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LM27342: EMI Concerns - Potentially caused by transient at "SW"

Daniel Cleveland
Prodigy 10 points
Part Number: LM27342

Using the LM27342 Eval board as well as in our own design we've found that there is excessive EMI from 30Mhz - 200Mhz that is not present without this converter in the signal path.
Scoping the chip at the "SW" pin, I can see a large transient which is likely the cause of this EMI.

I am looking for ways to reduce these radiated emissions. Is this normal behaviour? I've just ordered 2 more eval boards just in case mine is a lemon.

On the EVAL board I changed R2 = 91 ohms. But the same problem is present with the original value of R2 = 187 ohms

This image is scoped on EVAL board at "SW" with R2 = 91 ohms. Everything else is stock. Vin=12V. Load is 800mA.

  • 0
    TI__Mastermind 46760 points

    Hi Daniel,

    1. Could you confirm if LM27342 converter is turned off in customer board, the EMI from 30MHz-200MHz noise is not present? Are all the boards have same behavior or only one board?

    2. Ensure using a proper switching node measurement technique. See attached file slide #2.

    3. Put probe with short lead on the D1, not on the SW test point. SW test point on the EVM has a long loop which may pick up some noises. Compare both EVM and customer design. Refer to attached file slide#3

    4. Try to add a 0402 package, 0.1uF or lower (2200pF) high frequency bypass capacitor placed as close as possible from Vin to GND. please refer to attached slide #4.

    5. You may send me customer schematic and PCB layout. I can help to review them.  nancy_zhang@ti.com

    LM27342 SW measurement.pptx

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    Prodigy 10 points

    1. Can confirm all have the same problem.
    2/3. Measurements have been done according to your outlined procedure. All measurements done like you suggest across D1. EVM photo here:

    My design looks identical.
    4. This changed nothing. Identical response.
    5. I've sent you a schematic to you email, though it's likely not helpful as it is identical to the EVM schematic.

    Please note, that my issue is not this transient. My issue is the EMI that this board is producing. I'm am just hypothesizing that this transient is the cause.

    Daniel

  • 0 in reply to Daniel Cleveland
    TI__Mastermind 46760 points

    Hi Daniel,

    We already discussed this issue offline. So I close this post.

    Thanks,

    Nancy