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LM3401: The method to reduce the noise of LM3401

takahiko yokota
Mastermind 7135 points
Part Number: LM3401

Hi

My customer use LM3401 under Vin=10~24V, Vout=3.4V and Iout=1.4A.

But when they evaluated LM3401 using EVM, there were the noise as attached document.

 Coukd you tell me to reduce it?

I think I use the bypass capacitor from ILIM to GND to reduce that noise.

Could you tell me recommending value or calculation method?

LM3401.pdf

Best regards,

Yokota

  • 0
    TI__Mastermind 25500 points

    Hello Yokota,

    Is that an AC coupled scope shot of the ILIM pin? That was not clear. In any case any time you probe anything on a switching regulator you will see switching noise due to the fast turn on/off of the FET and the reverse recovery of the diode. It's pretty normal and nothing to worry about.

    That said a lot of it can be from measurement method as well. If you are using a standard 10x scope probe with a ground lead and clip it creates a large loop. High frequency noise will couple into this loop and make it look much worse than it actually is. You can use differential probes or just remove the probe ground lead and wrap a short length of wire around it for the ground point. That way you can eliminate the large loop and ground as close to the pin as possible to the pin. You will likely notice a significant reduction just from that. But you should not need a cap on the ILIM pin, it will not cause issues.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Mastermind 7135 points

    Hello Clint-san

    I shortened the wire and tried the remeasurement, but the noise didn't reduce.

    If there are the methods to resolve, could you tell me it?

    Best regards,

    Yokota

  • 0 in reply to takahiko yokota
    TI__Mastermind 25500 points

    Hello Yokota-san,

    As I mentioned, the noise is real. It can just look worse if you probe pins with a long ground lead, but it does exist and is normal in any switching regulator. It is high frequency noise generated by the slew rate of the FET and the parasitic capacitance of the diode. However, if you really need to reduce it you can try the following:

    1. Use a gate resistor to slow the FET turn on/off

    2. Use an RC snubber across the diode and/or FET

    3. Place a ferrite bead in series with the diode and/or FET

    Those are the most common ways to reduce the high frequency noise in a switcher.

    Regards,

    Clint