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BQ24170: bahavier of system voltage during 2 cell battery abcent with input=9V

Di Wang
Expert 1740 points
Part Number: BQ24170
Other Parts Discussed in Thread: BQ24172EVM-706-15V

Hi experts,

Below is the schematic. I found, when the 4.2V x 2 cell battery is abcent, the system voltage is unstable after I changed the input voltage from 12V to 9V. The back-to-back MOSFETs (U21,U22) are switching on and off. I think it might be the 9V input voltage is close to the configured 2 cell battery voltage, but I don't know the specific reason/behavier of bq24170.

Could you help check the reason and method to avoid it?

Thanks.  

  • 0
    TI__Guru*** 146695 points

    Hi Di,

    I can't see the OVPSET resistors but the charger is likely in UVLO per datasheet section 9.3.8 below:

    9.3.8 Input Overvoltage/Undervoltage Protection ACOV provides protection to prevent system damage due to high input voltage. In bq2417x, once the voltage on OVPSET is above the 1.6 V ACOV threshold or below the 0.5 V ACUV threshold, charge is disabled and input MOSFETs turn off.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1740 points

    Thanks, Jeff.

    I attach the full schematic here. It seems not due to the ACUV. We are still checking if it is due to the 9V input is to close to the 2 cell battery voltage. The issue only happened during battery is absent. When the battery cells presented at VBAT，there is no problem for the 9V system voltage. 

  • 0 in reply to Di Wang
    TI__Guru*** 146695 points

    Hi Di,

    With battery absent and no TS fault, the output at BAT pin should be like below:

     

    Is above what you mean by unstable?  If not, can you provide a scope shot of 9V input voltage, PVCC and VBAT? Q1 BATFET should be off when VCC=9V is present.  Is that the case?

    The charger goes into SLEEP (turns off the input FETS) only when the sensed battery voltage is higher than the input voltage per the values below:

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1740 points

    Hi Jeff,

    ACDRV/Vgs(CH1) and BATDRV(CH2) is shown as below. We found when input=9V is close to the set Vbat, the BSTS/bootstrap can't refresh high-side gate drive well during battery absent, so that Vbat voltage is unstable. Does the unstable Vbat trigger the fault detection?

    We added a 450ohm dummy load at Vbat to help refresh, and it is fine now. However, would the dummy load cause farther problem when battery presents?

    Thanks.

  • +1 in reply to Di Wang
    TI__Guru*** 146695 points

    Hi Di,

    BATDRV and ACDRV only turn off when input voltage > input OVP or < input UVLO or <V(BAT)+90mV sleep threshold.  The BQ24170 can reach 97% duty cycle = V(BAT)/PVCC without issue. I suspect your issue is input voltage<V(BAT)+90mV due to battery capacitor hitting OVP.   I tested on the BQ24172EVM-706-15V EVM, set for BATREG=12.6V, and can take input voltage down to 12.9V (300mV headroom) volts before I see some issues but even then ACDRV and BATDRV do not turn off.   Adding the load to BAT is likely preventing the overvoltage on BAT.  Adding a load on BAT should actually make a BTST issue even worse.  The charge can't distinguish the load on BAT from from charge current so termination current will be off by that load current.  I recommend adding more capacitance on BAT instead.  The additional capacitance should absorb the charge current that is causing the BAT OVP.

    Regards,

    Jeff