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BQ24725A: When BQ24725A is powered on, ACFET cannot be turned on

Lee Tan
Prodigy 50 points
Part Number: BQ24725A
Other Parts Discussed in Thread: BQ24725

Dear Sir，

The voltage of the charging adapter is 19.3v, the nominal voltage of the battery is 14.8v, and the maximum voltage is 16.8v.

There is a very strange problem. When BQ24725A is connected to 19.2v power supply, ACFET cannot be turned on. We found that the voltage of ACOK  is 2.8v, VCC is 19.2v, SRN  is 0V, REGN is 2.8v. ACFET cannot be turned on, so MCU cannot read the register of BQ24725A.

Before connecting the battery, the impedance of the PHASE is 1M ohm. Plug in a fully charged battery with  voltage of 16.7v. The battery voltage  instantly becomes 1.5v and the battery capacity becomes 0. The impedance of the test PHASE is 0 ohm, HIDIV is 150k ohm,  LOIDIV is 6.7 ohm, REGN is 7.2 ohm, and  ACOK, SDA, and SDL is 5M ohm.

How to solve this problem? What caused the abnormal work of BQ24725A?

Can you help me? Thank you very much.

  • 0
    TI__Expert 5330 points
    Hi Lee,

    The fact that REGN is 2.8 V when VCC is 19.2 V would be a good place to start debugging this issue. The REGN is an LDO off of VCC that should be regulating to 6V. With a 19.2V input, failure to come up to 6V would seem to indicate a short on the board, which is pretty consistent with the rest of your description.

    I will look more thoroughly through your schematic to see if I can locate an issue, but from a high level, everything seems normal. But I think the failure of REGN to come up is a good place for us to start debugging as this is probably the root cause of the ACIN not turning on and possible some of the other symptoms you describe.

    If not too much trouble, could you (temporarily) remove the diode from REGN to BTST just to perfectly isolate the output and see if it is still at 2.8V? Also, have you looked at it with a scope or just DMM? It is possible that this voltage is oscillating, and that could be helpful to know.

    Regards,
    Steve
  • 0 in reply to Steve Preissig
    Prodigy 50 points
    Hi Steve，
    Thank you for your detailed reply.I will try to remove the diode from REGN to BTST to see if it is still at 2.8V.and I will look at it with a scope .
    Unlike the reference circuit, I added a comparator LMV7239M7 to the ACDET pin. Because I only want to limit the maximum input voltage, don't want to limit the minimum voltage . Is it possible that the comparator output overcurrent causes the BQ24725A to burn out. The previously designed circuit is identical to the reference circuit. Without this comparator, it works fine. The current circuit described above will work with the battery for a period of time (more than 10 hours) ，then the ACFET will not turn on and all power off. All power supplies are not shorted to GND.
    After the ACFET could not be turned on, I tried to get the circuit working by plugging in the battery. But when the battery is connected ,the battery is shorted. The description of the PHASE impedance of 0 ohm is not accurate. Later, the V32 was found shorted with DMM. The V32 burnout may be due to the LODRV error opening after the battery is inserted, causing the V32 to over-current. Why is this happening?
    Regards,
    Lee
  • 0 in reply to Steve Preissig
    Prodigy 50 points
    Hi Steve，
    Thank you for your detailed reply.I try to remove the diode from REGN to BTST but it is still at 0V.It looks like it has burned out.Replace the 24725 and the circuit is functioning normally.

    Unlike the reference circuit, I added a comparator LMV7239M7 to the ACDET pin.Is this right?

    Plug in the power adapter and everything works fine. Once the battery is plugged in, without the adapter, the sound of the diverter switch will be heard. BQ24725 is not working properly and seems to have burned down.It's hard to imagine what happened.At this point, there is no adapter, the ACOK voltage is 3.3V, giving an indication of the error. The voltage of BATDRV is the same as the battery voltage, and there is no 6V above.

    Plug in the power adapter,the ACFET will not turn on and all power off.REGN is 0v, not 2.8V .I am sorry for the fisrt reply.Can you help me find the problem? thank you very much.
  • 0 in reply to Lee Tan
    TI__Expert 5330 points
    Hi Lee,

    It looks like you have replaced the RBFET (reverse blocking FET) that is normally placed between your C86 and R213 with diode V13 that is placed at the adapter input. The diode will block current flow from battery out through the adapter input (the same as RBFET would), but its placement is not blocking current flow from the battery into CMSRC and ACDRV. Normally the RBFET is soft-started when either battery or AC adapter is plugged into a cold system.

