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Using bq24780S in conjunction with a Buck Converter

Gerrit Telkamp
Prodigy 190 points
Other Parts Discussed in Thread: BQ24780S, BQ24770, BQ24735, BQ24075

We would like to use a bq24780S battery charger in an application with 24Vdc input power from the ac adapter (Vin). The system power supply (Vsys) should be 17Vdc max. The battery is a 4 cell Li-Ion back, ranging from 12..17Vdc.

The bq24780S can be supplied with up to 24Vdc, charging the battery pack with up to 16.8Vdc in charging mode. In battery powered mode, the bq24780S switches the battery pack voltage directly to VSYS using an external N-Ch FET. This could all be done by the example circuit  shown in the data sheet or evb.

But when the system should be supplied by the ac adapter, the bq24780S would switch the adapter voltage of 24Vdc to Vsys in the example circuit, Hence, we need to insert a buck converter into the power path to get 17Vdc max.

I'm not sure if the bq24780S will support a different power path topology like that onr described above, or if it will detect a problem when Vsys differs from Vin. Could you give me some hints? Maybe there is an example circuit available? 

Thank you!

  • 0
    TI__Guru 56650 points

    Please check bq24770 batttery charger which can meet your requirement.

  • 0 in reply to Wang5577
    Prodigy 190 points

    Thank you for the hint - bq24770 might be an option.

    Additionally, we need the option to turn VSYS completely off. In the datasheet of the bq24735, there was an application example using a PNP transistor (IMD2A) to switch off the battery voltage from the bq24735. I think this will also turn off the BATFET, so VSYS off.

    We have tested this using the bq24780S, but it does not work. It seems that leakage current flows into the bq24780S (the leakage current flows into the pins PHASE or BATSRC, SRN/SRP). At the end we measure 0.7V at the VCC pin of bq24780S, even if it is not supplied by the battery nor by the AC adapter (probably due to the voltage drop of the internal protection diodes). The BATFET is still turned on in this state.

    My question is: Is there a simple way to turn off the BATFET, so VSYS will be turned off completely? (P Channel BATFET in case of bq24770 or N Channel BATFET in case of bq24780S). I know from other battery chargers like the bq24075, providing a SYSOFF input to turn VSYS completely off.

    Thank you!