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BQ25713: CMPIN pin pulls voltage to 1.4V in low power mode

Part Number: BQ25713

I noticed an unexepted behavior of the CMPIN pin if I set the BQ25713 into low power mode.

The CMPIN min is connected to 3.3V trough a 100K pullup resistor, also the alert open drain output from my gas gauge MAX17261 is connected to it.
So the gas gauge is able to interrupt the charging process, if something goes wrong.

As soon I set the BQ25113 into low power mode ( set EN_LWPWR of REG01 to 1) The voltage at the CMPIN pin is pulled to 1.4V.
I tried to enable EN_PROCHOT_LPWR (REG31 [6] or [5] to 1) but this does not help.
As soon I leave the low power mode the voltage at CMPIN is 3.3V again.

This behavior costs additional 20uA (1.9V / 100k pullup) in low power mode, I have to find a solution for this.

How can I avoid this?
Should i connect the Pullup Vcc to a switchable source, or is there anything I can change in the configuration of the BQ23713?

Thank a lot.

BR Stefan


  • Hi Stefan,

     

    Can you share your full BQ25713 register settings? /PROCHOT during low power mode uses ~150 uA of quiescent current consumption. If this function is not needed, then I suggest disabling it by setting REG31[6:5] = '00'.

     

    Also, to help reduce the quiescent current, is it acceptable for you to increase the CMPIN pull-up resistor to a larger value, such as 1 MΩ?

     

    Best regards,

    Angelo

  • Hello Angelo

    Ok, so enable the PROCHOT is not an option.

    Since, I can not use the BQ-Studio anymore I can't provide you the full register settings easily.

    But, I read the ChargeOption registers, befor and after enter low Power mode:

    before EN_LWPWR set to 1 after  EN_LWPWR set to 1
    ChargeOption0() 10'0111'0000'1110 1010'0111'0000'1110
    ChargeOption1() 10'0101'1000 10'0101'1000
    ChargeOption2() 10'0011'0111 10'0011'0111
    ChargeOption3() 0 0

    Specialy I configured to comparator to the following settings:

    ChargerOption1(): CMP_POL -> 1b: When CMPIN is below internal threshold, CMPOUT is LOW (external hysteresis)

    ChargerOption1(): CMP_DEG -> 01b: Independent comparator is enabled with output deglitch time 1 μs <default atPOR>

    ChargerOption1(): FORCE_LATCHOFF -> 1b: Enable this function

    If the gas gauge pulls the CMP_IN net to 0V, the charging process will be interrupted. And can only restarted with a high signal on CMP_IN.

    If you need more information, I can certainly provide it.

    I can try to increase the pull-up resistance to a higher value, but I would like to understand why this happens.

    BR Stefan

  • Hi Stefan,

     

    Thanks, I will try to reproduce this behavior on an EVM using your ChargeOption register settings and let you know the results.

     

    Best regards,

    Angelo

  • Hi Stefan,

     

    I have tested this on an EVM, but low power mode does not affect the CMPIN voltage that I'm seeing.

     

    Is it possible for you to disconnect the path from the MAX17261's ALRT signal to the BQ25713's CMPIN pin? This will show us whether the gauge's ALRT pin is somehow pulling the CMPIN pin to 1.4V, or whether the charger is doing this internally.

     

    Best regards,

    Angelo

  • I cutted the open drain ALERT pin away from the CMPIN net. But this has no influence.

    But what I can say, this behavior only occurs if th system is powered from battery only. If the Adapter input voltage is present the CMPIN voltage stays at 3.3V even  I activate / deactivate the low power mode of the BQ. 

    If I set the BQ into low power mode and halt the execution of the firmware with a breaktpoint. I can "toggle" the CMPIN voltage by activate/deactivate the adapter input voltage.

    MCU halted:

    adapter input voltage [V] CMPIN [V] VCC [V] low power mode BQ
    20 3.3 3.3 on
    0 1.4 3.3 on
           
    20 3.3 3.3 off
    0 3.3 3.3 off

    I tested another pcb also, with the exact same behavoir. So from my point of view I do not have anoter explanation than this behavoir is related to the BQ25713.

    Best regards

    Stefan

  • Hi Stefan,

     

    After doing some more testing on an EVM, I found that the CMPIN voltage you are seeing is due to the finite impedance of the CMPIN pin, which allows a small leakage current to flow into the IC. At first, I wasn't able to reproduce this behavior since the EVM has different resistors tied to the CMPIN pin compared to your schematic, but I began to see similar results after changing the resistor values.

     

    In order to decrease the quiescent current in this state, please increase your CMPIN pull-up resistor (R313) from 100 kΩ to a larger value, such as 1 MΩ. This should cause the CMPIN pin voltage to be lower than 1.4 V, but the quiescent current should still be lower due to the larger resistor value.

     

    By the way, low power mode is for battery only mode, so it makes sense that this behavior does not occur when the adapter is connected.

     

    Best regards,

    Angelo

  • Hello Angelo

    Thanks a lot for you investigation.

    I will increase the value of R313 so that the current consumption is only slightly affected.

    I think it would be helpful, if the behavoir/input resistance of this pin is listed in the datasheet of the affected devices.

    BR Stefan