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charging in low currents from 7-14V source

Ran Aloni
Expert 2386 points
Other Parts Discussed in Thread: BQ2057, TPS560200, BQ24031, BQ24079, TPS62125, BQ24123

Dear all,

I have an application that receive an AC from a generator, and after a diode bridge I receive 7-14VDC and 30mA - 100mA.

is there a charger IC with very low consumption when not charging, and during charge, in order to charge on Li Ion cell (4.1V).

is there a chip you could recommend for my usage?
I tries to use switch step down (worked badly) and LDO.
I'm trying to avoide using a Zener to set the voltage.

what would be your suggestion for the best solution here?

best regards,

  • 0
    Expert 2386 points

    any comment please?

  • 0 in reply to Ran Aloni
    TI__Guru**** 166996 points

    bq2057 or bq24031 (with USB input grounded) would provide a linear regulator solution.  The most efficient solution would be a step down buck converter like TPS560200 followed by a linear charger like bq24079. 

  • 0 in reply to Jeff F
    Expert 2386 points

    thanks Jeff,


    I made an initial test with an EVM I already have BQ24120EVM.
    it fits for 1.5A charge, but I tried it anyway and canceled the timer and current limitation (grounded TTC jumper).

    I can control the generator output, and increase its voltage and currents.
    it worked nice for the low currents, but for some reason the current couldn't get up for more than 70mA charge.

    by checking the generator on a resistor, I know that it produces enough mW to give a lot more charge than 70mA.


    what makes the buck + linear solution so efficient? I had problems using a buck before since it couldn't handle the low currents (TPS62125), so the DC/DC forum sent me here.

    what makes the IC you suggested (BQ2057 and BQ24031) suitable, how can they improve my problem?

    thanks,

  • 0 in reply to Ran Aloni
    Expert 2386 points

    Dear Jeff,

    I think I know what is the problem...

    I looked at the efficiency graph of the BQ24123 -

    it is logical, because for this chip, for low currents the efficiency rapidly decline as high as the voltage goes up.

    do you agree?
    can you recommend a better chip?
    in addition to my last post, what is the advantage of a linear charger for this solution?

    thanks,

  • 0 in reply to Ran Aloni
    TI__Guru**** 166996 points

    With any buck switching charger, efficiency drops as the Vin to Vout differential increases and as the output load drops closer the converter's supply current.  I do not directly support the bq24123 or any of the higher powered switching chargers, only the lower power linear and switching chargers.  I am not aware of a switching charger that can give you high efficiency for your high input voltage and  low current.  Therefore, a two chip solution with a high efficiency, lower current buck converter followed by a linear charger may be the best solution if efficiency is high priority.  The two options I see are:

    -bq2057 linear - Pros: low cost, single chip (ignoring the external FET)  Cons: low efficiency -> low charge current -> longer charge time

    -Low cost buck converter (e.g., TPS560200) + bq24079 (only charger with 4.1V BATREG): Pros: high efficiency -> higher charger currents -> shorter charge time; Cons: two ICs

    Board space will likely be comparable given the size of the bq2057 and the externals FET and supporting passives compared to the small footprints of the buck converter and charger.