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BQ2057W: Issue with current supplied

Keerthivasan Vijayakumar
Prodigy 30 points
Part Number: BQ2057W
Other Parts Discussed in Thread: BQ24005

Good afternoon,

I am looking to use BQ2057W to charge 8.4 volt Li-ion battery .There are a no.of issues that I faced:

a)I Used 0.2 ohms as Rsns  in high side voltage sensing configuration. The current drawn was only 200mA  and the voltage across Rsns was only 40 milli volts .

b)Then I tried changing Rsns to 0.1 ohms which means the current should double to 400mA .But instead it drew only 200mA and voltage across the Rsns was 20 milli volts.

I read the datasheet and it says that it tries to maintain a voltage of 125 milli volts (TYPICALLY) in constant current mode .But I wasn't able to figure it out why it couldn't achieve that. 

Second of all if Rsns is changed ,CURRENT should change and not the VOLTAGE across Rsns .But it occurred the other way round i.e the voltage changed keeping the current constant to 200mA.

I hope Experts help me out and find a solution as soon as possible.

 

  • 0
    TI__Guru* 75816 points

    Hello

    BQ2057W is an older device with large number of external parts.  Recommend looking at BQ24005 which is a new part with fewer external parts.

    Things to look at on the BQ2057W design:

    1.) What is the CC pin doing, is it pulling low to fully turn on the pass FET/BJT -- it would indicate the device know current is not at regulation point.

    2.) Double check input voltage make sure you have enough head room ( input to output voltage)

    3.) Double check battery simulator voltage setting.  For the BQ2057W fast charge starts at 6.3V, below this point current is limited to about 10% of fast charge.

  • 0 in reply to Bill Johns
    Prodigy 30 points

    I thank you for suggesting the above points . 

    The CC pin has a voltage of 1.62 volts.But According to the datasheet the CC pin must have maximum voltage of 1.5 volts.What possible amendments do we make to make this charger charge our battery?(We really don't have time to buy another charger as it shall take time for shipping ) . 

  • 0 in reply to Keerthivasan Vijayakumar
    TI__Guru* 75816 points

    Hello

    For the BQ2057W (Dual Cell) you will need an input voltage of greater than 8.4V on input to pass transistor.  1.62V on the CC pin looks very low, this would be a lot of current thru base and still not getting output current you need.

    Do you have schematic you can send?

  • 0 in reply to Bill Johns
    Prodigy 30 points

    We followed the exact schematic (of PNP) given in the datasheet.

  • 0 in reply to Keerthivasan Vijayakumar
    TI__Guru* 75816 points

    If you have used the EVM schematic as a reference in your design check the voltage drop across R1 (1k ).  This should give an indication of the base current and if the BQ2057W is trying to increase output current.  See attached as a reference.

    Also what input voltage are you testing with?

    How are you simulating a battery?

    EVM BQ2057 sluu056a.pdf