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Original question:

BQ76952: Switching observed on DDSG pin with LOW side FET switching BMS design

BQ76952: BQ76952

Anandteerth Wadavi
Prodigy 105 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQSTUDIO

Hi TI Guru,

1. What is the rationale behind necessity of actual cell between Cell16 & Cell15 ? How is it related to the oscillations of DDSG and DCHG lines ?

2. What are the other necessary design guidelines to be followed to prevent the Oscillations of DDSG & DCHG lines other than an actual cell across cell16 & cell15 AND having increased the body diode threshold current for discharge MOSFETs to 100mA ?

Reason for the above question is that, in spite of adhering to above design guidelines, the DDSG & DCHG Lines are toggling. 

Thanks and Regards,

anandteerth

  • +1
    TI__Guru* 81034 points

    Hi Anandteerth,

    Connecting a cell between Cell16 and Cell15 is important, but it is not related to the issue you describe. The structure of the internal balancing FET circuits requires this. If VC16 was shorted to VC15, then the cell balancing switch in the top cell (between VC15 and VC14) could not be enabled, since that switch is an NFET and needs a higher voltage for its operation (it draws from BAT).  VC16 uses a PFET for its cell balancing switch, so it doesn’t need the higher voltage, but it is the only cell which uses a PFET cell balancing switch, all the rest use NFETs. In Section 8.2 of the datasheet, if you take a close look at the Functional Block Diagram, you will see this.

    For your issue, it would be helpful if you can collect a log file to see why the DDSG and DCHG signals are toggling. If you are able to connect to BQStudio, you can click the Start Log button on the registers screen to log this behavior. It would be good to check to see if protections are being triggered or if the body diode protection is turning on. It would also be good to see your register settings (click Export on the BQStudio Data Memory window).

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 105 points

    Thanks Matt for the detailed reply. It has helped me resolving the issue.