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BQ78350-R1: BQ78350-R1 driving BQ769200 and BQ24725: ground connection with low-side FET

Nicolas Bouquet
Intellectual 435 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ769200, , BQ24725, BQSTUDIO, BQ78350, BQ76920, BQ76940, BQ76200, BQ76930

Hello,

I have successfully designed a BMS+charger for a 4S Li-ion pack :

- PCB1 : AFE (BQ769200) + SMBS companion chip (BQ78350-R1) + CHG & DCHG low-side protection FET.

- PCB2 : charger (BQ24725), configured by a tiny CPU

Everything is working fine, I can configure and monitor the whole system, the charger is also working great with a 95-97% efficiency :)

I realized that the BQ78350-R1 can broadcast ChargeVoltage and ChargeCurrent data to the smart charger. This is a good news, the tiny CPU used to send 2 frames on the SMBus becomes useless, and the charging becomes more smart (configurable preconditioning eventually temperature-dependent, ...).

For test purposes, I have connected theses 2 components and the charging current/voltage becomes configurable from BQstudio, great! To do that, I had to make all GND shared so I shorted the BAT- and PACK- power signal, short-circuiting the FET protectors.

What is the safe way to do that?

- In the first phase of this project, the battery was protected by CHG/DCHG FET from faults (chargers or load faults).

- If the charger is drived by the BQ78350, what about ground connections ? The charger is also a power-path selector. Should I connect the charger after the complete {battery+BQ769200+BQ78350-R1+FET} and links the SMBus signals through a I2C isolator ?

Any advice or suggestion appreciated :)

Thank you!

  • 0
    TI__Guru* 77343 points

    Hi Nicolas,

    This is one of the challenges with low-side FETs. When the FETs are open, you can lose communication between pack electronics and the outside world. An I2C isolator is one solution to the problem. Another option is to use a high-side FET driver. 

    Here is the link to a TI Reference Design using the BQ78350-R1 with the BQ76940 (15 cell version of the BQ76920) along with the BQ76200 high-side FET driver:  http://www.ti.com/tool/TIDA-00792

    Here is the link to another TI Reference design that uses an I2C isolator:  http://www.ti.com/tool/TIDA-01093  The isolator is used in this case to communicate with a 2nd BQ76930 device to create a 20s battery.

    I hope this helps.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 435 points

    I have another question concerning the top FET solution. Looking closely to the schematic of the app note you mentioned, the system GND is taken at BAT- (C0), not PACK-, using NT1 on the schematics. 

    So all signals are referenced to BAT- instead of PACK-, including the SMB signals that may be usefull for host or charger connection. The difference between BAT- and PACK- is very low (Ibat*Rsense), but why do not connect the GND to the other side of the sense resistor?

    Thank you

  • 0 in reply to Nicolas Bouquet
    TI__Guru* 77343 points

    Hi Nicolas,

    The voltage difference between BAT- and PACK- is low, but SRP needs to be connected to GND because it has a min/max of -/+10mV, while SRN has a min/max of -/+200mV.

    Best regards,

    Matt