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BQ77904: Load Detection Issue

Yasin Kocaoglu
Prodigy 20 points
Part Number: BQ77904

Hi,

I am currrently working on a 6 series battery management system  using 2 BQ77904 in series. Each device has 3 cells and LD pins are connected to the GND with 470k Ohm Resistor and Diode. When the battery has overcurrent fault, it is not enough to switch off the load to remove overcurrent fault. Battery doesn't work properly until I remove the load cable. 

Please give me an advice. Thanks.

  • 0
    TI__Guru 54715 points
    Hi Yasin,
    See figure 24 of the bq77904 data sheet, although this shows current flow for UV it is helpful. When the load causes OCD, PACK- will be pulled up by the load. When an electronic load is turned off it will still provide a load as it tries to measure the voltage, it may be around 1 mA. When connected to the circuit it may show a very low voltage since the load pulls up PACK-. The RLD_INT is typically 250 k ohms, and the VLDT is typically 1.3V. So more than 5.2 uA of load will prevent the load removal detection.
    Check your load's specifications to see if it has a lower current setting, but typically you will need to disconnect the cable.
  • 0 in reply to WM5295
    Prodigy 20 points
    Hi WM5295. Thanks for your answer.
    I need to remove OCD fault when system draws 230uA. Is there any solution for that?
  • 0 in reply to Yasin Kocaoglu
    TI__Guru 54715 points
    Hi Yasin,
    The bq77904 does not have an adjustable threshold for LD comparator (VLDT). You could adjust the external resistor value to LD, this will affect UV recovery as well as current recovery. You could make your own threshold with a comparator circuit, hold LD above the threshold and when the voltage is within some limit your pull LD low. Your circuitry could measure current and similarly pull LD low when recovery is suitable. Pick a method which works well for your system.