LM2941CT Linear Regulator

Discussion in 'General Electronics Chat' started by k7elp60, Jul 7, 2012.

  1. k7elp60

    Thread Starter Senior Member

    Nov 4, 2008
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    Looking for positive or negative comments and experience with thw LM2941 regulator. I still of some stock and was thing of using as adjustable regulator with an output of 5-11VDC instead of a LM317T for a load of about 100Ma.
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The 2940 - 2941 series can be cranky about the ESR of the output capacitor. Aluminum electrolytics will make it oscillate at cold temps because the ESR is too high.
     
  3. k7elp60

    Thread Starter Senior Member

    Nov 4, 2008
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    I am not sure if understand what you are saying. If the electrolytic capacitor has a temperature range, does the ESR change with temperature?
     
  4. #12

    Expert

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    No. ESR might change with temperature but that's not how you find out. You have to read the specs on every capacitor you buy.
     
  5. k7elp60

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  6. #12

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    I don't see any promises about ESR on this one. That means, if you need a certain ESR, don't use these.
     
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  7. k7elp60

    Thread Starter Senior Member

    Nov 4, 2008
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    I see on page 2 an impedance at certain frequencies. Doesn't the impedance of the capacitor have a bearing on the ESR?
     
  8. bountyhunter

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    Absolutely. The ESR of an aluminum cap goes skyward at temps below about 10C. Cap makers show the curves.

    Best way to go is get a good X7R ceramic cap of the size and voltage needed which will have almost zero ESR, and add external resistance in series to satisfy the ESR requirement of the IC regulator.

    Here is the ESR limits for the output cap printed on the 2941 data sheet: note that about 0.5 Ohms is dead perfect. Two 1 Ohm resistors paralleled in series with a 22uF ceramic is perfect.
     
  9. bountyhunter

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    A cap has a certain amount of inductance, capacitance, and resistance at any frequency. ESR is the resistance (real) part of the impedance. You can read ESR directly on a network analyzer by sweeping out until the impedance goes minimum (the resosnance point) where inductive and capacitive parts are equal so the cap looks purely resistive. Above resonance, it looks more inductive. Below it looks more capacitive.
     
  10. #12

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    Please excuse the geekitude but, the graph in post 8 is a perfect example of a question I have thought about before.

    When looking at a printed log graph, the physical center between (2) 1's is about pi/10.
    Would the center of the useful range in that graph be closer to .5 or .314?
    Or have I been standing too close to my nephew while he was smoking something?
     
  11. bountyhunter

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    I think you're right.... put 3 one Ohm resistors in parallel.:p
     
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  12. #12

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    Are you cereal? I could kick this up to the math forum if you're just joking with me. We have some major math geeks here.
     
  13. k7elp60

    Thread Starter Senior Member

    Nov 4, 2008
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    I will understand the resistor part of the answer, but are you sure about 22uF ceramic capacitors. I have seen 1.0uf, but 22uF.......I don't know.
    One other question if the input to the regulater is pure DC, like from a battery do we still need the capacitor on the output?
    Thanks,
    Ned
     
  14. bountyhunter

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    Read the LM2940 data sheet for the value. I recall 22uF was the minimum but verify on the data sheet to be sure. There are high capacitance ceramics available from makers like Taiyo-Yuden.

    YES! The capacitor is for loop compensation. It is absolutely required and the ESR must be in the specified range. I would (seriously) estimate I must have gotten thousands of calls over 20 years regarding parts oscillating because the wrong capacitor was used.
     
  15. bountyhunter

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    It doesn't matter if you get the exact "center value", it just has to be in the stable range.
     
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  16. #12

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    I was hijacking the thread. Not in particular about this chip. More about whether physical things that respond in a logrhythmic way see 5 as halfway between 1 and 10.
    I'll kick this to the math forum where the math geeks will see it.
     
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