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LM2674 External power feed parallel to output

Gregg Herzog
Prodigy 10 points
Other Parts Discussed in Thread: LM2674

We  have been using the LM2674 in a standard application where feed voltage (+VP) is 24VDC and output (+12V) is 12VDC at 300mA.  See included schematic.  We are looking at using the same circuit board but in an application where the input (+VP) to this regulator will not be connected and the output (+12V) will have a alternate 12VDC source connected in parallel.  We have tried this on the bench and everything appears to work OK but I do not have a detailed schematic of the internals to the LM2674 to confirm.

Would like to know if the LM2674 will live quietly under this condition?

I do not mind if the part is damaged as long as it does not become a current sink.

Thanks for your help,

Gregg

  • 0
    TI__Guru 72035 points

    Referring to the block diagram we see the power MOSFET connected from the VIN pin to the VSW pin. 

    This FET has a diode from VIN to VSW (cathode at VIN).  This diode will be forward biased if a voltage is present

    at VOUT and the input is floating or shorted to ground; the path will be through the inductor.  This may cause trouble in your

    system.  Also, a current flow through this diode may damage the LM2674.

    Therefore, we cannot recommend this type of operation.

    If possible some kind of diode-OR should be used on the output.