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LM25011: Inserting ferrite bead in series with Pin 1 for EMI control blows up the IC. Solutions?

Dr. Ramkumar Ramaswamy
Intellectual 410 points
Part Number: LM25011

Greetings TI Engineers! I've been using a LM25011 in a product that recently went for an FCC pre-scan. It failed the pre-scan and the cause of EMI violation was traced to the LM25011. Sure enough, we went about tightening the current traces etc, but as a quick fix to see if it would help clear the pre-scan, we added a ferrite bead in series with Pin 1. It seemed to reduce EMI quite a bit so we decided to keep it in the next design revision. However, now we found the IC repeatedly blowing up. After a series of controlled diagnostic tests I have done with and without a bead (and no other changes), I see that with no ferrite bead, subjecting the IC to a sharp startup by abruptly plugging in the board's input to a hot 24V supply (and I did this 50 times) does not harm the IC despite nice sparks at the input connector. However, the moment I introduce the ferrite bead, the IC blows up if I perform even one or  two such hot plugins.  The bead is 1k.

The design was taken from a Webench simulation. Input is 24V and output is 7V@2A. The above test was done with no load.

What could cause this? Any advice/insights/suggestions?

TIA!

Best - Ram

  • 0
    TI__Mastermind 27470 points

    Hi 

    Adding a bead in PIN1 is adding a inductor which will make the main fet over stree when it turn off.  you can try to add the bead in front of the input ceramic cap to see the EMI improvement.  it will helpful to the EMI and cause the input ceramic cap close to the VIN PIN, the stress of main fet will be smaller compare the bead in front of the VIn pin.

    Thanks

  • 0 in reply to Daniel Li14
    Intellectual 410 points

    Thank you @Daniel Li14. I did wonder whether a harmless little ferrite could throw up enough inductance to cause issues but chose to dismiss the thought. I should trust my instinct a bit more I guess :-). Your suggestion of course worked perfectly. The input cap was 1uF and I added a couple more 0.1uF in parallel for added safety. It worked fine with a 1k ferrite. I'll probably try a 2k ferrite too.

    Thanks again!

    Best - Ram