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BQ25175: Seems to be stuck in pre-charge mode?

Kevin Parker
Prodigy 20 points
Part Number: BQ25175

Hello,

I'm testing a custom board with BQ25175YBGR on it and a 4.35 li-ion cell with a capacity of 545mAh. The PCB has Riset at 750 Ohms, which would indicate a charge rate of 400 mA. Measuring with a multi meter, the battery voltage is 3.4v, yet only approximately 100 mA flows into the battery when measuring. 

Looking at the DS I would assume pre-charge is ended when the voltage reaches above Vbat_low but this never happens, at least for 10 minutes, when I decided to stop testing.

Any advice?

  • 0
    TI__Guru 67666 points

    Hi Kevin

    Current meter between battery and BQ25175 has caused problems before, try measuring current at input.  Also output current can be monitored at ISET.

  • 0 in reply to Bill Johns
    Prodigy 20 points

    Hi Bill,

    Thanks for your reply.

    Interestingly enough, if I push the bench supply to 6v, then I do see that approximately 1.386 W are being consumed, so 1.386 / 3.918 (at the battery) = 354 mA... About 12% shy of 400 mA.

    Is it normal behaviour for the delivered power to the battery to drop with voltage?

    I was expecting this part to be fine with 5v as will be supplied by another TI part BQ51003YFPR... 

    I'm only seeing 0.810 W on the bench set at 5v @ 5A, so 0.810 / 3.895 = just 207 mA... 

  • 0 in reply to Kevin Parker
    TI__Guru 67666 points

    Hi Kevin

    Check the voltage at ISET -- 1V = 400mA (750 ohm)  the ratio is 300 to 1.

    "Is it normal behaviour for the delivered power to the battery to drop with voltage?" -- Bill J - The unit has a thermal regulation feature, increase voltage will result in higher temperatures, to compensate the output current is decreased.  Should see voltage at ISET decrease to indicate lower output current.

  • 0 in reply to Bill Johns
    Prodigy 20 points

    So the device isn't warm, unless the voltage is increased to 6v. At 5v, the voltage at ISET is 0.300v...

  • 0 in reply to Kevin Parker
    TI__Guru 67666 points

    "At 5v, the voltage at ISET is 0.300v..." -- Bill J -- This would be 120mA output current.

    Other thing to check is the TS pin voltage, it should be between 1.04V and 0.680V.  If it is 0.680 to 0.188V the charge current will go to 20%.