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LM317 LDO pin compat replacement for CC op?

  1. Apr 15, 2009 #1
    Hi. I use an LM317 in constant current mode to vary the current from ~20ma to ~400ma, the voltage use is about 3 to 5. I just don't like that there's a 3 volt drop out for the 317 design.

    I'm wondering if anyone knows of any pin compatible replacements with a lower drop out? I wouldn't mind if I had to pay 1 or 2 $ each.

    I'm trying the LM1086, but I have not done extensive tests to see the benefits yet. They're supposed to have a lower drop. I like the specs of the LM2941, but they don't seem to make them in TO-220 3-pin. The pin distance needs to be the same as the 317 (~2.54mm hole pattern) and preferably a drop in replacement.

    Also, I'm not sure how the drop out is calculated for the voltage regulators used in constant current mode. Do you have to account for the 1.25 reference voltage used plus the rated drop out? For example, a chip rated at 1 volt drop out in CC mode. Would the drop out be 2.25 for the whole design?
  2. jcsd
  3. Apr 15, 2009 #2


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    Staff: Mentor

  4. Apr 15, 2009 #3
    The LM317 is a 3-terminal adjustable regulator. Many years ago I had a similar problem, and I realized that the mfgr was probably using a NPN pass transistor which requires another transistor (probably a PNP common emitter) to pull the base up, as well as having the base-emitter voltage drop in the pass transistor. I designed a discrete regulator circuit with a PNP pass transistor, thus being able to pull the base down, and eliminating the base emitter drop because the output is the collector and the DC input is the emitter..
  5. Apr 20, 2009 #4
    A "surf board" does sound nice. But way too much labor involved for what I'm looking for, as I deal with hundreds of LM317s. Has anyone done tests on the LM1086 or something similar?
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