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BQ25700A: Review on the circuit implemented using BQ25700A in BOOST mode

Bivin George
Expert 1057 points
Part Number: BQ25700A

Hi

I'm using BQ25700A IC for charging Battery in 4S configuration (16.8V) from a 12V supply. The circuit used is given below.

I powered up the circuit using 12V with 0.2A current setting, and without connecting battery, we observed that VDDA is generated, CELL_BATPRESZ is 75% of VDDA but the voltage at EP3 test point is only 12.2V.

L1 is not mounted.

1. I have not connected battery, is it because of this the 16.8V is not appearing on EP3. ?

2. Do i need to set a minimum current at 12V source for boost operation ?

3. Will the decaps added create an issue. More decaps added to reduce ripple.

  • 0
    TI__Expert 4780 points

    Hello Bivin,

     

    I took a look at your schematic and question. After powerup, the part will go through the following steps:

    • 50 ms after VBUS above VVBUS_CONVEN, enable 6 V LDO and CHRG_OK goes HIGH
    • Input voltage and current limit setup
    • Battery CELL configuration
    • 150 ms after VBUS above VVBUS_CONVEN, converter powers up
    • After CHRG_OK goes HIGH, the charger sets default input current limit in REG0x3F() to 3.30 A.
    • The actual input current limit is the lower setting of REG0x3F() and ILIM_HIZ pin.
    • Charger initiates a VBUS voltage measurement without any load (VBUS at no load). The default VINDPM threshold is VBUS at no load – 1.28 V.

     

    In your case, powering up without a Battery connected, the following applies:

    When the battery is below minimum system voltage setting, the BATFET operates in linear mode (LDO mode). As the battery voltage rises above the minimum system voltage, BATFET is fully on when charging or in supplement mode and the voltage difference between the system and battery is the VDS of BATFET. System voltage is regulated 160 mV above battery voltage when BATFET is off (no charging or no supplement current).

     

    If you have not set MinSysVoltage Register (0x3E) then the default is 12.288V for 4S configuration. In this case with BATFET operating in LDO mode and the default MinSysVoltage setting of 12.288V it could explain why you see this voltage on EP3.

     

    1. Please confirm CHRG_OK is HIGH as expected
    2. Based on your schematic and inputs, the Current Limit should be 3.3A and the VINDPM should be 10.72V. You don't need to set up anything to use these defaults.
    3. Please confirm if you have set MinSysVoltage if you intend the system to run at 16.8V

     

    I will check on the capacitors you mentioned and get back to you tomorrow.

     

    Thanks,

    Khalid

  • 0 in reply to Khalid Bairuti
    Expert 1057 points

    Hi Khalid

    Thanks for the reply.

    We checked CHRG_OK net and it is high.

    Please find our comments below.

    1. As mentioned by you, based on schematic, current limit for input source is 3.3A. If the input source 12V limit is set to 1A, will this affect turning on battery charging in BOOST mode. Any minimum current, i need to set in my 12V power source ?

    2. If i set MinSysVoltage Register (0x3E) corresponding to 16V without connecting battery, will the voltage at EP3 become 16V, will this damage IC ?

    3. If i connect battery to this circuit without writing any registers of IC , will it properly charge the battery in BOOST mode ?

    4. Battery used has recommended operating condition as 0.2C upto 16.8V in CC mode & 16.8V upto 100mA in CV mode. But battery can be charged with maximum of 3A current.

      a) How to choose CC or CV mode in this IC ?

      b)  To achieve 100mA in CV mode, do i need to write Chargecurrent() 0x14H register with value 0000001 . Will the IC take care of charging current automatically ?

    5. How can i check whether battery is charging or not,  how to check how much percentage battery got charged ?

    6. Any alert signal is generated in case of fully charged battery, will the circuit draw current to EP2 from battery once it become charged ?

    7. In schematic EN_OTG is directly connected to 3.3V net, is this an issue. Is there any internal series resistor for the same. If not pullep up, will it affect above operation.

    Please reply to above queires in addition to decap question asked in previous mail.

    Regards

    Bivin

  • +1 in reply to Bivin George
    TI__Expert 4780 points

    Bivin,

    1. 0.2A is OK for the input supply. Minimum is 50mA.

    2. This should not damage the IC. 

    3. The default battery voltage will be set to 4.2V/cell, but you need to program the ChargeCurrent(). Since your configuration is 4S, this will translate to charging in BOOST mode.

    • Based on CELL_BATPREZ pin setting, the charger sets default battery voltage 4.2V/cell to ChargeVoltage(), or REG0x15(). According to battery capacity, the host programs appropriate charge current to ChargeCurrent(), or REG0x14(). When battery is full or battery is not in good condition to charge, host terminates charge by setting REG0x12[0] to 1, or setting ChargeCurrent() to zero.

    4. See answers below

    a. There is no choice between Constant Current and Constant Voltage mode. Which mode the charger in relies on the settings programmed and the status of the battery. If the battery voltage is below the programmed ChargeVoltage() target and the ChargeCurrent() is programmed to something other than 0A, the charger operates in CC Mode. Otherwise, it operates in CV mode. See the attached charge profile below as an example of CC and CV mode. 

    b. If you set Bit 7 of the ChargeCurrent() register, this corresponds to 128mA of charger current. If all conditions for charging are met then the charger will start charging with this current. In CV mode, the charger decides the charge current based on the battery voltage. 

    5. To confirm the IC is in charging mode,  you can check the ChargerStatus (0x20) register. Bit 10 indicates whether the IC is in Fast Charge, and Bit 9 indicates whether the IC is in Pre-Charge. You can monitor the battery voltage by using the ADC. See ADCOption() register as well as ADCVSYSVBAT() in the datasheet.

    6. This device relies on the host to monitor battery voltage and terminate charging. Regarding the second part of the question, when the Battery voltage is above the MinSysVoltage, BATFET will be fully on so current can be supplied from the battery to the system.

    7. If the EN_OTG pin is tied high, and register 0x32[12] = 1, the charger will switch to OTG mode. If the pin is tied high but the register 0x32[12] = 0, the charger will not be in OTG mode. If the EN_OTG pin is not high the charger will not be in OTG mode. If you are intending to use OTG mode then this pin can be tied high, and you can set the enable bit when needing to go into OTG mode. If you are not using OTG mode then this pin does not impact anything, as long as you do not enable OTG Mode in the register setting. I would suggest a 10KOhm pullup resistor instead of direct short.

    Regarding the capacitors you asked about, it is OK.

    Thanks,
    Khalid