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LM5145: CE Issue

Phil Chapman
Prodigy 55 points
Part Number: LM5145
Other Parts Discussed in Thread: LM5146-Q1

I have a CE problem at the regulator switching frequency on my network port. Input Power port is ok.
I've yet to go into lab to investigate. I'm suspecting that switching noise is getting onto the ground or power plane. It is a 4-layer design so kept the planes solid and kept layout very similar to EVK. I'm considering increasing the BST resistor value and adding a snubber at the SW node. I have unpopulated components there, 0603 footprints. I'm trying to avoid re-spinning my board unless absolutely necessary. I'm still awaiting data from the EMC test lab, but it has a peak at 550kHz (approx) 15dB above the line. Any help would be appreciated. I no longer have any tech support contacts at TI.

  • 0
    TI__Guru 73072 points

    Hello,

    The boot resistor and snubber are more to reduce the higher frequency emissions. The boot resistor reduced the rising dv/dt of the SW voltage and the snubber reduces ringing on SW.

    Is 550kHz the switching frequency or 2nd/3rd harmonic? If so, check that the inductor is shielded and all switching traces (SW, HO, LO) are kept away from the network port.

    Regards,

    Tim  

  • 0 in reply to Timothy Hegarty
    Prodigy 55 points

    Hi Tim

    The 550kHz is the switching frequency. The switching nodes are away from network port.

    The snubber is not populated yet, I provided footprints for it. The boot resistor is currently 2R1, I was considering increasing the value.

    Do you have recommendations for the R and C for the snubber?

    Regards

    Phil

  • 0 in reply to Phil Chapman
    TI__Guru 73072 points

    Hi Phil,

    Given the Fsw of 550kHz, try 2.2 Ohms and 220pF for the snubber. Also, take a look at the rising edge of the SW voltage to see if it rings - if so, the snubber should help that, but you can also increase the Rboot resistor if needed.

    Take a look at the layout guidance in the LM5146-Q1 datasheet (the automotive version of the LM5145) for more detail.

    Regards,

    Tim