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BQ25798: CC/CV Threshold

Jeff LaBundy
Intellectual 300 points
Part Number: BQ25798
Other Parts Discussed in Thread: BQ40Z50

Tool/software:

Hi there—our design sets VREG = 4.0 V and ICHG = 5.0 A. According to our measurement, the BQ25798 moves from constant-current (CC) mode to constant-voltage (CV) mode once the battery voltage reaches approximately 3.7 V:

For my own understanding, what criteria does the BQ25798 use to judge when to move from CC mode to CV mode? The datasheet explicitly defines the criteria for moving from pre-charge mode to CC mode, but I could not seem to find any information regarding the transition from CC mode to CV mode—please let me know in case I have missed it.

The purpose of this question is simply to check whether our measurement matches the expected behavior. In case I can clarify our application, please let me know.

  • 0
    TI__Intellectual 995 points

    Hi Jeff,

    The threshold to switch from CC to CV mode is when VBAT reaches the regulation voltage (which is set in the register). Where are you measuring the battery voltage? The part regulates the voltage at BATP. If BATP is at 4V, then the 0.3V drop is from the impedance from BATP to the battery. 

    Best Regards,

    Jibin

  • 0 in reply to Jibin Biju
    Intellectual 300 points

    Hi Jibin—thank you for your prompt support; this answers my question in full.

    The voltage and current shown in my previous message are reported by a BQ40Z50, which resides in the battery pack. The difference in voltage is therefore caused by the series impedance of the charge and discharge FETs in the battery pack, along with interconnect between the BQ25798 and the pack.

    Thanks again for your help, and I'll reach out in case I have any additional questions.