

Q1. 

I want to use 4.7uFH

Please review whether it is okay to use an inductor of 4.7uH/

LM25085A_Quick_Start.xls

Q2.

I checked the reference PCB, but the part spacing or NET was designed too wide.
Is it possible to design more compact? (20 x 20mm)

Thanks

  • 0
    TI__Expert 3766 points

    Hi Yongki,

    It's not recommended to deviate widely from the design calculator values. With a smaller value inductor, you may run into subharmonic oscillation at high duty cycles and the output will have an envelope rippling at a low frequency. The TI reference PCB incorporates good practice for thermal distribution and low noise particularly with the sense lines. However, feel free to request a layout review.

    Regards,

    Henry

  • 0 in reply to Henry Kou
    Intellectual 515 points

    Hi, Henry

    As I attached the wrong file in my previous question, I am attaching a new calculator.

    6.8uF is the best value I can use.
    I would appreciate any advice on what to improve in the attached file.

    LM25085A_Quick_Start_230718.xls

    Best regards,

    Jeong.

  • 0 in reply to Yongki Jeong1
    TI__Expert 3766 points

    Hi Jeong,

    In order to use a smaller inductor, 6.8uH, with your design you will experience greater inductor current ripple over 20% (more like 30%) of maximum current IOUT when you reach max VIN of 24V. I would recommend testing on an EVM or using PSPICE simulation to double check stability over your entire VIN and IOUT ranges with this higher inductor current ripple.

    Regards,

    Henry

  • 0 in reply to Yongki Jeong1
    TI__Expert 3766 points

    Hi Jeong,

    In order to use a smaller inductor, 6.8uH, with your design you will experience greater inductor current ripple over 20% (more like 30%) of maximum current IOUT when you reach max VIN of 24V. I would recommend testing on an EVM or using PSPICE simulation to double check stability over your entire VIN and IOUT ranges with this higher inductor current ripple.

    Regards,

    Henry

  • 0 in reply to Henry Kou
    Intellectual 515 points

    Hi, Henry

    So, in the calculator I gave you, if I limit the VIN so that the value in 'Step 4 - Inductor Selection : Minimum Recommended value for L1' is less than 6.8(Ex. VIN_max = 15V), is it okay to use the values designed by the calculator?

    Best regards,

    Jeong

  • 0 in reply to Yongki Jeong1
    TI__Expert 3766 points

    Hi Jeong,

    The calculator is designed to recommend an inductor value so that your inductor current ripple stays under 20% of your max current output with the design parameters you've input. The calculator is just an estimate using datasheet equations, so I would verify this using the PSPICE simulation or on the device EVM.

    Regards,

    Henry