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Load switch selection

David Cowan1
Prodigy 30 points
Other Parts Discussed in Thread: TPS2115A, LM5050-2, CSD88537ND, CSD88539ND

I am working on a battery powered application that may be plugged into an external power source.

I would like the unit to detect when an external source in provided and always select that source as long as it is within a certain voltage range.

In other words I want to use the external power even if it is at a lower voltage than the batteries in the unit as long as the external source is above the minimum threshold and below the maximum threshold.

This unit will not provide battery charging.  I just want to preserve the batteries if there is an external power supply.

Any suggestions on a TI part or group of parts that will provide this functionality?

Thanks for your help.

  • 0
    TI__Expert 5275 points

    Hi David,

     

    I think the TPS2115A is worth looking in to, but it will depend on what voltages and currents you are using.

    Can you tell me what is the battery voltage range, the voltage range of the external power supply, and the desired min and max threshold for the external power supply?

    What is the maximum current that will be drawn by the downstream circuitry?

    Also, the minimum threshold seems easier to implement than the maximum threshold.  Is the maximum threshold a hard requirement?

    Should the behavior depend at all on the battery voltage?

     

    Thanks,

    Adam Hoover

    Load Switch Applications Engineer

     

  • 0 in reply to Adam Hoover
    Prodigy 30 points

    Adam,

    Sorry I didn't provide those details before.

    Nominal power requirement is 28VDC at about 150W and acceptable voltage range is 20VDC up to 36VDC.

    The upper threshold can be controlled with a different circuit so it is not a hard requirement for this circuit.  My main goal for this is to always use the external power if it is available and at or above 20VDC.

    The 2115A looks like it performs the right function but does not cover the voltage range I need. 

    Is there another part that works similar to the 2115A but uses external FETs so it can be used for a wider voltage range?

    Thanks for your help.

  • 0 in reply to David Cowan1
    TI__Expert 5275 points

    David,

     

    Thanks for the additional info.  Have a look at the LM5050-2.   You'll probably want to use back-to-back FETs instead of the 1 shown the datasheet typical application.  You can use a resistor divider from Vin to the OFF pin for UVLO (no upper threshold).  Also, I think you can find a way to use the nFGD pin to enable the battery supply when the external power is not in range.   Let me know what you think!

     

    Regards,

    Adam

  • 0 in reply to Adam Hoover
    Prodigy 30 points

    That looks like it will work.  Should the back-to-back FETs be of the same type or is there a different criteria for each one?

  • 0 in reply to David Cowan1
    TI__Expert 5275 points

    You probably want them identical so they turn on at the same time.   Please do carefully read and follow the MOSFET SELECTION section in the datasheet.  A dual channel TI NexFET such as CSD88537ND or CSD88539ND might be a good option.  Just connect the Gates and Drains.  The 2 sources can then be used interchangably as input and output.

     

    Kind Regards,

    Adam

     

     

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

  • 0 in reply to Adam Hoover
    TI__Expert 5275 points

    Hi David,

     

    Have you been able to confirm this combination will work for your design? 

     

    Best Regards,

    Adam

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