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BQ24125 datasheet error, design problems

Pablo Davalos
Prodigy 30 points
Other Parts Discussed in Thread: BQ24125, BQ24040

Hello! 

I have a problem with the design equations on the datasheet.

Eq (17) y (18) are the same, when they're setting different parameters. 

Eq (17):    R(iset1) = (Kiset1 x Viset1) / (Rsns x Icharge)

This eq seems to be OK, because pin ISET1 on BQ24125 sets the Output Current (Fast charge).

Pin ISET2 (according to PIN FUNCTION Table) sets the output current in precharge mode, or termination mode.

However, Eq (18) is EXACTLY the sam that Eq (17)

Eq (18): Eq (17):    R(iset1) = (Kiset1 x Viset1) / (Rsns x Icharge)

And finally Riset1 = Riset 2!! That means Icharge = Iprecharge!

Which one should i take?

(There're several mistakes in datasheet)

PD: I have to design a 250mA battery charger,  anyone have the values of Riset1, Riset2, Cout, Lout and Rsns?


Thanks!

  • 0
    TI__Mastermind 20570 points

    To answer your question, equation 18 in the datasheet has RISET2 = KISET2 * VISET2/(RSNS*ICHARGE)

    VISET2 is 100mV instead of 1V. The calculation part of equation 18 needs to be corrected, but the equation it self is correct. You can refer back to the datasheet section 8.3.3.

    Are you charging a single cell of multi cell li-ion battery?

    If you are charging a single cell li-ion battery, I would suggest to use linear charger such as the bq24040 for low current charging (250mA).

  • 0 in reply to Jing Zou
    Prodigy 30 points
    Thank you!
    Anyway, ther's also a contradiction between Eq (18) and (5). Both set RISET2, but (5) uses ITERM and (18) uses ICHARGE, where

    VISET2=100mV=VTERM

    Again, which one sould i take?
  • 0 in reply to Pablo Davalos
    TI__Mastermind 20570 points
    Equation 5 as indicated in the datasheet is for termination current. Please also refer to the pin function section for ISET2 function. ISET2 sets up both precharge current and termination current. You will not able to set these two values separately. You can find all the values, such as KISET1, KISET2, VISET1, VISET2, VTERM in the electrical characterization table.
  • 0 in reply to Jing Zou
    Prodigy 30 points

    It's alive! Finally, the charger is working. I had to recalculate the values of RISET 1 and RISET2, which leads me to conclude that EQ (18) is wrong, both the equation and the calculation.

    This is the right EQ (18):   RISET2 = (KISET2 x VISET2) / (RSNS x IoPRECH)

    Where IoPRECH = ITERM.

    If you select    IoPRECH = IoCH / 10  

    then    RISET2 = RISET1,

    because    VISET2 = VISET1 / 10

  • 0 in reply to Pablo Davalos
    Guru 13040 points
    Pablo,
    It is interesting that there is no post about these before Nov 2016!
    I had the same suspicion when I started calculating the resistors for this charger. Indeed, EQ(18) is wrong - whoever made the datasheet simply copied/pasted the previous set of equations...
    Did the IC work well for your project in the end?
    Bruno