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LV2862: Tolerance to back feeding power

Mike Ciholas
Prodigy 170 points
Part Number: LV2862

For the LV2862, what does it do if it is not operating (in SHDN or Vin absent) and you back feed power to the output?

For example, say LV2862 makes 5 volts from a higher voltage source.  Another power supply makes 5.5 volts and diode mixes that on the output of the LV2862 circuit.  Will the LV2862 tolerate that?

To say it another way, what is the LV2862 output leakage?

I can design the circuit with a diode in the LV2862 output, but would prefer to eliminate the cost and efficiency loss of that diode.  An ideal diode is straight out for cost reasons.

Thanks.

  • 0
    TI__Mastermind 31315 points

    Hi Mike,

    It sounds like you have tied VIN to SHDN. If the part is not operating, the output would probably feedback through the SW node into the VIN pin via the body diode of the high-side FET. In this case, I do not think it'll be that much, maybe in the uA range, since the shutdown current is also in the single-digit uA range. 

    In other cases, I would probably recommend adding a external diode if possible. 

    The other concern is to make sure that you do not violate the abs max value of the FB or SW pin, which is located in the datasheet. 

    Regards,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 170 points

    SHDN is not tied to Vin, it is controlled by a Vin UVLO divider so it starts at about 30 volts.  29 volt zener from Vin to SHDN does the trick nicely.

    There is a large cap on Vin, in the range of 1000 uF to 2200 uF, for energy storage during brief power interruptions.  The reverse flow through body diode from SW to Vin might be substantial current flow, perhaps serious enough to be an issue if the back feeding power is not current limited.  The block diagram implies this is a normal N MOSFET from Vin to SW, so that will be a hard diode drop.

    An extra diode hurts efficiency all the time, sad to do that for a one time factory test.

    SW max is -0.3 to 65 volts, no issue.  FB is -0.3 to 7 volts, no issue.  Won't violate those.

    Best plan is to inject on the Vin side and add circuitry that makes the SHDN UVLO change when the extra supply is added, or make the extra supply be > 30 volts.  This does mean the LV2862 circuit has to work for the very first board power up test, but that's not horrible.

  • 0 in reply to Mike Ciholas
    TI__Mastermind 31315 points

    Hi Mike,

    Your setup is now clearer, and I think the approach is reasonable in order to have two different rails. Let me know if you have any more questions.

    Regards,

    Richard