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BQ24232: Precharge safety timer doesn't activate

NGCDG
Prodigy 45 points
Part Number: BQ24232
Other Parts Discussed in Thread: BQ24230

I am working to test the precharge safety timer on our BQ24232. The fast charge safety timer works as expected, and cuts off the charge current if the timer elapses. Currently I have an RTMR of 20 k-ohms, which translates to a nominal fast charge timeout of 160 mins. My test setup has the BQ24232 powered from a DC power supply, and a programmable load set to constant voltage mode hooked up to the BAT pins.

I have the load set to a voltage of 2.4 V. The charger seems to be in precharge because the charge current in this case is ~30 mA, and if I were to increase the voltage programmed across the load above 3 V, the charge current switches to the programmed fast charge current.

Given the 20 k-ohm timer programming resistor, the precharge safety timer should trigger after 16 mins. However, I have let it run for 4+ hours, and the precharge current never changes.

Any ideas for why the precharge timer is not kicking in, and for how I could debug it? Any ideas at all would be greatly appreciated.

  • 0
    TI__Mastermind 21590 points
    Hello,
    What signal is seen on the /CHG pin?
  • 0 in reply to Raheem
    Prodigy 45 points

    Hi Raheem,

    Thanks for the quick reply. /CHG remains low at all times while the charge current is ~30 mA, including after 16 mins elapses and the precharge timer should have triggered. Any thoughts on this?

  • 0 in reply to NGCDG
    TI__Mastermind 21590 points
    Can you measure the resistance value from TMR to GND to ensure you have the appropriate value.
  • 0 in reply to Raheem
    Prodigy 45 points

    Hi Raheem,

    Thanks for the reply. TMR to GND is about 20 k-ohms. This is consistent with the fast charge timeout I measured of ~160 mins.

    Anything else you would suggest probing?

    Thank you,

    Noah

  • 0 in reply to NGCDG
    TI__Mastermind 21590 points
    Hello Noah,
    Can you post your schematic. Also, lets try leaving the TMR pin floating and see if the /CHG pin is flashing after 30 minutes. Double check that your silicon is BQ24232 and not BQ24230.
  • 0 in reply to Raheem
    TI__Mastermind 21590 points

    Hello Noah,

    Any update on this issue?

  • 0 in reply to Raheem
    Prodigy 45 points

    Hi Raheem,

    Thanks for checking in. It seems as though the original issue we were seeing was a function of our test setup. We came up with a more realistic test setup, and can see the situation more clearly.

    We hooked up a battery charged to 2.75 V to the charger, with a 79 ohm resistor in parallel with the battery. This allows us to test the precharge timeout without letting the battery voltage increase significantly.

    As shown in the below oscilloscope screenshots, the timer does trigger charging to stop after the expected amount of time (in this case, at about 28 minutes with RTMR floating). The battery voltage lowers because of the parallel resistance, and shortly thereafter the battery's internal protector trips, dropping the voltage to zero. However, it seems like the charger resets and starts charging again when the battery voltage drops to zero. Is this expected behavior?

    Note that /CE is grounded in our application. In the event of a shorted battery, the voltage would actually drop to zero, in which case the charger turning back on would defeat the purpose of the precharge safety timer.

    Could you confirm whether the above behavior is expected? Thanks again for your help.

    Thank you,

    Noah

  • 0 in reply to NGCDG
    TI__Mastermind 21590 points

    Hello Noah,

    When the battery voltage goes to 0V and then recovers, a new charge cycle is started. This is the reason why the timer resets. 

  • 0 in reply to Raheem
    Prodigy 105 points
    Hi Raheem, Thanks for clarifying. Since the principal purpose of the precharge timeout is to cut off the supply of current to a shorted cell, this is a pretty severe design flaw. If the cell is damaged, it will always discharge after the timeout expires, and therefore the BQ24232 will effectively never cut off charge to a shorted ccell as long as the VIN supply is connected. I can't fathom that this is intentional; the datasheet describes restarting based on output voltage falling "once a charge cycle is complete" in 8.4.1.2, but 8.4.1.6.1 describes a sustained fault state after the precharge timeout expires. If the fault state is automatically cleared when the cell discharges, which a shorted cell always will, how is this effective? Ultimately, I suppose it doesn't matter whether this is behavior is intentional or erroneous; the part is fundamentally flawed, and we will have to find another solution. Best, Carl
  • 0 in reply to cdt148
    TI__Mastermind 21590 points

    Hello Carl,

    To clarify my statement, the timer fault isn't cleared when the cell discharges but when the recovery occurs as shown in your scope capture. If shorted cell discharges after the timeout expires, the protector will trip and the voltage will not recover hence when the battery short routine is run and charging will not begin because the battery voltage wont rise above 1.8V. That is why the charge current stayed at zero when the battery voltage dropped to 0V. The device has battery short protection to check specifically for this. Since the battery wasn't shorted in your test, the device sources current to close the protector. When the protector is closed, a new charge cycle begins.