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BQ25895: nQON as Battery Switch

Andrew Baek
Intellectual 915 points
Other Parts Discussed in Thread: BQ25895

We are developing a portable charger solution with BQ25895.

In an effort to save the battery consumption, would like to see if we can disconnect the battery completely from BQ25895 when not charging or discharging.

One option is to use a load switch with VBUS detection.  However, it looks like nQON may serve the purpose of a load switch.

Another question is we have a wireless charging solution attached to PMID.  We would like to detect when a user places his device on the top of the wireless charger so our module can start charging.  Would it be possible to detect such an event?  I think there may be several things that need to be considered/explained but just wanted to open up for discussion.

Thanks,

Andrew

  • 0
    TI__Guru**** 169466 points
    You can perform an I2C write to REG09 b5 to disable the internal BATFET between BAT and SYS. The /QON pin can then be used to turn the BATFET back on.
    Regarding the wireless charging solution, are you planning on connecting a wireless transmitter to PMID? Do you want the bq25895 to not enter OTG boost mode until the device is placed on top? If so, the detection would have to be performed by the transmitter (or some other circuit). We typically recommend an external diode from SW to PMID for maximum efficiency in OTG boost mode. This means that PMID will always be ~0.3V below the battery voltage. Will that be enough voltage for the transmitter to perform the detection and pull the OTG pin high (thereby enabling OTG mode)?
  • 0 in reply to Jeff F
    Intellectual 915 points
    For clarification, would I still be able to do an I2C write when the BATFET is off? The battery is still connected to BAT so I think it may be possible. Then BQ25895 would still be on or half-on, consuming power. I plan to have a low-power microcontroller (PIC), which can stay in sleep mode until the interrupt button is pressed. It can also control a load switch to completely disengage the battery. However, it adds to the cost and size.

    In terms of wireless charging, I would need 5V. So PMID staying 0.3V below the battery voltage is not an option. The current would be 1A. I am including 4 x 10 uF on the PMID output to support 5V at 1A. Would it be sufficient or should I add more caps?

    To permanently enable OTG, I followed a recommendation somewhere in this forum that OTG can be connected to REGN thru 10k resistor.

    Lastly either /QON or load switch, it needs to be controlled by both VBUS and micro-controller. The VBUS voltage level can be shifted with resistors not to overload the PIC's pin. The question is whether I can tie the shifted VBUS and the PIC output (3.3V) together on either /QON or load switch control pin. I think it should be ok because they can share a common ground but I am not completely sure.

    Thanks again,

    Andrew
  • 0 in reply to Jeff F
    Intellectual 915 points
    Jeff,

    I can send you a copy of the schematic if that helps to clarify the questions.
    Thanks,

    Andrew