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LM25576 adding alternate power

Jim Conrad
Intellectual 360 points
Other Parts Discussed in Thread: LM25576

Our production device is powered by an LM25576 circuit virtually identical to the reference design.  We expect 12v in with wide allowance, and produce 5v out.  Our device has a USB port but we have not used the available power from the USB.  The question is regarding adding the USB power to the output of the switcher, through a schottky diode so the device can be powered by USB during initial programming and field configuration.

Will this arrangement of the USB power applied to the switcher output (with no power into the switcher) harm the LM25576 in any way?  If a second isolating diode is required on the switcher side, should it be located between the feedback resistor and the rest of the switcher circuit?

  • 0
    TI__Mastermind 19891 points

    Hello Jim, 

    When the switcher output is biased, this bias voltage will show up all the way on the input of the switcher through the inductor and the high side power FET body diode. In this case the bias voltage is not enough to clear the VIN UVLO threshold and the switcher will stay off, which is good. However,  if the switcher input supply can sink current, it will drag down the EXT USB power when the input of the switcher is between 0V and 5V.

    Assuming the 12V supply can only source current (not sink) and you don't expect any short circuit on the input of the switcher while the USB is connected, you should not need a second isolating diode on the switcher side. The switcher will not get damaged and it will start up with prebias on its output.

    I happened to have an evaluation board in the lab, here is a startup test with a 12V supply and a 5V (-diode drop) prebias on the output:

    CH1 is the input of the switcher, initially showing the prebias voltage from VOUT and then ramping up to 12V.

    CH2 is the output of the switcher, initially showing the 5V external bias - diode drop and then ramping up to 5V when the switcher kicks in.

    CH3 is the output current coming from the switcher. 

    CH4 is the current coming from the external 5V supply. 

    Note that the output of the switcher should be configured to be slightly greater than the USB voltage minus the blocking diode drop, so that the switcher takes over once the switcher is ON and USB is still connected.

    If the input supply of the switcher has sinking current capability (or if there is a input short condition while the USB is connected), then it makes sense to place a second blocking diode. Since the issue is really about current going from the VIN of the switcher back to the switcher supply when the USB is connected, I would suggest placing the extra blocking diode on the input of the switcher between the switcher input capacitance and the switcher supply. That way you will see less DC loss from the extra blocking diode because the input current to the switcher is less than the output current and the diode is going to conduct lower average current. Also, that way you will not have to compensate for the extra diode voltage drop on the switcher output.

    If you choose place the extra blocking diode on the output of the switcher, I believe it is OK to place the feedback sense point after the diode to compensate for the diode drop. Still, the output of the switcher should be configured at slightly higher voltage than the USB-diode drop so that the switcher always takes over. 

    I hope this helps. Let me know if you have any additional questions. 

    Regards, 
    Denislav 

  • 0 in reply to Denislav Petkov
    Intellectual 360 points

    Thanks, Denislav,

    Your answer is exactly what I was looking for.  The anticipated use would not involve 12v supply sink capability, so I'm good there.  Just to be on the safe side, I think I'll add the extra isolation diode just prior to the feedback sense point and short it with a trace unless needed.  All depending if I have room on the board.

    Thanks again!