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BQ21040: Charging current not as per theoretical calculation

manish sharma2
Prodigy 150 points
Part Number: BQ21040

I am using BQ21040 part for charging Li-Ion battery with following hardware specification.

Input capacitor: 1uF/16V ceramic X7R, 10% tolerance

Output capacitor: 1uF/16V ceramic X7R, 10% tolerance

RISET: 1.2K ( corrosponding to 450mA)

RTS: 10K, as i am not using NTC feature of it.

Pulled up CHG pin with 10K.

but practically seeing the charging current of the battery is 214mA, which should be 450mA for 1.2K resistor.

please help me in resolving the issue.

Regards

Manish

  • 0
    TI__Genius 13700 points
    Hello Manish,

    Where are you measuring this battery charging current and what voltage is the battery at what you are measuring that current? If the battery is up at or near 4.2V you will probably be in the Constant Voltage portion of the charge curve which has the charging currently slowly decreasing from the set charging current of 450mA to the termination current at the end of the charge cycle. See figure 7 of the datasheet.

    The other question is if there is anything else connected to the VOUT net that could be pulling the remaining ~240mA of current from the charger away from the battery?
  • 0
    Prodigy 20 points
    Why are you connecting CHG pin to VOUT with 10K. It is charge indication pin, either connect LED with its series resistor to it or left open if not required to use.
  • 0 in reply to Mrugesh Goriwale
    TI__Genius 13700 points

    Hello,

    It is possible that for their system that they do not need to visual indicator but they wanted to use the /CHG pin as an Open Drain Logic indicator.

    Eliminating the LED saves cost, power, and board space so if it is not truly needed and only the controller needs to know if the bq21040 is charging then a simple pull up to VOUT works great to create an active low logic level indicator for when the bq21040 is charging.