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BQ27320: Problem Board

G Cant
Intellectual 315 points
Part Number: BQ27320
Other Parts Discussed in Thread: BQ25180,

Hello, I designed a custom board using the BQ27320 and a BQ25180 as BMS. I am monitoring some strange behavior in my board and can not find where the problem might be. 

A USB is used as charger, which goes to the BMS that charges the battery. There is a Reverse Voltage circuit with some mosfets just after the battery and then VBAT (after-mosfet) goes to the Gauge and to the BMS. PACK - is also protected by a MOSFET and then goes to the sense resistor after which my main GND for the full Device is generated. Attached you will find schematics for everything.

I am having strange issues when charging the Device. When I plug the Battery into the Evaluation Module for BQ27320 and connect the LOAD output to my Power Input (PACK+, PACK-) everything seems to work. I am able to charge the battery, monitor the current and use my Device perfectly.

But when I disconnect the EV Module and plug the battery into my Device, the battery won't charge. I used some wires attached to my I2C line and ground so I can monitor the behavior of the Gauge from BQ Studio. I am able to calibrate the Gauge from there and it measures current consumption and voltage perfectly. But when I attach the USB, the charging is not detected and the battery is not charging.

I think that the BMS and the reverse voltage protection are not the problem because I am able to charge the Battery when I use it out of the Device with the Ev Module. 

Battery is not charging when I don't use the BQ Studio in case you were thinking that maybe the I2C is causing some problems.

Can you think of what mistake I might have made? If you need more info/ schematics /pictures please let me know.

Thank you.

  • 0
    Intellectual 315 points

    Okay, I bypassed the Reverse Voltage Circuit (just PACK - line) and the full system works now. I connected the PACK - signal from the sense resistor directly to the PACK- connector.

    When I used the EV Board, the Gauge was placed between the Battery and the Input of my system. When I used my own board the Gauge is placed after the Reverse Voltage Circuit.

    I really wanted to use the Reverse Circuit in case someone connects the Battery Backwards so it does not damage the rest of the system.

    Do you know how I should properly do this?

    Thank you very much for your help.

  • 0 in reply to G Cant
    TI__Guru 51366 points

    The gauge will work as long as its powered up (i.e. the voltage REGIN-VSS is within the recommended operating conditions). I don't see why your reverse circuit would prevent this as long as Q303 is on.