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BQ76920: some type of isolation needed?

D3jun58
Intellectual 370 points
Part Number: BQ76920


Hello,

below is attached simplified schematic how to use BQ76920:

Because I don't need to use CHG/DSG and current measure functionality, can I left pins CHG, DSG, SRP and SRN pins just floating or do you recommend to use external pull down resistor (for ex. 1 MOhm)?
Consequently this means that PACK- symbol on schematic above will be connected directly to GND (the same as GND of mCU, BQ76920 and GND of first battery cell).
Exactly - for measurements balancer connector of LiPo battery will be used, the load will be then connected to the main wires of the battery. But because the GND in balancer connector is on the same potencal as GND which is part of battery main connection, I'm worry that some type of return current path will establish also via GND line (and will affect with the measurements) which goes through balancer connector. Please note that mCU which will be in use for communication with BQ76920, will be also in use for controling the main load. Would you suggest to use some type of isolation here (I2C isolation) to disconnect GND which comes from balancer connector from GND which comes around via main wires? - in that example GND of first cell and of BQ76920 will come through the balancer connector, the other part of circuit will be isolated via I2C isolation.

Many thanks!

  • 0
    TI__Guru 54715 points

    Hi Dejan,

    SRP and SRN are inputs and should be connected to VSS of the IC.  Normally you can route these directly to VSS, if you have a long routing path and expect a large signal may be coupled onto the traces 100 ohm as shown in the diagram is ok.  DSG and CHG are outputs and may be left open, they will be pulled low by the driver when the part is on.  If you keep the part in shutdown and are concerned about signals induced on the pins a 1M pull down to VSS is ok.  Shorting the outputs directly to VSS is not usually recommended since it will draw a lot of current if software ever turned on the pins.  The pins can be safely shorted and software should never turn them on if you do choose to short them.

    Isolation may be required by your system specifications, or from desire to control currents as you describe.  In low voltage systems it is often avoided unless necessary since it adds cost and current consumption which is undesired in a battery powered system.  Check your regulatory requirements, but in general I would expect your GND connection may be low resistance. If the MCU is referenced to the system load the I2C lines may be modulated by the current in the return path.  When tolerable this is often allowed to reduce the voltage margin on the interface.  If it is large enough to induce communication errors then isolation may be desired.