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BQ24193: Is the safety timer stopped when charging completes?

Southern Engineering
Intellectual 715 points
Part Number: BQ24193

Hi,

We have an issue with devices that are bricked due to BQ24193 stop providing output power on SW node in the field. The issue is being investigated, but one assumption is that EN_HIZ has been set, followed by the battery discharging. After that there is nothing left to power the host since the battery is too low and SW is disabled. The fixed by removing the battery to reset BQ24193 (which is not possible in the field).

Reading the datasheet there is a section about the safety timer and that EN_HIZ will be set when the safety timer expires in default mode. However, there is a couple of details we fail to find info on:

1. If a charging cycle completes (normal conditions), does that stop the safety timer, so that it does not expire? 

2. If a charging cycle is started, and VIN (a 12V adapter in our case) is then disconnected, does that mean the safety timer will continue and eventually expire since the charging cycle will not have a chance complete? That is, will it be important to always have the host pull CE_ high when VIN is lost, not to risk have EN_HIZ set and brick itself once the battery depletes?

  • 0
    TI__Guru 57341 points

    Hi,

    1. The charging safety timer does not matter after the charge is terminated.

    2. When valid VBUS is disconnected, the charger will not charge and the charging safety timer does not matter.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Intellectual 715 points

    Hi,

    The datasheet states "At the end of the 5 hours, the EN_HIZ (REG00[7]) is set". When you write "does not matter", does it mean it will not set EN_HIZ in any case where charging has stopped (either due to being done or VIN disappearing)?

  • +1 in reply to Southern Engineering
    TI__Guru 57341 points

    Hi,

    It is called "Charging Safety Timer". The safety timer counts and may expire when the device is charging. When charge is terminated and VBUS is not present, the charger does not charge and charging safety timer will not count or expire.

    To debug effectively, please read all the BQ24193 register values before and after VSYS drop then check the differences.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Intellectual 715 points

    Hi,

    We are aware of its name. The problem is that there is no obvious reason to why we get this behavior. Therefore we needed to clear this not to simply make assumptions.

    We intend to read the registers, but have so far failed. The problem has occured in a number of units globally, but only a few have arrived in the lab this far. The problem with them is that they have been sensitive in the way that once opened and hooked up for measuring they work again. The assumption is that the battery is depleted enough to kill the MCU, but not enough to reset the charger IC. If EN_HIZ then is set nothing will happen when we attach VIN and the only way to wake the system is to open the (glued) unit to remove the battery or wait a potentially long time for the battery to deplete enough for the charger IC to reset so that EN_HIZ becomes 0 again. Perhaps setting up the measurement triggered that final depletion level. 

    This is just an assumption. We see no clear reason as of now why EN_HIZ would be set. The SW is not touching that bit and we assumed the safety timer should not act (which we now confirmed with you). Unless it charges until it expires.