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BQ24171: BQ24171 reverse current when operate with 4.2V battery

Nestor Rambaldo1
Prodigy 50 points
Part Number: BQ24171

Hi,

I am using the BQ24171RGYT with 4.2V battery.  I designed my circuit taking for reference the following dev board: bq24172EVM Evaluation Module (Rev. A) 

When Vin is 0V (power adapter removed from main but dc jack plugged into the charger). the current  flow from the battery to Vin (when AVCC <3.6V and the circuit that operate the power path mosfet is disable).

Is the only want to stop this current draining to the input is using a diode? any configuration available on the BQ24171RGYT to operate the power path mosfet with 1 cell battery? a UVLO  of 3.6V is too high for 1 cell lithium battery. 

Thank you

  • 0
    TI__Guru**** 166076 points

    Hi Nestor,

    Is the problem that the input back to back AC FETs are not turning off and so you are seeing a voltage on the input side of the FETs (i.e. at the dc jack)?

    If so, then you can add a 500kohm resistor from ACDRV to CMRS for those FETs as shown in d/s figure 21 on page 27.  This provides a discharge path for the AC FET charge pump that doesn't properly turn off with a 1s battery.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 50 points

    Hi Jeff,

    Thank you for you reply, I really appreciate it.

    I do have this resistor in the circuit (see below), something is not working correctly.

    From the datasheet: "If AVCC is below the UVLO threshold, the device is disabled". 

    From you previous comment, I understand ACDRV ground the mosfet gates, even when AVCC<UVLO? Block diagram shown UVLO as control input for the circuit that drives ACDRV and CMSRC (9.2 Functional block diagram).

    Thanks again

  • 0 in reply to Nestor Rambaldo1
    TI__Guru**** 166076 points

    Hi Nestor,

    The input FETs are n-channel so they should turn off correctly if that resistor is there.

    Do you not have the diode OR to allow the battery to power the IC down to 3.6V as indicated by the red X?  If not, then the IC will not function properly without input power.  And you are correct in that the UVLO being at 3.6V means that the charger will not function after the battery discharges.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 50 points

    HI Jeff, 

    Thank you for that.

    The OR diode is not needed for 1S battery, taking into account that the nominal voltage is ~3.7V minus  ~200mV drop across the diode, the voltage will be under UVLO. 

    For 1S batteries, a diode is required to stop current flowing to the input. 

    If you want to add a comment, please do. Then I will close the issue.

    Thank you again

  • 0 in reply to Nestor Rambaldo1
    TI__Guru**** 166076 points

    Hi Nestor,

    Yes, if you don't have the diode OR, you have to add a diode at the input.

    Regards,

    Jeff