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Original question:

BQ25792: Charging stops as a result of input voltage change (Autonomous Charging Cycle).

BQ25798: VINDPM operation

Chuck Jenkins
Prodigy 70 points
Part Number: BQ25798

Tool/software:

Hello Jeff

WE have been working with the 798 now for a few years and still are surprised with the VINDPM operation.  This variable needs its own application note.  Recently we noticed that VINDOM will be set a few volts lower that Vbus and if Vbus drops below VINDPM various strange things happen. 

We were under an indicated fast charge (011) with a few hundred mA of Ibat current on the ADC when Vbus was above VINDPM.  Dropping below VINDPM on Vbus we noticed that the battery charge current dropped to zero yet the indicated fast charge status stayed at 011 and the status LED was still on.  I would think that if battery current were reduced to zero we would indicate a no charge condition.  No fault was set (that I noticed).

Another strange VINDPM behavior was experimenting with MPPT.  I would have thought that IINDPM would be adjusted to set the input current limit keeping Vbus (solar volts) at the Mpp voltage but instead I saw what appeared to be control of VINDPM to regulate (adjust) Vbus to MPP.

Is there a better description of VINDPM operation that what is in the data sheet?

  • 0
    TI__Guru**** 154136 points

    Hi Chuck,

    VINDPM has been in our chargers for over 10 years.  The functionality hasn't really changed since then. VINDPM threshold is autoset at power up to a fixed voltage below the open circuit voltage before switching starts.  The VINDPM analog feedback loop clamps the converter duty cycle when the input voltage droops down to the VINDPM threshold so that the charger does not pull more current and crash the input source.  The loop essentially regulates the input voltage to the VINDPM threshold.  There is a fault flag and status bits that report VINDPM in REG0x22 and REG0x1B respectively.  The charger state machine is still in fast charge, waiting for the event (high SYS load?) to be removed so the VINDPM fault flag and status tells you that the charger is not giving max charge current.

    The simplest MPP algorithm is fractional open circuit voltage regulation, which uses the VINDPM loop to regulate input voltage.   After periodically stopping switching to get the open circuit voltage, the charge set the VINDPM threshold to a fixed % of that open circuit voltage.   

    There may be some documents on ti.com from a few years back.

    Regards,

    Jeff

  • 0
    Prodigy 70 points

    We are emulating our solar input to Vbus with a current limited power supply.  Dropping below the VINDPM must not allow the analog servo to track and we get 0 charge current.  This is with MPPT disabled.  If enabled will the 2 minute (or whatever is set) refresh interval restore a more current value of VINDPM to keep the charge process working?  Prior to looking at MPPT we were periodically setting VINDPM to the min value of 3.6V to keep it from impeding the solar panel output during cloud or other panel obstructions.

    I had not noticed the fault flag when Vbus dropped below VINDPM. We will investigate using that event to set a new value of VINDPM should MPPT not pan out.

  • 0 in reply to Chuck Jenkins
    TI__Guru**** 154136 points

    Hi Chuck,

    If you apply a voltage < VINDPM threshold, the charger essentially turns off.  

    The EN_MPPT bit resets to 0 if V(BAT) < MINSYS voltage.  If EN_MPPT is enabled, the charger periodically resets VINDPM = fixed % x measured open circuit voltage. If the panel is too weak to provide > VIN_UVLO = 3.6V and the poor source current of up to 20mA, the charger goes into HiZ mode for ~10min and then tries again.  Even if EN_MPPT bit = 0, the charger always performs its normal VINDPM routine which is to set VINDPM to a fixed voltage below measured input voltage (e.g. 0.7V for low voltages). 

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 70 points

    That confirms our observations.  I was hoping with this part to have a VINDPM disable. Instead we frequently set VINDPM to 3.6V.  There appears to be a slope requirement on the VINDPM sampling. A slowly rising Vbus will set a low VINDPM where a fast rise to a high voltage sets VINDPM to a few volts below Vbus.  For solar applications we see both slow (clouds) and fast (moving foliage) transitions.  

    Using MPPT to do a VINDPM correction at its programmed interval may remove the requirement to continuously send a low voltage to VINDPM.  

  • 0 in reply to Chuck Jenkins
    TI__Guru**** 154136 points

    Hi Chuck,

    You are not the first customer to want to be able to disable auto VINDPM.  MPPT should help with that.

    Regards,

    Jeff