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BQ25700A: Combining a BQ25700A and a BQ40Z50-R2

Ruben Hoebee
Prodigy 100 points
Part Number: BQ25700A
Other Parts Discussed in Thread: BQ40Z50-R2, BQ24650

I'm interested in building a power system which normally operates on a 24V power line but sometimes is required to be battery operated for a few hours.

If the systems runs on battery power I need to be able to estimate the battery life.

For the battery power a 3S2P pack is going to be used with li-ion batteries.

If there is a battery failure but the power line is still available I want the system to still operate.

The output voltage of the power system is can be between 3.6V and 28V.

I'm a bit confused about which chips I should use to get this done.

Can I combine a BQ25700A for charging and BQ40Z50-R2 for fuel gauging and protection? And do I need a BQ296103 for overvoltage protection? (it's used in the bq40z50EVM)

  • 0
    TI__Mastermind 25800 points
    Hey Ruben,

    The BQ25700A will not support a 24V input. That charge controller only supports 20V input with a 24V over-voltage protection circuit.

    To estimate battery life, I think the BQ40Z50-R2 is a good choice. The gauge does provide secondary over-voltage protection, its always recommended to use a primary protector like the BQ296103 but that based on the risk you want to assume.

    In terms of the system voltage going up to 28V, again this will be a limitation from the charger side for two reasons. First, the maximum system voltage on this charger is 19.5, which trips our protection. Additionally, the system voltage tracks with the battery voltage above a minimum system voltage threshold. So unless the 3S battery you have can go up to 28V, the system voltage will be capped at the battery voltage.

    In fairness, you may need a combination of chips to achieve what you want. Likely a buck to drop the 24V input down to a usable operating range. And a boost on the output to get the 28V you are looking for. You can also look at the BQ24650 which operates up to 28V, but you will still need an output boost to get SYS up to the values you are looking for.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 100 points

    Hey Joel,

    Thanks for the response.
    It's good that you tell me that charge controller doesn't charge at 24V, this would indeed require the use of a buck converter on the input stage.

    The output voltage doesn't have to be 28V. The voltage range I gave was the total acceptable output range.
    What I do require is that the output voltage is always between this range, so even without a battery I need a minimum of 3.6V output. I don't think the BQ24650 does this.