    The datasheet notes that the 4K resistors on each are required to limit inrush current. This is under the scenario I describe above where the FETs are soft started. In your circuit, the battery plug in provides a path back to CMSRC and ACDRV without a soft start, which may be damaging these pins.

    The CMSRC/ACDRV would be my first suspicion of where the overstress is occurring, but the comparator may also be overstressing the ACDET pin. In our reference design, the high side of the resistor-divider leading into the pin is 430K. Even for a 20V adapter, this would limit the max current into the pin to 46 uA. The comparator may be driving the pin much harder than that and damaging it.

    Regards,
    Steve
  • 0 in reply to Steve Preissig
    TI__Expert 5330 points
    Hello Lee,

    I haven't seen any more feedback/questions from you since the last post, so I am assuming you were able to resolve your issue and I will close the ticket.

    Regards,
    Steve
  • 0 in reply to Steve Preissig
    Prodigy 50 points
    Hi Steve
    I am very sorry that I did not give you feedback in time. My problem has not been solved, I have been trying it recently, but I have not succeeded.

    A very strange phenomenon has recently been discovered. With the adapter and battery connected, the system works normally and can be charged normally. But after a few hours , the ACFET turns off, the BATFET turns on, and the HIFET and LOFET turn off. Charging is disabled and the entire system is in battery power. At this point, ACOK is always high, indicating the presence of an adapter. The CPU detects that there is an adapter and a battery, and sends an open charger and sets the charging voltage and current commands to the BQ24725A. Why is this? I can't find the answer in the data sheet, I hope you can help analyze it.

    Disconnect the adapter and battery, reconnect the adapter, and the ACFET turn on，return to normal. When the battery is connected, the charging also returns to normal.

    When this abnormality continues until the battery power is zero, that is, when both the adapter and the battery are present, the ACFET is turned off, the BATFET is turned on, and the HIFET and LOFET are turned off, causing the BQ24725A to burn out? Can no longer return to normal. The final phenomenon of several previous burns was when the battery was zero, and both the adapter and the battery were present, but the ACFET was turned off.

    I hope that you can give some suggestions to help us solve this problem, thank you very much.

    Regards,
    Lee
  • 0 in reply to Lee Tan
    TI__Expert 5330 points
    Hi Lee,

    Some of the fault conditions detected by the device can shut down charging in the manner you describe:

    8.4.12 Input Over Current Protection (ACOC)
    8.4.14 Battery Over Voltage Protection (BATOVP)
    "If BATOVP last over 30 mS, charger is completely disabled.
    8.4.16 Thermal Shutdown Protection (TSHUT)
    8.4.18 Inductor Short, MOSFET Short Protection
    "After seven times of short circuit events, the charger will be latched off and ACFET and RBFET are turned off to disconnect adapter from system. BATFET is turned on to connect battery pack to system."

    A watchdog timeout can also halt the charging. The longest watchdog timer setting is 175 S, so if you are charging for 2 hours, it is not a likely culprit unless your uC watchdog "feed" routine misses for some reason over that time period.

    The inductor short, ACOC and watchdog timer can all be disabled, so the simplest first step would be to disable all three and rerun your test to see if charging still halts after 2 hours. If so, then you can eliminate them as possible causes. For the BATOVP and TSHUT, you would need to monitor the voltage and temperature when the charging stops and make sure that neither of them is out of range.

    As an additional reference, the datasheet lists these as possible causes for charging termination. Most of them are repeats of what I listed above, but I wanted to give you the complete list:
    One of the following conditions will stop on-going charging:
    • Charge is inhibited via SMBus (ChargeOption() bit[0]=1);
    • ILIM pin voltage lower than 75mV;
    • One of three regulation limit DACs is set to 0 or out of range;
    • ACOK is pulled low (See the Adapter Detect and ACOK Output section for details);
    • ACFET turns off;
    • VSRN exceeds BATOVP threshold;
    • TSHUT IC temperature threshold is reached;
    • ACOC is detected (See the Input Over Current Protection (ACOC) section for details);
    • Short circuit is detected (See the Inductor Short, MOSFET Short Protection section for details);
    • Watchdog timer expires if watchdog timer is enabled (See the Charger Timeout section for details);


    Regards,
    Steve
  • 0 in reply to Steve Preissig
    Prodigy 50 points

    Hi Steve,

        Thank you very much.