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CHG-FET is off when BQ2084 is requesting charge current

John Klaus
Prodigy 20 points
Other Parts Discussed in Thread: BQ29312, BQ3060

I have a three cell pack using the BQ2084 fuel gauge and BQ29312 AFE with no pre charge FET.  There seems to be a fundamental contradiction in the operation of CHG-FET.  Any time charge termination is indicated CHG-FET turns off.  But the data sheet also indicates that Fast Charge Terminate includes a followup maintenance charge.  Indeed the fuel gauge will ask for maintenance current from the level 2 charger while it has also commanded CHG-FET off resulting in no charging current flowing to the cells.  There is a CHGFET bit setting in misc. config. data flash that can keep CHG-FET on in charge termination.  When this is setup this way all charge current commanded by the fuel gauge (including 0mA requests) are properly delivered and charging appears normal. 

Why does BQ2084 turn off CHG-FET when it is also asking for current from the level 2 charger?  Why is the CHGFET option provided?  What is the downside to keeping the CHGFET on in charge termination?  Is turning off CHG-FET under charge termination indication the intended way to shut off the charging current instead of commanding the level 2 charger?  When a level 2 charger is in use should CHGFET always be set to keep CHG-FET on in charge termination?

  • 0
    TI__Genius 10225 points

    The CHGFET bit is there in case you have a dumb charger. The gauge can cut off charging at the end of termination. Without it, the dumb charger would just keep going. If a smart charger is used, there is really no need to turn off the CHG FET at termination since the gauge will be broadcasting requested current. The downside of keeping the FET on is overcharging the pack, but for LION applications you typically set maintenance current to 0 unless you are trying to offset some standby current from your system. LION cells have very low self discharge.

  • 0 in reply to Chase7071949597
    Prodigy 20 points

    Thanks for the quick response.  The explanation makes sense.  I could not find any design guidance like this in the data sheet.  Is there any other resource you could recommend for application information on the fuel gauges?

  • 0 in reply to John Klaus
    TI__Genius 10225 points

    Unfortunately, the datasheet is the best DF design guide we have for this older device. Newer devices, such as the bq20zxx devices or the bq3060 have a much more detailed technical reference and a useful help text that can be accessed from the EVSW.