Which 3.3V linear regulators can withstand 3.3V on the output & no voltage on input?

Discussion in 'General Electronics Chat' started by blueroomelectronics, Mar 14, 2011.

  1. blueroomelectronics

    Thread Starter AAC Fanatic!

    Jul 22, 2007
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    I'm looking for low dropout three pin regulators that are designed for use when there is a voltage on it's output pin and power is disconnected.
     
  2. tom66

    Senior Member

    May 9, 2009
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    I think you are looking for "mirror-image insertion protection"; these are regulators which can tolerate the input being grounded and output being at a higher potential than the input (but often not higher than the maximum input voltage of the regulator.) Be aware that only some can continuously withstand this situation; others might only manage a minute (it will usually be specified in the datasheet.)

    EDIT: Are you looking for regulators where the input is grounded?
     
    Last edited: Mar 14, 2011
  3. blueroomelectronics

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    Actually the input will be open, no voltage present. It's for a serial adapter where when built will have either an external power supply via the regulator or be used as a daughterboard on a 3.3V motherboard.

    Adding a diode on the output pin would work BUT it would drop the output by 0.2V not desired.
    [​IMG]
     
  4. tom66

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    Okay, well I'd say most if not all regulators can withstand that situation. Think about it: when power is removed, the output capacitor would basically be supplying 3.3V and the input cap would be open - yet almost (if not all) regulators are fine with this.
     
  5. blueroomelectronics

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    The old LM78xx series needed a diode across the in & out pins. I would have figured modern regulators would have something similar built in.
    And the input will not be shorted.
    [​IMG]
     
  6. tom66

    Senior Member

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    I would think that you would not need a 4700µF capacitor on any modern regulator.

    An example TC1017 shows a body diode across the main fet, which might cause the discharge problem as mentioned.
     
  7. blueroomelectronics

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    I've just went through the LD1117V33 datasheet and they show a battery charger with a pair of LD1117 outputs connected together. Seems it's not a problem for that regulator.
     
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