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Original question:

BQ25886: Short Circuit on The Charger Circuit

BQ25886: 2-Serial Cell Charging Through USB

AE104
Expert 1090 points
Part Number: BQ25886

Hello,

I am designing a portable system with two 3.7V Li-poly rechargeable battery powered. The plan is simple that the batteries will be charged (BQ25886) through USB or 5V adapter, and there should be protection (S-8252) to the batteries for overcharging and overdischarging. I have attached the design. I have two questions: First, the n-mosfet (for power controls -at the right bottom in the figure-) S1 goes to the negative side of another cell which is eventually going to ground, and S2 goes also ground (designed the protection circuit with referencing S-8252 datasheet). So, how do these mosfets control charging and discharging? Second question, is there any missing connection on the design especially VDD at the protection to the SYS pin of the BQ25886? Could you share your feedback? 

Thank you,

  • 0
    TI__Genius 10385 points

    Alpaslan,

    I cannot offer support on the S-8252, but it seems Table 11 of their datasheet offers some functionality of the FETs. Connecting VSYS to VDD seems unnecessarily redundant. Is there a reason you made this connection?

    Attached for your reference is a schematic and layout guide

    Thanks,

    Ricardo

    0207.BQ2588x_SchematicChecklist_AddPCBcomment (1).pdf

  • 0 in reply to Ricardo
    Expert 1090 points

    Thank you Ricardo for your comments!

    You are right. I removed the VSYS to VDD connections. For FETs, in the MOSFET datasheet (DMG6968UDM), the gate threshold voltage is 0.5-0.9V, then the gate-source breakdown voltage +/-12V, so based on the table 11, the MOSFET is fit, isn't it?

    Actually, I asked a different question about BQ25886 in this forum previously. One of the expert recommended to place a overdischarge protection circuit as an addition to the BQ25886. Thus, I placed this one. Do you think it works with this design?

  • 0 in reply to AE104
    TI__Genius 10385 points

    Alpaslan,

    I cannot comment on the MOSFETs associated S-8252. I do think that this protection is a good fit for the application with BQ25886.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Expert 1090 points

    Hello Ricardo,

    Do you have recommondation for the overcharge, overdischarge, and overcurrent protection IC rather than S-8252?

    Also I have a question about the evaluation board design of the BQ25886. There is a 0.01Ohm resistor at the VBUS line. What is the function of this resistor? Should I use like it? Note: I have attached related part of the evaluation board design.

  • 0 in reply to AE104
    TI__Genius 10385 points

    Alpaslan,

    In the future please create a new thread when asking a new question.

    For a recommendation on a protector IC please create a thread in the appropriate forum.

    The purpose of both of these sense resistors is to provide a way to measure the current without interfering with the performance of the EVM. The sense resistors are not necessary for operation.

    Thanks,

    Ricardo