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BQ7710A

Lohit Gupta
Prodigy 30 points
Other Parts Discussed in Thread: BQ77910A

Dear Sir/Madam,

I am working on BQ77910A for the application of 10 cell Li-Ion protection.

I am doing test on this EVM and I have a question or two to ask.

If I want to avoid the wake-up resistor then what can I do for the schematic design.

And also during the Under Voltage test when the cells went under the UV condition the Dsg FET turned off,its ok, but i could not observe the recovery even after increasing the voltages to above hysterisys. Load was connected at the output and after that dsg voltage did not recovered before I used wake-up resistor again. so can you Please suggest me a design in which wake-up resistor can be avoided as it wont be feasible in practicle circuits.

If possible provide a schematic different from the EVM as I am already working on that circuit ,provide me a circuit in which the there is no need of wake-up resistor.

Thank You.

  • 0
    TI__Guru 54715 points

    The bq77910A is designed for systems having 3 or more terminals, one of those being a charge present signal from a charger. If you do not have a signal line for charger detection, some circuit is needed to provide CHGST to the IC.  Most circuits will have some limitation to the device performance.

    When the device goes to UV with the default setup and CHGST is low, if it does not recover within 8 seconds it will shut down and needs a CHGST signal to wake up again.  There is an EEPROM setting to prevent shutdown with under voltage.

    One circuit which we have used for CHGST is included in http://www.ti.com/lit/slua637 figure 6, page 7.  When the bq77910A is off, applying a charger voltage Q20's Vgsth larger than the battery voltage turns on Q20 which turns on Q19 and provides a signal to CHGST through a resistor divider connected to Q19's drain.  When the bq77910A wakes up and turns on CHG, Q21 comes on latching Q19 on.  CHGST is held high until there is a fault which turns off CHG. Basically the part operates as if it were in a charger until there is a fault.  The transistors, resistors and capacitors can be adjusted to provide a load, thresholds and response time suitable to your system.  Since the pack enters UV in the charger, it does not shut down with UV. Balancing settings must be considered since CHGST may remain high for the life of the pack.

    Always test this circuit or any other thoroughly to determine if it is suitable in your application.