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BQ25050: Bypassing OUT pin for system rail

Nicholas Carley
Expert 8875 points
Part Number: BQ25050

Hello,

We have an application where the system pulls ~6A, we have a battery in the system to allow for a safe/slow power down once the input supply is gone. 

We need a simple battery charger ~500mA to charge the battery, it will sit at full charge most of the time. But when the input supply falls the battery needs to supplement the system while it shuts down.

The BQ25050 has an external FET which is useful as 6A won't work on an integrated FET. But most of our chargers have the input --> output through the device which also presents a current limitation.

Can the BQ25050 be used in the below way to bypass the OUT pin (bypass the internal IN-->OUT FETS) while still providing it the necessary information to switch over to the battery when the input supply falls? The resistor is there to current limit the OUT pin but still provide the voltage sense functionality.

Thanks,

Nick

  • 0
    TI__Mastermind 23220 points

    Hi Nick,

    This isn't recommended as the BQ25050 may be bypassed and the input may go directly to the system even if the current draw is low. This would be much better suited using a switching charger. 

  • 0 in reply to Anthony P Pham
    TI__Expert 8875 points

    Hi Anthony,

    The 4.5Vin is a regulated load already (output from DC/DC) and bypassing the VIN--->SYS was the goal. We'd like to utilize the BQ25050 for just the battery charging and its FET control when the VSYS rail drops below the battery voltage.

    Would we be better off using a non-power path device and using external ORing/Muxing devices?

    Thanks,
    Nick

  • 0 in reply to Nicholas Carley
    TI__Mastermind 23220 points

    Hi Nick,

    The device will enter discharge mode if the IC has no input power or in supplement mode.

    Is the system going to be continuously drawing 6A? I believe this will work regardless of an input power but I just wanted to see if charging the battery is still possible if that is a concern.

  • 0 in reply to Anthony P Pham
    TI__Expert 8875 points

    When the input rail falls, a voltage monitor will detect that and tell the system to start shutting down, so the current during shutdown will start at ~6A but decrease as parts of the systems turn off.

    When you say " I believe this will work regardless of an input power but I just wanted to see if charging the battery is still possible if that is a concern."  You mean when the input rail falls and the battery is connected to VSYS, current could flow back to the VIN pin and begin charging again like shown below?

    I think the CTRL pin could be set low to disable charging to prevent this from happening, correct?

    Thanks,
    Nick

  • +1 in reply to Nicholas Carley
    TI__Mastermind 23220 points

    Hi Nick,

    Yes, that was the main concern on my part although I did word it oddly. If there is control over the CTRL pin this may be fine. I'd also recommend a diode to prevent current in the path from OUT to IN just in case. This would ensure no current from the battery is backfeeding if the IC is in a BAT only state.