LDO Doesn't Dropout?

Discussion in 'General Electronics Chat' started by philipm, Sep 9, 2013.

  1. philipm

    Thread Starter Member

    Jun 27, 2012
    47
    3
    I'm using a LiPo battery with a 3V fixed MCP1700T LDO to power an Atmel ATMega microcontroller curcuit. It's connected as per the data sheet with just two 1uf Caps (for Cin and Cout).

    When the battery voltage is above 3.2V, the regulator perfectly regulates at 3V.

    However, I noticed today that it doesn't drop out and below 3V it becomes linear (the output from the LDO matches the battery voltage).

    Surely it's meant to drop out (i.e. cut off) below 3V + LDO voltage?!?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,145
    1,791
    No. Whatever gave you that idea? Dropout refers to "dropping out of regulation" -- NOT dropping to zero volts.

    RTFDS my friend.
     
  3. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,686
    2,747
    On the datasheet, there is a graph of dropout voltage vs. load current. You'll see that the dropout voltage is linear wrt load current, crossing 0 volts at 0 mA.

    What is your load, and what is the dropout predicted by the graph at that load?
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    The dropout voltage refers to the minimum difference between the input voltage and the output voltage for the regulator to do its job.

    Supplying less than Vin + Vdropout will result in no regulation...
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,145
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    There are devices, such as the VN340 high side switch, that have undervoltage lockout. When Vcc is less than 8V the outputs will not activate even when commanded to do so. Such a feature is possible in a regulator, but it has to be designed to do this. Some regulators also have a shutdown input so that you can provide this behavior with external circuitry.

    One more time with feeling. Read The Freaking Data Sheet
     
  6. philipm

    Thread Starter Member

    Jun 27, 2012
    47
    3
    Well there you go! I learned something new today. Thanks all.
     
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