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BQ24171: output capacitor selection

Kai Nomiyama
Expert 5575 points
Part Number: BQ24171

Hi Team,

I have a question about bq24171 output capacitor.

due to output inductor and capacitor, hearing LC resonance. 

in order to avoid that LC resonance, is it ok to use 10uF Electrolytic capacitor at C9 instead of ceramic capacitor?

If there is any concern about it, please let me know.

Background

Battery pack doesn't have thermistor so put 10kΩ resistor to TS pin.

therefore, when battery is not inserted, bq keeps switching and hearing LC resonance.

Regards,

Kai

  • 0
    TI__Guru**** 166626 points

    Hi Kai,

    When there is no battery but NTC is replaced with 10kohm, the charger charges the output capacitors to BATREG voltage.  The charger periodically switches to both keep the BTST capacitor charged and to prevent the output capacitors from drooping below BATREG-VRCH.   The ringing that you hear is likely because the charger is switching at a frequency in the audible range, causing a piezoelectric effect in the ceramic capacitors.  If so, then you could increase the capacitance value or lower the FB resistor values to force the switching frequency out of the audible range.

    Changing to electrolytic capacitors will also prevent the same issue.  However, the charger's internal compensation is based on low ESR (~10mohm) ceramic output capacitor that do not generate a small signal zero.  Using an electrolytic capacitor with high ESR generates a small signal zero that could lead to control loop instability. Also, the ESR will cause a large signal switching ripple.  Low ESR electrolytic capacitors are generally much more expensive than higher ESR electrolytic or ceramic capacitors.

    Regards,

    Jeff