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LMR14030: Switching Frequency noise on the RF board

JH Shin
Mastermind 8475 points
Part Number: LMR14030

Tool/software:

Hello,

My customer is using the LMR14030 as a power supply for RF power amplifier board.

They have an issue with unwanted spurs related to the switching frequency of the LMR14030 as shown below.

They confirmed that the spur signals disappear when the output voltage of LMR14030 is supplied externally.

spur.pptx

Q1. Please review the design below for any issues.

LMR14030SDDAR_CD.pdf

Q2. They built the boards at the same time using LMR14030SDDAR of the same LOT, but as above, unwanted spurs occur in various forms depending on the board.

        Have you ever reported experiencing spurs due to DC/DC switching noise on your RF board?

Q3. Could you please advise on the causes and solutions of various forms of noise that may be generated by DC/DC switching frequency?

Q4 Please let us know if there is a way to reduce the external influence of DC/DC switching frequency.

Thank you.

JH

  • 0
    TI__Genius 12505 points

    Hi JH,

    Thank you for sending your schematic. I reviewed it and here are my comments:

    • Please confirm that your inductor follows equations 9 and 10 in the datasheet. An inappropriately sized inductor can cause considerable current ripple and negatively affect EMI performance. Also please ensure that the inductor current rating is higher than the current limit of the part (6.6 A) to avoid saturating.
    • Please check that your capacitors are following equations 11-14 of the datasheet. 
    • TI recommends that the capacitors used are rated for at least 2x the voltage that they would experience to avoid significant de-rating. 
    • Please ensure that you are following the layout guidelines in section 11 of the datasheet. The key care-abouts with layout are ensuring that the SW node is minimized and the CIN are placed as close as possible to the VIN pin and GND. Also note that it is preferable to have a one spot ground connection point for CIN, COUT, and the diode to minimize the noise within the ground plane. You can find an example layout in section 11.2 of the datasheet or the device's associated EVM. 
    • Finally, it can be helpful to add an input EMI filter to improve conducted EMI performance. Please see this app note for designing an EMI input filter.

    Once you have checked these, feel free to reach out if you require further assistance and send your layout for us to review. 

    Thank you,

    Joshua Austria