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Original question:

LM5176: Using LM5176 as Battery Charger it happens that there is an unwanted reverse current flow

LM5175: Dither function in case of low frequency regulation loop.

Pierannunzi Francesco
Intellectual 350 points
Part Number: LM5175

Dear Sir.,

I have a battery charger developed with the LM5175, the loop bandwidth is lower than 1kHz (about 400Hz) to achieve stability for a large input and output voltage and current range. My switching frequency is 100kHz in DCM with RMode=49.9k and Cslope of 100pF.

I tried to use a Dither capacitor of 10nF value, but when the charger works in Buck-Boost and Buck mode the output oscillate at about 5kHz (Dither frequency) and I can hear a noise from the charger, if I remove the Dither capacitor the oscillations disappears and the noise too.

In my case where the loop frequency so low, does it makes sense to use the Dither function? If yes, What should be the correct Dither frequency to use in my case?

What should I do? Should I try to increase the loop frequency?

Please can you help me?

Thank you in advance.

B.R.

Francesco Pierannunzi 

  • 0
    TI__Guru 65170 points

    Hello Francesco,

    According to the formula in the datasheet, with 10nF, you get about a dither frequency of 4.2kHz. To effectively reduce EMI, the dither frequency should be a lot lower than the switching frequency, so you could try to get even lower than your crossover frequency.

    In general if you hear a sound, something on your board has exactly this frequency as resonance frequency, so you need to try which dither frequency does not cause this resonance.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Intellectual 350 points

    Hi Brigitte,

    thank you for your answer but since I have a very low crossover frequency if I select a Dither capacitor for 400Hz Dither frequency or lower, what do you think could it makes sense to use it? Or it is better to disable it? What is your advice in this situation?

  • 0 in reply to Pierannunzi Francesco
    TI__Guru 65170 points

    Hello Francesco,

    The dither feature helps to reduce EMI at a certain frequency, so if this is not needed for your application, I recommend disabling this feature.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    TI__Guru 65170 points

    Hello Francesco,

    As we did not hear back from you, I expect the problem is solved and will close the thread now. If there are further questions, please post below or start a new thread.

    Best regards,
    Brigitte