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LM22676: EMI issue of LM22676

Sara Jia
Expert 2975 points
Part Number: LM22676

Hi team,

My ctm use LM22676 as the buck dc/dc in their system, which is only dc/dc. 12 Vin and 3.3  V out, the processor is NXP RT1052, PHY 8081.We can't pass GBT 9254_ClassB  EMI test.

the testing data \ sch and PCB file are as attatchment.

Would you pls give us some recommendation on this issue?

Thanks very much.

BR

Saradocument.zip

  • 0
    TI__Mastermind 25590 points

    Sara,

    Thanks for providing the schematic/layout/results.

    Schematic:

    • FB17 appears to be a resettable fuse, not a ferrite bead. Replace with a ferrite bead and test again.
    • CIN should have more ceramic capacitance like the EVM. It's currently 0.1uF ceramic and 100uF electrolytic. The ceramic isn't large enough to suppress VIN ripple and the electrolytic's ESR will have ripple as well. I'd recommend 6.8uF ceramic to accompany the 0.1uF cap.
    • L4 is a "shielded" inductor but does not have shielding around the whole inductor. Fully shielded inductors provide better EMI performance.

    Layout:

    • The GND pad under the IC is not fully connected. This should be a solid polygon under the IC.
    • Should place vias at the GND pin of the input cap. That way return current can go directly back to the cap.
    • C110 and C111 should switch places. C110 has a lower inductance package which will allow more high-frequency noise to redirect back to the input caps.
    • Try rotating L4 180 degrees. Sometimes the asymmetrical construction of the inductor can cause big changes in EMI performance.

    -Sam