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Dynamic regulation of charge current via ISET pin

David Dean
Expert 2305 points
Other Parts Discussed in Thread: BQ24095

Hello,

  I am looking at the BQ24095 Li-ion battery charger, but I think this question applies to many of the current-limited chargers available.

  I am charging a battery in a system where the input source sometimes may not be able to provide the current set by Rset.

  For example, my nominal input voltage is 5V, and I set my current limit to 500mA.  If my input power droops, Vin will drop.  If the drop continues, Vin will fall below the minimum voltage, and charging will be disrupted until Vin rises above the threshold.

  What I would rather happen is for the charge current to decrease, allowing Vin to stay at 5V.  I imagine a circuit something like this, using some feedback from Vin to drive ISET:

   

This would make sure the battery does not draw enough current to drag down VIN when it cannot provide the full 500mA.

Does this seem like a reasonable idea?  Am I asking for trouble, maybe by messing up the charging algorithm or risking some sort of oscillations?  Am I overlooking an existing TI part that can already do this?  I have seen app notes and forum posts showing that the charge current can be adjusted by switching in a resistor, so I have some hope that this can work.  However, I haven't seen anything addressing feedback or constant adjustment of ISET, so I don't know what to expect.

Thanks,

Dave

  • 0
    TI__Genius 13700 points

    Hello Dave,

    There is a feature on the BQ24095 that performs much this very same thing. The Dynamic Power Management (DPM) feature looks at the input voltage to the charger and in the event that the input voltage begins to collapse do to source limitations it will scale the charging current back to allow the input voltage to rise again.

    Are you needing something in addition to this integrated feature?

  • 0 in reply to Ryan May
    TI__Guru*** 148825 points

    If you need a feature like the integrated IN-DPM but at a higher voltage, your circuit should work.  You might consider an NFET instead of BJT due to the BJT higher saturation voltage.  Also, you might need a series R (~1k) and C (~50nF) from ISET to ground for stability as ISET is part of the current regulation control loop.