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BQ24105: TTC charge safety timer not working

PDeng
Prodigy 20 points
Part Number: BQ24105

we are using a 68nF capacitor on the TTC safety timer pin, which should stop the charge after approx 3 hours. With aged NiMH batteries, the charger continues to run in voltage regulation mode for over 8 hours and we have not observed the timer stopping the charge. We can see an oscillating waveform on the TTC pin approx 0.5V - 1.5V voltage range. Any idea why the safety timer is not working as expected?

The circuit is as per app notes and runs off a 5V supply, the rest of the behaviour is as expected

  • 0
    TI__Mastermind 40475 points

    Hello,

    Can you please supply a schematic?

    Thanks,

    Mike Emanuel

  • 0
    Prodigy 20 points

    Hi Mike,

    I've discovered the "problem", which is due to the design. The safety timer is actually working as designed. However, the regulation voltage was set to 3.0V. NiMH typically meaure 2.8V when charged. Therefore, as soon as the chip times out, it detects that the battery voltage is less than Voreg - Vrch and commences the charge cycle again. The change of the status pins is only present for approx 1 second, so it was difficult to capture and appeared to be a glitch in the logger data. The solution is probably to reduce Voreg to 2.85V - but will need to test in order to check that fast charge is still possible

  • 0 in reply to PDeng
    TI__Mastermind 40475 points

    Awesome!

    Glad you could figure it out!

    Thanks,

    Mike Emanuel