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BQ25798: Schematic Review and ShutOff Question

Don Pitchford
Intellectual 255 points
Part Number: BQ25798
Other Parts Discussed in Thread: TPS25750, , CSD87501L, TPS25751

Tool/software:

Hi,

I am trying to design a system that is powered by either USB-C input or aux barrel connector.  I am needing maximum power (60 watts 20V @ 3A).    The attached schematic uses TPS25750 for PD to BQ25798 to provide battery charging (4 cell Li-Ion 18650).   

Questions:
1.  What is the best way of shutting off the SYS_POWER (system power to the system), but allow for the battery charging via the USB / AUX power source?     

2.  I didn't tie the EXT_POWER (barrel connector) to the VBUS.  I was thinking I could just add a wire from TP2 to TP10 and the CSD87501L would prevent reverse power to VBUS from the EXT_POWER.  Or do I need to add a ORing chip? 

3.  Does this system look like it would negotiate USB-C to provide the 20V @ 3A?.  I am trying not to write firmware and was hoping to achieve this hardware. 


If you see anything else, I would appreciate feedback.     This USB-C is powerful, but, a lot of moving parts.  

5807.output.pdf

  • 0
    TI__Guru**** 168436 points

    HI Don,

    BQ25798 hits thermal regulation before it can use all of the 60W input to power system load and charge the battery. The system load is prioritized. 

    Regarding 1, there is no way to disable SYS power and still provide charging since the system load is in parallel with internal BATFET to the battery.  I recommend adding an external FET in series with system load.

    Regarding 2, unless your equipment physically prevents both input sources from being connected at the same time, you will need the MUX FETs to MUX the input sources or a diode OR circuit.  Otherwise the 12V  could back feed the USB-C port and vice versa. On the schematic, assuming the MUX FETs or diode OR is not needed, EXT_POWER must connect to VBUS in addition to VAC1.  VACx are only high impedance sense lines.  Test point traces are generally only 6-10mil so not suited for high currents.

    Regarding 3, the TPS25751 + BQ25798 firmware allows for 20V/3A negotiation.

    Regards,

    Jeff