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BQ29701 not turning on

Nick McMillan
Prodigy 30 points

I am using the BQ29701. I have it hooked up as in the sample simplified schematic. I have not included the resistors to Cout and Dout, but rather just the resistor, capacitor between the terminals and a resistor from V-. Everything was working as expected for about a month on a 19mA hour Lipo battery. It was then that I switched to a similar battery, but different brand. Now when connected, the battery will not go high.

I can force it high by temporarily shorting the battery to ground and then it will stay high for a time (before randomly going low again). It is on a jumper as well, so I can not disconnect and connect the jumper and expect it to stay high. It then needs shorting again. I saw in the datasheet that on the first time it may need to be shorted to ground so I am wondering if this has something to do with it.

I have also tried changing back to the old battery as well as a different battery with no luck. The problem persists (on three seperate boards). I have disconnected from the rest of the system, so the power is just going to an LDO and nothing else. I am also curious if the need to short to ground is common. This seems like a hassle for manufacturing on small boards.

Thanks,

Nick

  • 0
    TI__Genius 10785 points

    HI Nick, 

    Sorry for the late responds. This device belongs to MnP forum not charger. This is why it wasn't notify to right supporting team. 

    Basically, all you did was change the battery with different brand and it stop working. 

    Quick questions for you. 

    1. What do you mean by battery will not go high? Are you saying voltage on BAT pin is not high? 

    2." I can force it high by temporarily shorting the battery to ground." Not clear on this. 

    3. Is it possible for you to capture your schematic and post here?  

    I want to review your schematic before I respond. 

  • 0 in reply to Roger Hwang
    Prodigy 30 points
    For 1 and 2, the battery is essentially disconnected from the circuit. There is still voltage on the battery, but the circuit sees 0V due to the protection circuit. This disconnect will not allow the battery to be reconnected at all in some cases. It is possible that this could be due to overdrawing current since this is a small 19mAh battery. This is my thought, but unfortunately this means we will not be able to get more than 50% use of the battery before the circuit disconnects the battery.

    3. I could email the schematic to you, but it is proprietary so I do not believe I should post it here.

    I am still concerned about the small excerpt on shorting the battery to ground upon first connection. This seems like a real hassle for manufacturing in HDI devices.
  • 0 in reply to Nick McMillan
    TI__Guru 54715 points
    The protector will turn off the FETs from one of the protections, or if damaged.

    The optional external gate resistors will keep the FETs off when the protector is shut down. The tradeoff is extra current to bias the resistors during normal operation, or the risk of extra current when the part is asleep/shut down if there is sufficient leakage in the FETs or IC to bring the voltage near normal so that the part starts to wake up. With a small battery you appear to have chosen the lower operating current.

    The part is designed for a re-chargeable battery. The part senses the presence of a charger through the V- terminal. When first connected, it is expected that the electronics would hold the pack voltage low and V- would be high with the part shut down. If a charger is applied, the charger voltage would be greater or equal to the battery voltage and V- would go low. The datasheet comment on shorting the battery to ground would temporarily pull the V- low to wake the part without requiring the charger. It is a convenient charger-free solution to waking the protector, but for a very small board the charger may be a better physical option.

    The part has no status, so finding why the part is turning off is the challenge. Additionally it seems you must determine if you have a difference from the cell, the rework, or both. Under voltage is certainly the expected shut down condition. If the battery is in a holder, look for intermittent connection, check the voltage at both the cell and the IC, etc. Check for damaged solder on the filter components.

    Agreed, don't post your proprietary information. If you can share your schematic with Roger privately it may help him identify any further suggestions.