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BQ2057: Charging 8.4V battery at 1A

steve wenner
Expert 2355 points
Part Number: BQ2057

Good afternoon....

I am looking to use a BQ2057W to charge an 8.4V LiPo battery...I would like to charge it at 1A during the constant current charge phase.  Can I do this with this device?  What would you recommend for the sense resistor and pass MOSFET if this can be done?

Thank you

Steve 

  • 0
    TI__Guru**** 165986 points
    Steve,

    Yes. You can size the sense resistor passed on the datasheet equation 1 on page 10. If needs to be in a package that dissipates the power computed by I^2*RSNS. Any PFET with high enough voltage rating for your input voltage and in a package capable of dissipating the power computed by d/s equation 10 on page 16.
  • 0 in reply to Jeff F
    Expert 2355 points

    Thank you! 

    One other quick question....I am interested in supporting a Li Iron Phosphate chemistry of 2S2P....I want to get the resistors correct for the BAT pin but I am somewhat confused to the meaning of terms in Eq. 2 page 11.....since I have a 2S I believe N = 2, but I am not sure of TI's terminology when it comes to voltage.... that is to say what is V(cell) in the case of a Li Iron Phosphate...is it 3.2V (nominal) or is it maximum 3.65V...The same with Vo(reg)?

  • 0 in reply to steve wenner
    Expert 2355 points
    Note to self....read data sheet first...ok I got Vo(reg) but V(cell) still eludes me, as it says 'Desired regulation voltage per cell'??
  • 0 in reply to steve wenner
    TI__Guru**** 165986 points
    LiFePO4 V(CELL) is 3.6V not 4.2V like LiIon and LiPo. If you have 2S LiFePO4, V(CELL)=3.6V and VO(REG) is either 4.1V, 4.2V, 8.2V or 8.4V depending on which BQ2057 you choose. The resistor divider can only be used to implement higher regulation voltages (i.e. N*V(CELL) < VO(REG)) so you will need to choose either the 4.1V or 4.2V BQ2057 or BQ2057C.
  • 0 in reply to Jeff F
    Expert 2355 points
    Jeff...
    Based on your formula I think I am good...I am currently using the W part (8.4V)...I have 4 LiFEPO4 cells (2 in series with 2 in parallel....effectively 7.2V with twice the capacity)....So according to your formula V(CELL) = 3.6 and VO(REG) = 8.4....given I have 2 in series this defines N....2*3.6 < 8.4

    am I missing anything?

    Thanks
    Steve
  • 0 in reply to steve wenner
    TI__Guru**** 165986 points

    Steve,

    The formula gives you a negative resistance if VO(REG) > N*V(CELL) so you have to pick a version of BQ2057 that has VO(REG) < N*V(CELL).  In orther words, you can use the resistors to set a higher regulation voltage than VO(REG) but not lower.

  • 0 in reply to Jeff F
    Expert 2355 points

    Jeff...

    Ahhhh.....I am laughing now.....I read that '-1' in the formula as raising to a power of -1 NOT subtracting -1.....Now I see....

    Thanks for your patience

    Steve