Minimum load current of LM317

Discussion in 'General Electronics Chat' started by glorimda, Nov 21, 2011.

  1. glorimda

    Thread Starter New Member

    Oct 12, 2011
    17
    0
    I'm using the LM317(adjustable voltage regulator). I had some problem getting the right output I want and finally I figured it out.
    At first, to get the output of certain value I want, I referred the specification document and followed the application schematic that I attached. I put the R1 with 3K and R2 with 300 to get 1.3V output. But it didn't work. And I've struggled to figure out the reason. Finally, my colleague told me I need to use R1 with 240ohm and find out the value of R2 with respect to R1.He also said it is related to minimum load current.
    And I did as he said and it's working now. And now I'm trying to figure out about what the minimum load current is and why I need to put 240ohm in R2.
    I'm now seeing the inside schematic of LM317(attached), but I can't understand.
    Can anybody explain about this?
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    You have things confused,R1 should be 120Ω. That will insure the minimum load current is 10mA. If you eliminate R2 the output should be very close to 1.25V.
     
  3. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Yes, it is related to minimum load--voltage references generally require at least 5mA, and this has an internal 1.25V reference. For better stablility, the 120Ω suggested by k7elp60 is better yet--that brings the min load up to 10mA.

    I have starved them successfully down to 3.8mA by using a 330Ω resistor, but this is an out of spec condition and not all devices may work. The ones I tested dropped out of regulation around 3mA.

    For an experiment, ground the adjustment pin and connect a 1k pot in series with a 100Ω fixed resistor across the load--increase the resistance until the voltage starts to increase--you are not running at the actual minimum resistance (and current) for this selected device.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    All the schematics in the datasheet use 240 ohms for the more expensive LM117. But the less expensive LM317 needs 120 ohms max so that its minimum load current is 10mA or more.
     
    mdgomes likes this.
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You can use a higher value for R1 as long as the current through R1 plus the load current will meet the minimum current requirements for guaranteed regulation; otherwise the circuit may not otherwise meet manufacturer's specifications over temperature.

    However, if you use a higher value for R1, in order to get the same Vout, you would also need to increase R2. Increasing R2 causes the current from the ADJ pin, Iadj, to have more of an effect on Vout. Iadj might be anywhere from 50uA to 100uA, but it's usually around 70uA. That doesn't sound like much, but if R2 is 10k, that's a 700mV difference in the output voltage.
     
    Last edited: Nov 23, 2011
  6. glorimda

    Thread Starter New Member

    Oct 12, 2011
    17
    0
    Sorry for late reply. I've struggled to figure this out and forgot to visit here. As you explained, I figured out that R1 determines minimum load current 1.25V/R1>5mA.
    SgtWookie/ thanks for the idea. I was wondering why Vout is changed when I change the R2. And that's because of that leaking current from ADJ terminal.
    Thanks for helping me!
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It's not just the Iadj; if the minimum current is not met, the output voltage will be higher than expected due to insufficient current flow through PN junctions in the regulator itself.
     
Loading...