Lm1117mp-adj

Discussion in 'General Electronics Chat' started by ArchillesElbow, Jun 11, 2012.

  1. ArchillesElbow

    Thread Starter New Member

    May 2, 2012
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    Anyone know how to adjust this regulator to 3.3v from input battery (4.2v charged 3.7v discharged), do I need a set of resistors? If so what impedance?
     
  2. BSomer

    Member

    Dec 28, 2011
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    Did you read the datasheet? Page 9, figure 1 gives the formula needed to calculate the resistors for your desiered output.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Read the datasheet carefully, the regulator has typically 1.2V dropout voltage, so to get stable 3.3V you need at least 4.5V input.
     
  4. ArchillesElbow

    Thread Starter New Member

    May 2, 2012
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    Mm it has a standard product (not adjustable) of 3.3v -the data sheet says this is achievedby internal resistors...so because this was all that the shop had in stock I assumed this would do it..
    I added some resistors but all I get is a differential of 0.4xxx volts no matter what the input voltage is...

    The product is probably right, I am Probably doing something wrong...
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Again please? Are you trying to get 3.7V out of the regulator, or are you trying to get 3.3V from a lithium battery?
    The first is possible if you connect the ground pin through a diode or a small resistor, the second is impossible.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You need a low dropout regulator such as the LM3940 which has a typical dropout of about 1/2V.
     
  7. ArchillesElbow

    Thread Starter New Member

    May 2, 2012
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    Will the LM give me a stable 3.3v through the batteries ie fripom 4.2 volts drained to 3.7v input?
     
  8. ArchillesElbow

    Thread Starter New Member

    May 2, 2012
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    3.3v out of the regulator... Can u tell me why it would be impossible (for my own interest:))
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Because the lm1117 has a droput voltage of 1.2V, meaning you need at least 3.3+1.2V on the input side to get regulated 3.3V on the output. Some other LDOs like crutshow pointed out have this limit lower.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Extrapolating from the maximum data sheet dropout values I estimate the LM3940 should work down to 3.7V input if the current is not more than 330mA.
     
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