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BQ77905: BQ77905 application consult for 5 cell protector

Jimmy Zhou95
Intellectual 870 points
Part Number: BQ77905
Other Parts Discussed in Thread: BQ7718, BQ77915, BQ78350

Hi,

I have one consult for BQ77905. Customer has used BQ7718 with only OV protection. Now, they want to implement OV/UV protection together and I have promoted BQ77905. However, here is some consult from Positec.                                                                        

a. Customer only want UV/OV indication signal without external MOS and do not need OCP, OT, UT and OW now. Can they only use DSG/CHG for indication?

b. BQ77905 should support hot plug, ex, when plugging first cell first or third cell first, BQ77905 is not broken.

c. OV indication signal(CHG) should be safely turned on when BQ77905 fail. As in UL2595 standard, battery should not be over-charged in case any of device fails.

 .

  • 0
    TI__Guru* 81035 points

    Hi Jimmy,

    The DSG/CHG pins can be used to indicate a fault, they will just need to translate the voltage levels to the appropriate levels for their controller. 

    The BQ77905 should be able to connect in different order when stacked without an issue.

    I'm not sure I understand the last question. The CHG signal will indicate a OV condition until the cell voltages fall below the OV voltage minus the hysterisis. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Intellectual 870 points

    Hi Matt,

    Still need your help for third question.

    For third questions, I have some details to address. In UL2595 standard, IC safety has been given out at 9.9 and 18.6.1. Overchaging will lead battery to explosion. Considering any BQ77905 failure, CHG should indicate a OV condition. Do you have some comments with it? Thanks! 

     Jimmy

  • 0 in reply to Jimmy Zhou95
    TI__Guru* 81035 points

    Hi Jimmy,

    The answer is the same as for the BQ77915 thread:

    The UL spec is a system level specification. The BQ77905 can detect and indicate OV. I am not familiar enough with the standard or the customer system to advise whether that is sufficient to meet the requirements.

    Some battery gauge devices (like the BQ78350) have an additional Overcharge Protection feature (in addition to OV), but this might not be required by the UL spec, I'm not sure.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Intellectual 870 points

    Hi Matt,

    To make things simple, cusotmer want CHG keep low at  cell voltage normal or cell overvoltage or , when BQ77905 encounter any failure. This is required for UL2595 std. 

    Jimmy

  • 0 in reply to Jimmy Zhou95
    TI__Guru* 81035 points

    Hi Jimmy, 

    For the BQ77905 and BQ77915, the CHG pin behavior will be exactly as described in the datasheet. For BQ77905, Table 5 in the datasheet gives a detailed set of conditions which will turn off CHG and the conditions for recovery. Since they are planning to send the CHG signal to the host MCU, the host can make the final decision of whether to turn the CHG FET on or off.

    Matt

  • 0 in reply to Matt Sunna
    TI__Intellectual 870 points

    Hi Matt,

    Another queston is whether CHGU can drive external circuit if they use only one BQ77905 for 5 cell monitoring. In our datasheet, we propose GHGU open when not stackable. Thanks!

    Jimmy

  • 0 in reply to Jimmy Zhou95
    TI__Guru* 81035 points

    Hi Jimmy,

    From the documentation, I think that could work. I recommended they test this on an EVM since this is not a standard recommended configuration. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Intellectual 870 points

    Here is one more request from Positec. Can we use CHGU to control external logic for under voltage indication when only for 5 cell monitoring? And how about the status of CHGU when CHG connects to VDD? In the same way, how about CHG status when CHGU connects to VDD? Thanks!

    Jimmy

  • 0 in reply to Jimmy Zhou95
    TI__Guru* 81035 points

    Hi Jimmy,

    The CHG and CHGU will only be turned off for over-voltage (not for under-voltage). If CHG is shorted to VDD, it might pull up all of the signals (CHGU and DSG) since the rail will be pulled high. I'm not sure this can work to satisfy the UL requirement if two independent mechanisms are needed.

    Matt