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bq24100 charger always on

daniel motsinger
Prodigy 80 points

I have a circuit that models the reference design we are having problems with. We are trying to identify if the problem is design or manufacturing or components but don't understand the operations that are occurring. In this circuit it appears as if the charger is going on and off all the time well above the expected thresholds.

In an operational unit the output waveform on charger looks normal, the voltages goes up to 4.2 the charger turns off and it decays until the 4.1v threshold. In the bad unit (same circuit) the output decays only slightly and the charger starts to charge again. I have probed the circuit with the scope and don't see any ripple in the input or output that would cause this. Can you give me some ideas? In the bad circuit I have put 3 different battery cells and the results are the same so it doesn't seem like a cell problem. The thermistor in the circuit is in the battery pack and the voltage levels seem correct so we are not sure what is causing the charger to start up so soon. the circuit pulls a battery measurement every 1 minute and then is smoothed slightly to display the curve.

expected waveform:

bad waveform:

charger circuit

  • 0
    TI__Expert 5565 points

    Daniel,

    Two things that could cause a new charge cycle in your scenario is /CE pin toggling or the TTC pin toggling (see page 16, below equation 8). Could you share the waveforms of those two pins for a two or three cycles for the bad unit?

  • 0 in reply to Smriti B
    Prodigy 80 points
    Ok so I have nCE grounded so no problems there. TTC pin looks correct a 1k saw tooth wave about .75v with a .5v amplitude. Never drops to the threshold. I am now suspecting that my system may just be pulling on my battery enough to drop it below 4.1v momentarily causing the charger to kick in. Although I don't see this on other boards. I did have a sense resistor inline with the battery for measurements and see it pulls about 700mA at pk so maybe the battery just cant hold the charge above 4.1 at that current for long enough. it's just strange that multiple cells have this problem but not all. although I have a 2.2Ah battery so I don't know why that would be. also strange my once a minute logging never shows a dip below 4.1v either.