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BQ2970: Low-current application

Part Number: BQ2970
Other Parts Discussed in Thread: CSD13381F4, BQ2980, BQ2982

I'm looking at this part for protecting a rechargeable lithium coin cell, with maximum charge and discharge current about 50mA. Sizing the FETs for this is difficult; the math says I want about 1.8 ohms for the current detection to work.

Would it make more sense to use relatively low Rds(on) FETs (e.g. CSD13381F4) with a fixed resistor in the path? Maybe put the resistor between the FETs (D to D) or between the S node of the DOUT FET, and the VSS pin of the BQ29707?

Thanks!

  • Hi Steve,

    You could add a sense resistor between VSS and the FETs. I think some other users have asked about this before in the forum, so you may be able to find older threads on this topic.

    Another option is to use the BQ2980 or BQ2982 family which use a sense resistor and high-side FETs. With those devices, you just need to make sure to select FETs with low leakage similar to what is used on the EVMs.

    Best regards,

    Matt

  • The issue with those parts is we need about 4.3-4.4V or less for OVP and 2.8-3.0V for UVP. The OVP of the BQ298012 is probably ok for protos; how would we get a custom part programmed? Lead time? Thanks!

  • Schematic review, please. Application requirements:

    * OVP 4.4V or lower (ideally 4.3V to 4.4V)

    * UVP 2.8V or higher (ideally 2.8V to 3.3V)

    * OCC and OCD to support about 50mA maximum for cell protection. Operating draw should be under 25mA.

    Thanks!

    Steve

  • Hi Steve,

    I think C530 should connect to BATT- (VSS), and R532 is typically 2.2k. See Figure 10-1 in the datasheet. Regarding a custom part number for the BQ2980 family, I asked our marketing person to reach out on your other thread regarding this question. He can get you information on required volumes that would be needed to create a new custom part and the lead times.

    Best regards,

    Matt

  • Thanks! Can we use 4.99k? We literally have no other 2.2k on the BOM and would rather not add another line. Thanks!

    Also from that other thread, what about using a small PTC in place of the 20 ohm Rbat?

  • Hi Steve,

    Unfortunately I cannot recommend 4.99k in place of the 2.2k without digging deeper into the architecture with the design team. I will dig into this along with your question about the small PTC on the BQ2980 and get back to you tomorrow.

    Matt

  • Hi Steve,

    The resistor really should be 2.2k. This resistor is specifically selected and tested to protect against a reverse charger connection. 

    I think the PTC on the BQ2980 BAT pin should be okay, but as the datasheet note in the pin table states, a larger value than 20 Ohms may affect the voltage detection accuracy, so it is best not to increase this resistor value too much.

    Regards,

    Matt