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BQ25798: Suitable solution for my use case

Michael Whittingham
Prodigy 10 points
Part Number: BQ25798
Other Parts Discussed in Thread: LP2951, LM117

Tool/software:

I wanted to find out if this part is suitable for my use case, or if there is another part that would be better suited.

Use case:

12v - 24v DC input.  25W power supply, so 2.5A @ 12V.

12v DC system voltage

3S LiFePO4 cells, 3.2V each, 9.6V total, needs ~10.8V for charging.

The battery is external and can be unplugged hot, but it will almost never be done.

The battery is for backup if power is lost, thus 99.9% of the time it will be running off the DC Input power.  If power is lost, it must be able to continue running off the battery without power interruption.

There is no need for fast battery charging; ideally, charging will be done at 100mA or less.

Currently, my design has a 12V-24V -> 10.8V buck converter, which then boosts the 10.8V and/or the battery voltage back to 12V.  With the BQ25798 am I able to get rid of both of these circuits?  I.E.  The BQ25798 can take 12V - 24V DC input and then provide a stable 12V system voltage, in addition to the 10.8V that the batteries need for charging?

  • 0
    TI__Guru**** 166176 points

    Hi Michael,

    Unfortunately, no.  The SYS output is the higher of the MINSYS voltage (for when the battery is deeply discharged) or VREG.  MINSYS must be less than VREG in order for the charger to properly terminate charge.  Also, in backup mode, the voltage on PMID pin, tied to the system, equals VBUS voltage.  So you would still need 2 separate converters.  TI doesn't have a practical, low current charger with operating voltage up to 24V.  I think your solution is the best option.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    The issue I have with my current design is that I was told the batteries we would use have their own current limiting capability, so if I put the batteries on my 10.8V bus, they would control their own charging, cc then cv, etc...  That is no longer the case, so I need some sort of battery charging circuit.  If I keep my 10.8V buck circuit and then 12V boost circuit, is there an IC you can recommend that would be suitable for handling the battery charging?

  • 0 in reply to Michael Whittingham
    TI__Guru**** 166176 points

    HI Michael,

    Most of our chargers use 100mA as trickle charge to start recharging discharged batteries.  In fact, all of our chargers that can take 24V input do not regulate charge current will high accuracy in the 100mA range because they are designed to charge with several amps.   If you only want to charge at 100mA and can limit the output current of your dc/dc converter or linear regulator in a well heat-sunk package, powered from the 12-24V rail, then that is safe enough to charge a battery, especially LiFePO4. For example, I searched for a 24V linear regulator with 100mA regulation but 200mA current limit and found LP2951 in a package that can easily dissipate the heat.  The linear regulator would run in current limit (or possibly thermal regulation) until reaching the set output voltage.  That is okay.  The only gotcha might be stability with such a large output capacitor (i.e. the battery).  The really old LM117/317 actually has adjustable current limit.

    https://www.ti.com/lit/ds/symlink/lm117.pdf

    There may be newer versions of dc/dc converters or linear regulators will controllable output current but in my quick search, I didn't find one that could take 24V input.  

    The BQ25798 can charge at 100mA but is overkill for only charging your battery.

    Regards,

    Jeff