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BQ76940: NT1 Damage

Part Number: BQ76940

Hi Tier

    There is a NT1 damage when using BQ76940 EVM .

     What is the E1 device?

   

  • Hi Tony,
    E1 is a spark gap. It was placed on the EVM but is a poor idea in a product since contamination would bypass the FETs.
    The NT1 is a small trace to connect the signal ground (IC VSS) to the high current ground. I don't know its fusing current but have seen it damaged by high current from improper terminal connection. If R1 has been installed C0 can carry pack current through R1 and NT1 to the load, it is uncertain what is damaged first. Connection of a voltage between GND and BATT- possibly from unexpected test equipment grounds is another possibility. You may be able to connect across the fused NT1 and have the board work, but check for other damage such as SRP and SRN.
  • Thank you for your reply!

    The input filter circuit of the battery is also damaged. The recommended parameters for the EVM are 1k (resistance) and 1uF capacitance.

    If you change the resis tance of the 1k to 3.3k will improve?

  • Hi Tony,
    BATT- to VC0 should normally be 0V, as would the voltage across the NT1. If the filter components have been damaged it would be good to find the cause of the large voltage and prevent it.
    The maximum value recommended in the bq76940 data sheet for the Rc filter resistor is 1k. Larger values would likely work but may start to show some error during measurement of the cell. The filter capacitor value may vary, the filter should be able to settle before measurement starts on cell 1 during balancing. The 1uF 1k combination will show some voltage error on the lower cells, a smaller capacitor is suggested.
    The VC1 capacitor should be connected to GND rather than VC0 as shown in the data sheet figure 8-3. The connection shown above has unfortunately been shown in some schematics and does not seem to have caused problems in those circuits, but it is not recommended.