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BQ24735 Switching between battery and supply

Andrew Price
Prodigy 130 points
Other Parts Discussed in Thread: BQ24735

I am using a BQ24735 and I have it successfully charging a 3 cell lipo pack to 12.6V.

The DC supply to the IC is currently sitting at 19.2V. When the supply is connected the System Voltage is sitting at 19.2V . When the supply is disconnected, the system Voltage is the battery voltage of 12.6V.

I would like to have the system voltage constantly at or somewhere near 12.6V. But if I decrease the supply voltage below around 18V the system voltage reverts to the battery voltage, even though the supply is  still connected. Can the Supply voltage be somewhere near the battery fully charged voltage of 12.6V ? If so, how is this achieved?

Thanks

A

Thanks

Andrew

  • 0
    Prodigy 130 points

    Modifying the voltage divider  on ACDET will reduce the point at which the BQ24735 switches from AC adapter to Battery for the system voltage. (set at 15.4V)


    However I noticed as I increase the voltage of the AC adapter (a bench top power supply) there is a period where the system voltage oscillates between the AC adapter input and the battery. This continues till about 18V  when the AC adapter voltage is always selected.

    When I reduce the voltage  of the AC adapter, the voltage must drop to below 15.4V before the battery will be selected.

    It appears I have some sort of start up issue for the charger. Could anyone shed some light please?

    A

  • 0 in reply to Andrew Price
    TI__Guru 56650 points

    When ACFET is turned on, the system voltage is connected to input source. To reduce the ACFET loss, it works on switch mode, not linear regulation near to 12.6V.

    If ACFET<2.4V or ACDET>3.1V, the ACDRV would be truned off ACFET, And, the system is disconnected from the input source; the battery supports the system operation. That is what you saw.