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Original question:

BQ25895: Difference between BQ25890 vs BQ25895?

BQ25895: BQ25895 with all load off PMID, none off SYS?

Steve Fenwick1
Prodigy 250 points
Part Number: BQ25895

Is this a supported operating mode, all of the internal loads off PMID, none off SYS? Target load is up to 1.75A, 5V.

  • 0
    TI__Mastermind 31100 points

    Hi Steve, 

    Connecting the internal load of your design at PMID instead of SYS is supported.

    During charging when 5V input adapter is present it will directly power load at PMID while also charging your battery through the buck converter. Then when adapter is removed the device will transition to OTG boost mode where boost output voltage at PMID is default programmed to 5.126V. Your target load is not an issue as the max boost current supported is 3.1A. 

    Please be aware the transition from PMID being directly powered from VBUS to being powered by the battery in boost mode is not instantaneous. There is a ~30ms delay for PMID source to switch from VBUS to battery. During this time PMID voltage will drop below 5V. 

    Best Regards,

    Garrett 

  • 0
    Prodigy 250 points

    Thanks for the prompt response.

    Since the charger can accept voltages up to 14V on VBUS, I presume the inductor is used as part of a buck converter to drop the voltage at PMID to the target set in the registers? And then there is an LDO function to charge the battery?

    Best,

    Steve

  • +1 in reply to Steve Fenwick1
    TI__Mastermind 31100 points

    Hi Steve, 

    Part of your understanding is correct and part is not. Please see my comments below to clarify. 

    Firstly, I recommend you refer to the functional block diagram on page 14 of the datasheet to help with your understanding of the charger architecture. The PMID pin is on the input side of the converter (switching FETs are Q2 and Q3). Therefore when VBUS input supply = 12V PMID voltage will also be approx. 12V. PMID voltage will only be the target set in registers during boost mode operation. 

    In terms of charging your understanding is correct. Q4 BATFET can function as an LDO to charge the battery while SYS pin is being regulated above SYS_MIN threshold via buck converter. Then when VBAT is above SYS_MIN Q4 will be fully on and duty cycle of the buck converter controls charge current to the battery.