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TPS65721: Unusual current draw after dead battery

Christopher Payne
Intellectual 665 points
Part Number: TPS65721
Other Parts Discussed in Thread: MSP430F5528,

We are experiencing an issue where our devices will occasionally have a higher current draw (between .5 and 1.5 mA when normally 70 uA) after the voltage drops below ~2V, and then is brought back up again. I first observed the issue when about half of the devices battery life were drastically reduced after the batteries were discharged once.

Our design uses the MSP430F5528 along with the TPS65721 for charging the battery and regulating the voltage. I tried reproducing the issue by connecting to a power supply while observing the total board current draw, and adjusting the voltage below 2 volts and then back up to 4. About 50% of the time, there would be this sudden jump in current, but the device appears to still operate normally. The only way to get the current back down is to remove power, resetting the MCU alone does not help. While I am not sure if the TPS is the source of this current draw, I am wondering if there are any conditions where a discharged battery could cause it to enter a mode where it is drawing constant current. And if so, is there any register settings or reset commands I could send to the TPS to prevent it?

  • 0
    TI__Prodigy 30 points
    This post has been assigned to the expert for the TPS65721 and he will respond as soon as possible to try to resolve your issue.
  • 0
    TI__Genius 9310 points
    Christopher,
    There are safety modes to protect the battery. Going below 2V will trigger one of them. At this point you can only charge at a pre-charge current rate.
    To better understand what's really going on can you provide a little more information?
    When you say the voltage drops below 2V and then is brought up again, are you saying that you have charged the battery from pre-charge to normal charge? and to what SOC or level are you charging it to? Is the higher current in standby or normal mode?
    Simply put a PB event would reset the circuit but that is not automatic and usually is not something you want a customer to do unless the PB is brought out for the customer.
    Does it still draw higher current with AC connected and or disconnected? Any other information would make it easier to help. having a schematic if possible would help, you can send it to me directly if you don't want it public.
  • 0 in reply to Gordon Varney
    Intellectual 665 points
    Hi Gordon,

    I just private messaged you our schematic for the TPS. The devices should have been charged from pre-charge to normal, and then to a fully charged state. The higher current is happening in standby mode, not while charging.

    After doing more testing, this issue appears to happen after I lower the voltage to below 2.3 V, which is the minimum for the TPS. Then the overall current drops to 20 uA. If I bring it back above 2.3 V, I will see this issue happen about 50% of the time.

    The reset button for our unit is only tied to the reset of the MCU. The PB on the TPS is tied to ground, which should keep the voltage regulator running.

    When AC is connected, the board does not draw the higher current, it draws about 16 uA which is correct. But it will go back to the higher current right when the AC is disconnected again. I can give you whatever other information you need, I am working with our hardware engineer on this issue now.

    Thank you very much,
    Chris
  • 0 in reply to Christopher Payne
    TI__Genius 9310 points
    Chris,
    I am have been talking to the chip designer about this. I will get back to you asap, as soon as the chip designer has some feedback. Usually they go back to the original simulations from inside of the IC and see if they can reproduce the problem. Then they usually can provide a solution.
    In the mean time one more test if you can. When in the higher current mode/issue and with the battery voltage above the 2.3V threshold, can you toggle the PB pin (pull it high then low) and see if the current gores back to normal?
  • 0 in reply to Gordon Varney
    Intellectual 665 points
    Thanks for the update Gordon. I talked to our hardware engineer about seeing if we can splice the trace to the PB pin so we can toggle it. It might take some time but I'll get back to you if we can do it.
  • 0 in reply to Christopher Payne
    TI__Genius 9310 points
    Chris,
    I have some info for you. I will put it together tonight with explanations, and send it out in the morning. I just wanted you to know that I am working on this.
  • 0 in reply to Gordon Varney
    TI__Genius 9310 points

    Chris,

     This is what I have for you. I am not sure that there is a problem, I feel like after the low battery condition the system thinks it's a new battery insert and needs a PB as if it's the first time startup. The UVLO protects the battery and the system.

    8.3.10.6.1 ENABLE for DCDC1 and LDO1

    The DCDC1 converter and LDO1 are enabled as soon as PB_IN is pulled LOW OR input voltage at pin AC is detected (<CH_PGOOD> = 1).

    There is a power-hold pin for DCDC1 (HOLD_DCDC1) and one for LDO1 (HOLD_LDO1). When HOLD_DCDC1 is pulled HIGH, DCDC1 is kept enabled after PB_IN was released HIGH. HOLD_LDO1 serves the same function and keeps LDO1 enabled after PB_IN was released HIGH. After first power-up by pulling PB_IN = LOW or applying voltage at AC, the HOLD pins HOLD_DCDC1 and HOLD_LDO1 can also be used as enable pins, such that they turn on LDO1 or DCDC1, respectively when they are pulled HIGH. This function is available as long as there is a voltage at the battery. After the battery was removed or was discharged, first power-on needs to be done by pulling PB_IN = LOW.

    There is a work around for an actual button. It's a small circuit that creates a PB event when AC is connected. If you have a low battery event removing the AC input and then reconnecting it will create a first time startup. You may need to adjust the caps and resisters to increase the time. Usually not needed.

    You can test this by creating a PB and see if the problem clears. If it does then add the circuit above so that you don't need to add a button.

    Let me know if you have any problems with this.

  • 0 in reply to Gordon Varney
    TI__Genius 9310 points
    Looks like the images did not make it. I will send them to you directly. There is an app note being created for this. It will be published soon.
  • 0 in reply to Gordon Varney
    Intellectual 665 points
    Thank you very much Gordon for the help. This will help us move forward with the issue.

    One more question, is it possible to create the PB event using an I2C command? We do have I2C communication to the TPS. I see that the CONTROL1 register has a PB_STAT bit that is both read/write. Would toggling this bit have the same affect as the circuit in the schematic you sent?