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BQ76920: Circuit Design with and without BQ78350

Niklas Baumgartner
Prodigy 50 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ78350

Tool/software:

Hello,

I have a few questions regarding a BMS design in which we use a BQ76920 AFE. It shall also be used in conjunction with the BQ78350 Controller in one design and without it in another.

Furhtermore, we implement galvanic isolation between the battery side with both ICs and the MCU controlling side. The digital isolator chip that will be used for this purpose will be powered as soon as the BQ76920 is booted and provides power via its REGOUT pin.

1) For the BOOT, I think I can only use the PACK boot-up circuit that is described in Figure 4 of  "SLUA769", as I cannot boot the AFE from my MCU, since before the BOOT there is no supply voltage for the digital isolator coming from REGOUT!? Is there another way to solve this; do you have another boot-up circuit that works as soon as I connect the battery to the AFE?

2) Another problem is now that for the interaction with the BQ78350, the BQ76920-version with the 2.5V REGOUT must be used (as stated in the BQ78350 manual) and the digital isolator needs a supply voltage of at least 3 V. For the 3.3V BQ76920-version this is not a problem but for the other one it is. Is there a way to also use the 3.3V BQ76920-version in conjunction with the BQ78350?

Otherwise I would have to drain the cells all the time and use an additional voltage regulator to provide the supply for the digital isolator.

3) Can I connect the REGSRC pin directly to the BAT+ (highest cell in the stack) connection without it first going through the power input filter (100 Ohms + 10uF) as it is done in the simplified schematic in "SLUSBK2I"?

Because last time I build it with REGSRC being connected AFTER the power filter, I noticed a wrong battery voltage measurement when I drew a few mA through the REGOUT output because of the low filter resistance!

Does this have any negative effects in case of a battery transient if I now connect REGSRC before the filter?

4) We are going to use an external balancing/filter circuitry that provides around 130mA at 4.2V cell voltage. This is of course only when the internal balancing short circuits VCn and VCn+1. In "SLUSBK2I" it is stated that the internal balancing current has an Rds_on of minimum 1 Ohm or typical 5 Ohm which would result in a huge balancing current? Also the maximum balancing current is limited to 50mA per Cell as stated in the same document? Are these values correct and does this internal balancing always happen parallel to our external balancing?

Thank you very much in advance for your help!

Best regards,

Niklas

  • 0
    TI__Mastermind 22290 points

    Hi Niklas,

    Niklas Baumgartner said:
    For the BOOT, I think I can only use the PACK boot-up circuit that is described in Figure 4 of  "SLUA769", as I cannot boot the AFE from my MCU, since before the BOOT there is no supply voltage for the digital isolator coming from REGOUT!? Is there another way to solve this; do you have another boot-up circuit that works as soon as I connect the battery to the AFE?

    Unfortunately, no. Without an external regulator to power the MCU, the PACK based circuits are what we have available.

    Niklas Baumgartner said:
    Another problem is now that for the interaction with the BQ78350, the BQ76920-version with the 2.5V REGOUT must be used (as stated in the BQ78350 manual) and the digital isolator needs a supply voltage of at least 3 V. For the 3.3V BQ76920-version this is not a problem but for the other one it is. Is there a way to also use the 3.3V BQ76920-version in conjunction with the BQ78350?

    Unfortunately, no. The abs max supply voltage of the gauge is 2.75V, so you would need a separate LDO to drive the isolator.

    Niklas Baumgartner said:
    Can I connect the REGSRC pin directly to the BAT+ (highest cell in the stack) connection without it first going through the power input filter (100 Ohms + 10uF) as it is done in the simplified schematic in "SLUSBK2I"?

    You can certainly try this, although a battery transient would be more likely to affect REGSRC without any filtering on the pin.

    Niklas Baumgartner said:
    We are going to use an external balancing/filter circuitry that provides around 130mA at 4.2V cell voltage. This is of course only when the internal balancing short circuits VCn and VCn+1. In "SLUSBK2I" it is stated that the internal balancing current has an Rds_on of minimum 1 Ohm or typical 5 Ohm which would result in a huge balancing current? Also the maximum balancing current is limited to 50mA per Cell as stated in the same document? Are these values correct and does this internal balancing always happen parallel to our external balancing?

    The maximum internal balancing current is limited to 50mA. I recommend reading Section 2 of this document (https://www.ti.com/lit/an/sluaa81a/sluaa81a.pdf) to get an understanding of the concept of internal and external cell balancing, and how to properly set the resistors to avoid exceeding the internal limits. The document is for a newer generation of the BQ76920, but the methodology is the same.

    Regards,

    Max Verboncoeur