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Minimum fast charging current on BQ24257

Mattia Benato
Prodigy 150 points
Other Parts Discussed in Thread: BQ24257, BQ24232, BQ24072

Hi,

I want to use BQ24257 in standalone mode, so without using I2C..

I see that for setting the value of the fast charging current  I need to connect a resistor on the ISET pin.

this pin have a minimum RISET value of 75 Ohm but it don't specify a maximum value so I theorically set fast charging current at very low value but on the electrical characteristics I found the information that the fast charging current can only be set between a value of 500mA to 2A.

So my question is: what limit the minimum fast charging current at the value of 500mA? there is no possibility to set the fast charging current at lower value with standalone mode?

Thanks

  • 0
    TI__Guru**** 166936 points

    The bq24257 was on tested for charge currents down to 500 mA.  The IC will provide lower charge currents as set by the resistor on ISET but TI cannot warranty charge current accuracy (based on the KISET range) for values lower than 500 mA.  Is it possible to move to a linear charger like bq24232?

  • 0 in reply to Jeff F
    Prodigy 150 points
    Hi Jeff,
    Thank you for your answer!

    My first choice was a linear battery charger but at the end I decide to use a switching technology because for my application I want to cover a wide range of charging current (from 100 mA to 1.5 A for example) and a linear battery charger can't work at high current like 1.5 A.

    for "not warranty accuracy" you mean that if you choose a RISET using the formula that you give on datasheet the real value can be very different from theoretical value or also if you find the current that you want to obtain this current can vary in any case?
  • 0 in reply to Mattia Benato
    TI__Guru**** 166936 points

    We only warranty the accuracy based on the KISET min and max values down to 500mA.  If you go lower, the KISET min and max values will be lower and higher, respectively.

    The bq24072 or other family linear charger members supports charge currents from 150mA to 1.5A. 

  • 0 in reply to Jeff F
    Prodigy 150 points
    the problem of bq24072 but also every linear charger is that also if they ensure for example 1.5A of fast charging current they can't charge the battery with this high current at all value of battery voltage because they dissipate much more power they can do.. the bq24072 for example has 2.3W max of power dissiaption at 25 °C so at this ambient temperature and 5V input voltage it can provide 1.5A only if the battery is at 3.5V or above , before this value it must charge the battery with lower current and I want to guarantee 1.5A also at 2.8 max 3 V.

    However KISET on BQ24257 from what parameters depends? for example I see from datasheet that it typical value is 250, minimum 232.5 maximum 267.5 but in what conditions it reach this values?
  • 0 in reply to Mattia Benato
    TI__Guru**** 166936 points
    The variation is over the full operating temperature range.
  • 0 in reply to Jeff F
    Prodigy 150 points
    Ok
    so while it isn't tested we can't know if this value stay in the range already posted or not.
    Thanks for answers I'll waiting for update about this IC.