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Paralleling LP5900 outputs

Gordon Hardman1
Intellectual 540 points
Other Parts Discussed in Thread: LP5900

I want to parallel a number of LP5900 outputs, not for increased current, but for better redundancy and avoiding a diode voltage drop. Each one is fed from a separate LiIon cell. Any single one can supply enough current, but if one cell fails, the 3.3V regulated should still be there.

Questions:

Normally parallelling regulator outputs is a no-no, unless something is done to encourage current sharing. I am more concerned about stability- would the circuit oscillate?

If any cell fails short, would there be any back current through the LP5900? There would be no diode (trying to avoid the extra drop).

I have a drawing, but don't see an easy way to add it..

  • 0
    TI__Mastermind 22610 points
    Welcome to the ~improved~ E2E format.

    To insert an image you need to click on 'Use rich formatting'. In the 'rich format' window look for the "Insert/Edit Media" icon. There are two, one looks like a square picture, the other looks like a film strip. Click on the the 'square picture' icon, then browse to the drawing on your drive.
  • 0
    TI__Mastermind 22610 points
    " ... would there be any back current through the LP5900? ..."
    Yes. If the LP5900 input is shorted to GND with 3.3V applied at the output, there will be reverse current (from OUT to IN) through the body diode in the PMOS pass element. Reverse current will be limited only by any DC resistance in the current path. Most likely failure would be either bond wires fused open, or melted metal in the PMOS pass element.

    As for oscillations, probably not a likely scenario. The device with the highest output voltage, if only a few milli-volts, will provide all of the current unless/until load regulation pulls the output voltage down to the next highest output (or thermal shutdown kicks in). Then some modest current sharing (80%/20%?) ~might~ happen. The device with the lowest output voltage might never give up any current.
  • 0
    TI__Expert 3715 points

    Hey there,

    Just wanted to add to Don's comments. It is possible to parallel the outputs but I fear it might be overkill for what you're trying to do. Check out this TI Design.

    Another approach that might work is using a reset IC to detect when the active battery voltage drops below a given voltage and send a signal to the secondary LDO to turn on. Of course you'd need a way to disable the primary LDO once this happens to prevent reverse current flow.

    Let me know what you think.

    ~ Aaron

  • 0 in reply to Aaron Paxton
    Intellectual 540 points

    Thanks- that is an interesting design. I am about to start testing a design with "diode ORing" from the cells. I am trying to decide if it is worth the complexity of one of the "ideal diode" chips, which has a charge pump to switch a FET.