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LM25085: EMI - High Frequency Ringing Noise

Nicholas Tyler1
Prodigy 20 points
Part Number: LM25085

We are seeing c 200MHz ringing on a design using this part.  Any advice on how to find and fix this?

WebBench simulations are nice clean switching waveforms.

  • 0
    TI__Mastermind 28980 points

    Hello Nicholas, 

    We are exciting a parastic LC tank circuit in the IC with a SW node waveform that approaches a unit step response with faster rise time.

    The energy in the ringing can be reduced by:

    1. Reducing the parastics

    -High frequency loop inductance can be reduced by placing small value (low ESL) input capactior closely to IC PVIN and PGND pins

    2. Reducing the rise time on switch node

    -A resistor in series with boot capacitor slows down the rise time on sw node...leads to degregation in efficiency with larger boot resistance (=slower rise time)

    Resource

    https://www.ti.com/lit/an/slua831a/slua831a.pdf

  • 0 in reply to Marshall_Beck
    Prodigy 20 points

    Thanks Marshall,

    Different part in the appnote I guess they must be very similar internally?

    I would prefer to fix this with component changes rather than modifying the PCB.  We have the reservoir 4u7 within 4mm of the power supply pins, and followed the datasheet recommendation for a very low ESR capacitor on Vcc.  

    Among other things, it would be good to be able to model this more accurately than WebBench.  Is there a spice model for the LM25085?

    Thanks

  • 0 in reply to Nicholas Tyler1
    TI__Mastermind 28980 points

    Hello,

    Buck theory doesnt change wether controller, converter, or module.

    Modeling high frequency EMI is challenging. S Parameters have be ran on PCB + IC, not always giving real life result.

    We have transient model available.

    https://www.ti.com/product/LM25085/toolssoftware