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BQ2982: About UVP function

teritama
Expert 1690 points
Part Number: BQ2982
Other Parts Discussed in Thread: BQ21040, BQ25170

Hi team,

Our customer designed below battery management system using bq298216, CSD87501 and bq21040.

And they are evaluating this system in low battery voltage(VBAT) condition. BQ298216 has UVP function and UVP value is 2.5V. But when they started charge with Charge Voltage is 5V and VBAT is 2.1V condition, battery was charged with fast-charge current. They checked the charge current was constant fast-charge current(around 90mA) by both side voltage of Rsns.

Our understanding is that when VBAT is below 2.5V, UVP is enabled and battery is not charged and discharged.

Is this understanding correct? If you have any possible causes this behaver, please let me tell.

below figure is simply schematic, and next is waveform of VBAT and VOUT when supplied charging voltage 5V.

Best regards,

teritama

  • 0
    TI__Expert 4465 points

    Hi Teritama, 

    That's correct the BQ298216 has an UVP of 2.5V, so if the battery voltage at the BAT pin is below 2.5V for longer than UV delay the DSG FET will open. The BQ298216 also has the UV_SHUT enable, which mean the device will enter shutdown mode till a charger is attached. Once the charger is attached, the CHG FET can be turned on and allow the battery to charge till the UV condition is not longer valid. On this scenario, as you are attaching the charger the PACK pin, the protector understand the charger is connected and should allow the battery to charge in order to exit the UV condition. 

    For more information you can refer to Section 8.3.2 Undervoltage (UV) Status of the D/S https://www.ti.com/lit/ds/symlink/bq2982.pdf

    Regards, 

    Arelis G. Guerrero 

  • 0 in reply to Arelis Guerrero
    Expert 1690 points

    Hi Arelis,

    Thank you for reply.

    You mean when VBAT is below 2.5V, the FET become open is DSG FET only and CHG FET is keep ON, right?

    Additionally, when only DSG FET is OFF, VOUT is larger than VBAT by body diode of DSG FET(around 0.7V). Therefore in waveform Vout is larger than VBAT when VBAT is smaller than 2.5V~2.7V(VUVP).
    This understanding correct?

    If we want pre-charge when VBAT is below 2.5V, should we choice the charger device that pre-charge to fast-charge exchange threshold is over 3.2V instead  BQ21040?
    If you know other way, let me tell.

    Best regards,

    teritama

  • 0 in reply to teritama
    TI__Expert 4465 points

    Hi Teritama, 

    • Yes, during an UVP event the DSG FET is the one that will be turned off.
    • Yes, while charger is attached (VOUT - VBAT) > 700mV. 
    • For the pre-charge, the charger needs to be able to pre-charge at your desire threshold, and the protector UVP should be at least UVP at the pre-charge you want your cell to be connected.
    • For the reason of why is fast-charging instead of pre-charge, is the device using a real cell on the BAT or is a simulator with a power supply? In case of a simulator, when the charger starts putting current in order to charge the cell, the bypass capacitor might get charged super quick and transition to full charge prematurely. I would recommend testing with a real cell attached or using a big capacitance alongside the battery simulator to test the pre-charge current of the battery charger. 

    Regards, 

    Arelis Guerrero 

  • 0 in reply to Arelis Guerrero
    Expert 1690 points

    Hi Arelis,

    Thank you for reply.

    I understand. I would proposal to the customer to use BQ25170/J.

    >For the reason of why is fast-charging instead of pre-charge, is the device using a real cell on the BAT or is a simulator with a power supply?

    　⇒They evaluated by real board using 7.5F electric double-layer capacitor. 

    Best regards,

    teritama

  • +1 in reply to teritama
    TI__Expert 4465 points

    Hi Teritama, 

    The BQ2517x is a good charger alternative and is a newer device on the portfolio as well, the 7.5F cap should be good enough to simulate a 1s battery.

    Glad to support, feel free to post another question if you need further help. 

    Regards, 

    Arelis G. Guerrero