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BQ24703 Behavior?

PPinto
Prodigy 190 points
Other Parts Discussed in Thread: BQ24703, BQ24745, BQ24702

Hello All,

I am planning to use a BQ24703 to do a UPS system with a 12V 4Ah lead acid battery. My questions are,

1. Is there a better TI IC for this task? I chose this one because of the System Power Selector feature.

2. What happen when the battery is fully charged? I know there is a Enable pin that controls the charging process. But if I set a pullup to this pin, the battery keeps charging with a low current? I read the datasheet from the first to the last page and I can't find a direct answer to this.

3. This is a more complex question, at least more difficult to explain.

If I select AC as main source and unplug the AC adapter, the IC will switch the power source to the battery and turn off the internal 5V Vref (page 15 of datasheet). Then when the AC is plugged back the IC will choose the power source according to the state of ACSEL (if high it switches back to AC).

My question is, can I pullup the ACSEL to Vref? In other words, since the Vref was disabled when I unplugged the AC, it will be enable when I plug the AC or just when the IC switch the power source to AC?

Or basically, the IC stay in sleep mode when power source is the battery, regardless the AC is plugged?

Thanks.

Best Regards,

Pedro

  • 0
    Guru 22270 points

    Hello Pedro,

    Welcome to the Forum!

    1. There are at least 3 (ACTIVE) options that you can choose from that have Power Path option and can be used with lead acid and multichemistry batteries. They have similar features with the Bq24745 having a SMBus interface.  Also, its operating temperature is from 0 to 125 whereas the other two BQ24702/3 operate within the -40 to 125 deg range.  The BQ24703, will automatically switch the load to AC when the battery level drops below the battery depleted level, while the BQ24702  will not. 

    2.  Pulling the Enable pin high will cause the PWM output to go high.  This will turn off the P-MOSFET and all charging will stop.  This is implied from how a P-MOSFET operates.

    3.  You can override the the automatic switching function using the ACSEL.  If you want to stay on battery power after the AC adapter is plugged back in, then set ACSEL low.  In the example circuit found in the data sheet, the ACSEL is pulled up to Vref so it would follow the process outlined in the Autonomous Selection Operation found on page 22.

    This is from the information gathered from the data sheet.  You can do a search on the forum to see what real world issues others have had with these devices.

    Thanks,

  • 0 in reply to Greenja
    Prodigy 190 points

    Hello greenja,

    Thanks for your answer.

    1. Regarding this question, I will use bq27403 since I don't want to connect it to an host controller. Thanks for you answer.

    2. As I understand from Enable description on page 13 of datasheet,

    "ENABLE: Charge enable. A high on this input pin allows PWM control operation to enable charging while a low on this pin disables and forces the PWM output to a high state. Battery charging is initiated by asserting a logic 1 on the ENABLE pin."

     it has the opposite functionality. A high on enable pin allows charging and a low disables charging. That is why I have doubts about the pullup, and the "state" of charging when the battery is fully charged.

    3. I agree with you. Just made the question because I was not sure if the Vref would turn on when AC is plugged or just after an Enable transition from Low to High.  

    Thanks again for your answers,

    Pedro

  • 0 in reply to PPinto
    Guru 22270 points

    Hello Pedro,

    Yes, for number 2, my response was incorrect.  I'm still trying to figure out how I got that answer. Maybe I had another data sheet open...

    Anyhow, there is always the edit option :-)

    As for why it is pulled high, that would probably be to allow for charging in case the enable signal is lost.  It would be better to have your battery charge than leave it depleted I almost all situations.  With the ENABLE pulled high, the device would follow all the limits for voltage and current that you setup.

    Thanks for the correction!

  • 0 in reply to Greenja
    Prodigy 190 points

    Hello Greenja,

    I agree with you. Since I am trying to build an UPS system, it is better to have the battery fully charged than half charged, or depleted. But that is why I have more doubts about the BQ24703 behavior.

    I've been reading about the process of charging lead acid batteries and there are three phases. First, the constant current charge, where battery voltage increase until the current starts to decrease. Second, the topping charge, where the voltage is set to about 2.35V/cell (depending on temperature, but this is the recommended value for 25ºC) until the charging current decrease to about 3% of the rated capacity. Third, the floating charge, where the voltage drops to 2,25V/cell (@25ºC). This stage avoid the self-discharging process.

    Once more, as I understand from the datasheet, the BQ24703 charging process is define by three parameters,

    "BATTERY CHARGER OPERATION
    The bq24702/bq24703 fixed-frequency, PWM controller is designed to provide closed-loop control of battery charge-current (ICH) based on three parameters, battery-float voltage (VBAT), battery-charge current, and adapter charge current (IADPT)."

    which means that the BATP voltage define the floating voltage. Does this mean that the charging process of BQ24703 skips the topping charge stage?

    Thanks,

    Pedro