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bq76920 operation issues

OFER ZOHAR
Prodigy 10 points
Other Parts Discussed in Thread: BQ76920, BQ78350, BQ24650, CSD17483F4

Hello,

 

We would like to use bq76920 as a second battery protection circuit and need your assistance with the below issues:

 

  1. Is the bq76920 detects between charge state or discharge state Automatically? which register indicates that this time is to charge or this time to discharge? Where this info can be found in the data sheet?

 

  1. We would like to combine bq24650 with bq76920, each in different circuits – is it correct to connect between this two circuits only with two wires (pack – and pack +)?

     

  2. is it possible that the 76920 will work at the same time in charging and discharging simultaneously?

 

Thank you,

  • 0
    TI__Guru 54715 points

    The bq76920 does not detect charge or discharge directly.  It has a coulomb counter which a gauge (bq78350) or MCU can use to determine if the current is charge or discharge.  See the register map.

    Generally yes if the 2 circuits are built to be autonomous. The bq24650 has a function to detect the missing battery if the bq76920 is in a fault mode. If you need communication with the battery you may want high side switching on the bq76920 for battery protection .  If you need special switching for your circuit, consider all operating modes needed.

    Yes, if you have a charger and load connected simultaneously to the battery the battery (bq76920 and its gauge/MCU) will see the difference of current.  There can be interactions depending on the load behavior where the load may cause a battery fault, the charger can't provide the load current so the voltage collapses.  Once the load shuts down the voltage returns and may again charge the battery causing a recovery and a cycle. A circuit which controls when the battery can be charged and when the load is powered may be desired.

  • 0 in reply to WM5295
    Intellectual 280 points

    Thanks for your reply.

     

    Please advice:

     

    1. need your recommendation for how to implement the high side with the bq76920?

    2. What kind of mosfet is recomended to use? Is this kind of a connection can reduce the functionality?

    Thanks,

  • 0 in reply to or nsl
    TI__Guru 54715 points
    1. See page 43 zone D5 of www.ti.com/lit/sluu474 for an example of P-ch high side switching. You will need to select FETs appropriate for your load and a divider resistors for the voltage. If you need fast turnoff it is provided by Q5 and Q6 in that example.
    2. The example would be for P-channel FETs, size them for your system and load.
    3. The bq76920 will lose load detection function with the high side switch.
    4. Load detection in the bq76920 is from a comparator on the CHG pin which is pulled up by PACK- during a fault. If using high side switching so that the 'ground' reference is not lost in fault so that you can talk to the battery electronics, PACK- will stay near VSS/Battery-/'ground', so that mechanism won't work. Trying to bypass the charge FET driver to feed a signal into CHG would likely turn on the external charge FET driver, so some other mechanism is needed. A comparator on the PACK+ signal or measurement of the PACK+ voltage by the MCU may be appropriate.
  • 0 in reply to WM5295
    Intellectual 280 points

    Hello and Thank you for your detailed reply.

      

    1. is it correct to use mosfets for pchanel protection?

       

    2. Can you please recommend relevant mosfet?

       

    3. Is it possible to control the ts1 leg with the controller?

     

    Thank you!

     

  • 0 in reply to or nsl
    TI__Guru 54715 points

    1. The microcontroller would have control of the Q1 and Q2 FETs through the bq76920.  Q4 would prevent discharge of the battery if switched off by the microcontroller, but would not prevent charge.  To prevent charge without using Q2, another P-channel would be needed.

    2.  The topology looks correct to avoid discharge, however you may want to provide a voltage divider resistor in the drain of Q3 to limit the gate voltage.  Many MOSFETs will have a 20V Vgs limit, some lower.  Be sure to plan for transients, you may want a 50 to 100% margin on the Vds rating.  With 5 cells you might have a 21V battery and want a 30 or 40V FET for Q4. TI has 20V P-channel FETs if those are suitable, There are also a number of N-channel devices which may work for Q3, the FemtoFETs look interesting such as CSD17483F4 or others with low gate voltage limits might be good for use with the microcontroller.  The FET forum may be a better place for recommendations, https://e2e.ti.com/support/power_management/power_stage/  

    3. Yes, if you have a 3.3V MCU, you might feed the TS1 pin through a diode, the diode will block any current from TS1 when it samples temperature.  If you have a higher voltage MCU you might need some zener and/or voltage divider as well.  Your MCU should not use REGOUT power from the AFE if it is to boot the AFE.