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BQ24650: Additional capacitance on MPPSET signal: good or bad?

Derek
Mastermind 9792 points
Part Number: BQ24650

Hello,

   I inherited a design that uses the bq24650 to charge a 13.8V LiPO4 battery. Working fine for the most part, but in the existing schematic I noticed they added a 100nF cap between MPPSET and GND. Is this good to have or should we remove it? What would be its effect? Input source is either a big wall-wart or a solar panel. Vin (main rail in our system) has a ton of capacitance, like 800uF.

The rest of the MPPSET pin:

Rtop (to Vin) = 619k

Rbot (to GND) = 40.2k

There's also an 18pF cap from MPPSET to VIN, which is correct (on the eval board).

Thanks,

Derek

  • +1
    TI__Mastermind 40325 points

    Dear Derek,

    The resistors confirm a MPPSET voltage of 19.67 V. Wanted to confirm this with you?

    Having the extra capacitance on MPPSET is up to you. It will influence how quickly the MPPSET pin charges/discharges its voltage by the according RC time constants of the resistors and capacitors. In other words, do you want the MPPSET pin to respond quickly or slowly to changes in the input voltage?

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answered your question.

  • 0
    Mastermind 9792 points

    Thanks - yes the MPPSET voltage matches. Regarding how fast the MPPSET pin changes: when the unit is powered from wall wart, it will never change the voltage so it doesn't really matter. But when it's powered by a solar panel, I'm guessing the voltage doesn't need to change very quickly? Clouds can only move so fast.

  • 0 in reply to Derek
    TI__Mastermind 40325 points

    How fast you want the MPPSET pin to respond to solar input changes is completely a design decision. Please decide appropriately for your application.

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answered your question.