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BQ24232: Max output capacitance

John Griffith
Mastermind 21769 points
Part Number: BQ24232

Tool/software:

Hello,

My customer would like to have more than the 47µF capacitance that is in the recommended range per the pin functions table in the datasheet:

"Bypass BAT to VSS with a 4.7-μF to 47-μF ceramic capacitor."

What would the concern with having more capacitance be? Is it in rush current or stability? Ideally they would like to have 100µF on the output of the device. 

John

  • 0
    TI__Expert 8120 points

    Hello John,

    I tested a BQ24232 EVM with 100uF on OUT and didn't observe any significant ringing or overshoot when enabling charge. I believe the customer is okay to have this much capacitance on OUT.

    Inrush current and regulator stability are both concerns, but stability is the bigger concern here.

    The more capacitance there is on BAT or OUT, the more inrush current there will be when the battery is connected to the charger IC. Depending on the inrush current level and battery specifications, the customer may want to use a battery protector IC to prevent discharge overcurrent or an eFuse to limit inrush current when connecting the battery.

    That said, regulator stability is the more critical concern. Could you clarify whether the customer wants 100uF placed as close as possible to the OUT pin, or if there are downstream components with capacitors on their input pins that add up to 100uF? If the downstream capacitors are far away from the OUT pin, the trace inductance in series with them will help decouple their resonant effects, which can improve stability.

    Let me know if you or the customer have any questions - I'd be happy to help.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    TI__Mastermind 21769 points

    Hi Alec,

    Appreciate you going in the lab to try this out. I am asking for clarification on how much capacitance is in different places on the PCB, but I do know that he said he was "considering additional capacitors which are far away from the chip but connected to the same "OUT" power plane." So I definitely think this is going to be distributed with some parasitic inductances between capacitors. 

    John

  • 0 in reply to John Griffith
    TI__Mastermind 21769 points

    Update from my customer:

    Actually, there is 66 uF (3 * 22 uF) connected closed to OUT pin, and there are additional three 10 uF capacitors connected closed to the three downstream DC/DC input.

    would you please help to explain why there will be the risk for regulator stability problem? And how can we check the stability?

    Do we have a test procedure that they can follow in their PCB to test that this isn't an issue? 

    John 

  • 0 in reply to John Griffith
    TI__Expert 8120 points

    Hello John,

    To clarify, I don't expect that the customer will experience stability issues with this amount of capacitance. At a high level, the BQ24232 is designed for use with ceramic capacitors and has internal compensation, which reduces its sensitivity to ESR and capacitance variations on the output.

    The customer can start by turning on their loads and checking for ringing or overshoot on the output. If they want to verify stability more thoroughly, they can perform a load step test with all capacitors populated on their board and their actual loads replaced with an e-load or resistor that draws the same current. In either case, they should monitor the output during the load step - if they don't see any ringing or overshoot, then they should be stable.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    TI__Mastermind 21769 points

    Thanks! I will follow up if my customer has any follow up questions. 

    John

  • 0 in reply to John Griffith
    TI__Expert 8120 points

    Sounds good! Let me know if anything else comes up.

    Best regards,

    Alec