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BQ25101: nCHG stays low when no longer being charged

Ricardo Smith
Prodigy 40 points
Part Number: BQ25101

Hello,

We are using the BQ25101 in our design, and we have noticed that for some devices, despite disconnecting the device from being charged, the charge LED remains on. Indicating that nCHG remains low, when no longer being charged. Below is our schematic for the charging our battery. Please let me know if you have any ideas for why this may be occurring.

 

  • 0
    TI__Expert 8245 points

    Hi,

    Are your cells reaching termination current before disconnecting the device? I would expect the LED to turn off if any of the following conditions are true:

    1. The device enters sleep mode

    2. OVP fault occurs

    3. Termination current is reached

    If you have not triggered any of the above conditions, this could be why the LED remains on.

    Thanks,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 40 points

    So despite VIN = 0, it is possible for the LED to remain on? What might cause this problem to occur inconsistently?

  • 0 in reply to Ricardo Smith
    TI__Expert 8245 points

    I also just noticed you have nothing connected to TS. This could be causing a temperature fault that may also lead to this LED behavior. If you do not wish to utilize the TS feature, place a 10k resistor between TS and GND. Try doing this and let me know your result.

    Thanks,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 40 points

    Ok! But by leaving the TS pin floating, shouldn't the IC enter TTDM mode? Could this still cause the temperature fault?

  • 0 in reply to Ricardo Smith
    TI__Expert 8245 points

    That is a fair point. TTDM should not allow this to be a temp fault related issue. I will get in the lab today to test this.

    I think another possible cause arises from TTDM functionality. When in TTDM, the CHG pin will remain low (LED on) until termination threshold has been reached. If you disconnect the IN connection, the battery is no longer charging so it would remain in that state indefinitely.

    I will let you know what I am able to find by the end of the day today.

    Thanks,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 40 points

    Thank you Jackson, Here are some of the troubleshooting we have done:

    • Charging to full - No entry️ LED still stays on even when device is charged for a few hours
    • Disconnect battery and connect USB: No entry️ LED turns on even when battery is not connected.
    • Disconnect battery & USB: turns off. But immediately we reconnect battery, turns back on.

    I hope this helps in your tests.

  • 0 in reply to Ricardo Smith
    Prodigy 40 points

    Hello Jackson,

    After doing some additional probing, we have found that nCHG is pulled low even when the battery is not connected. This might indicate a soldering failure on our end, and perhaps nCHG and VSS are shorted

    -Ricardo

  • 0 in reply to Ricardo Smith
    TI__Expert 8245 points

    Hi Ricardo,

    I tested this behavior on an EVM in the lab and was not able to see this behavior. I think now the question is whether this is PCB related or IC related for the units exhibiting this behavior.

    Can you share your PCB layout with me and also do an IC swap from working to non working units to determine if the culprit is the IC or the PCB? It is possible that the PCB may have an unwanted short or other defect that is causing this behavior.

    Best Regards,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 40 points

    Hello Jackson,

    Thank you for looking into this, we are in the process of ordering a revised PCB as we do believe that PCB is the culprit. I'll update you when I receive the new PCB's.

    On another note, for the devices where everything seems to be working fine, we noticed that the IC heats up to about 130ºF when charging, is this normal? We are charging the device at it's max (250mA) from a 5v device, so it makes sense that the amount of power being dissipated is causing the IC to heat up, but we just want to confirm that this is typical.

    -Ricardo

  • 0 in reply to Ricardo Smith
    TI__Expert 8245 points

    Hi Ricardo,

    With using the device at its maximum current setting I am not surprised to see the die heating to this temperature. This is right near the thermal regulation threshold where the device will cut the timer clock in half and reduce charge current to keep it from exceeding this temperature. If the temperature were to raise to around 150C, the device would trigger TSHUT and stop charging entirely until the TSHUT fault is recovered.

    When the new PCB comes in I would however recommend doing this same thermal test to ensure that the behavior is consistent.

    Thanks,

    Jackson