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bq24157S output capacitor

Ronald Jansen
Intellectual 520 points
Other Parts Discussed in Thread: BQ24157S

Hello,

The bq24157S datasheet specifies that an output capacitor of 4.7-47uF will give the best stability. However, the schematic (figure 28) shows 2 capacitors. One of 22uF and one of 33uF. This totals up to 55uF. I though that one of them might be electrolythic, but the schematic does not show this. Am I missing something?

I was planning on using a 22uF ceramic capacitor X7R, probably a 10V type. Is there any reason why I would have to add an extra capacitor?

Thanks in advance for your help.

Best regards,

Ronald

  • 0
    TI__Guru*** 143980 points
    The 22uF capacitor should be okay. The additional capacitance is recommended but not required for improved transient response.
  • 0 in reply to Jeff F
    Intellectual 520 points
    What I do not understand is that the sum of both capacitances exceeds the recommended maximum for stability. That seems like a contradiction to me.
    If the only purpose of the two capacitors is to gain more output capacitance I would have placed a single 47uF.

    As the circuitry behind the battery will have an extra regulation down to 3.3V small dips should be ok. On the other hand if the battery is completely discharged it will give around 3.3V. In that case a dip might cause a problem. So just to be sure I will place a 47uF rather than a 22uF then.