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Original question:

BQ77915: Design Not Working

BQ77915: output abnormal with load connected

laisan zhang
Intellectual 315 points
Part Number: BQ77915

Dear,

our customer designed with BQ77915, 4 cell, schematic as follows, please help to review if anythig wrong here:

now we have two problems need help to solve:

1. 4 cell with 3.7V each, total is about 15V, but the output is about 10V without load connect,

2. output drop to about 0.8V if load connected

please help to advise, thanks.

  • 0
    TI__Expert 7465 points

    Hi Laisan,

    I have reviewed your schematic and have some suggestions:

    1. Since you are not using external cell balacing FETs, the Rin values should be 33 Ohm, not 1k.
    2. CCFG should be pulled high for the 4S configuration, it seems that you have it connected to AVDD and ground. Double check the value on this pin.
    3. R47 needs to be removed. VC5 should not be connected to VC3 for this configuration.
    4. Could you explain why you have added extra caps and resistors on the SRP and SRN pins? I do not believe they are needed.
    5. The CBI pin needs a 10k Ohm pull down resistor.
    6. Rts_pu (R50 in your schematic) should be 10k, not 100k.

  • 0 in reply to Shawn Hinkle
    Intellectual 315 points

    Dear,

    customer check item 6 and changed the R50 to 10K, and it works.

    please help to explain why?

    and customer will modify the rest of your advice and to check again,

    thanks.

    b/r

    vincent

  • 0 in reply to laisan zhang
    TI__Expert 7465 points

    Hi Laisan,

    I'm glad I could help fix your problem!

    The Rts_pu is a pull up resistor that is used to determine the temperature protection threshold. This behavior is described in section 10.1.1.5 of the datasheet. Changing this resistor value will impact other fault functions such as OTD, UTC, and UTD. I cannot say for sure what happened, but it is possible the device was in some sort of false fault condition.