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LM3485: Buck Controller Noise

Julius Klose
Prodigy 10 points
Part Number: LM3485

We are using your PFET Buck Controller LM3485 and have a few questions about noise problems and their solutions.

At 7.2 V – 7.4 V we are currently facing some noise problems on the input current.

The output voltage is 7 V.

 

In the attached files you see the measurements for 7.2 V and 7.4 V. This noise is noticeable by ear.

 

Is it possible to get rid of that noise by ESR?

We have been trying with a 50mOhm and 1.3 Ohm ESR but were not able to solve this.

We are using MLCC capacitors with Flexible Termination at the Output with 10uF each. (you can see in the attached schematic).

 

Is it possible to solve the problem by adjusting capacitor values at the feedback pin ?

  • 0
    TI__Mastermind 22299 points

    Julius,

    Sorry, I did not see your schematic anywhere. 

    You switching frequency has fallen due to VIN getting close to VOUT.

    The reduced switching frequency reduces inductor ripple current and ripple needed for hysteresis.

    Use equation 4 of the LM3485 datasheet to approximate switching frequency.

    Switching frequency should be larger than 20kHz to avoid audible tones.

    A feedforward capacitor CFF parallel to upper feedback resistor can be used as well. See figure 21 in typical characteristics curves.

    Hope this helps.

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 10 points

    Hello Orlando,

    sorry I forgot to send the schematic..

    You are Right, I can measure less than 20 kHz at this Voltage.

    So I tried to use different Capacitors for the Feedback capacitor CFF.

    I have tried values from 100pF up to 680pF. This did not make the noise better unfortunately.

    I guess I have to switch the value of my inductor if I want to reduce that noise ?

  • 0 in reply to Julius Klose
    TI__Guru 54712 points

    Hi Julius,

    I'll let Orlando comment further, but one recommendation is to keep the FB trace as short as possible to minimize noise pickup. Place the FB resistors close to the FB and AGND pins.

    Regards,

    Tim