Please help me understand LM431 operation!?

Discussion in 'General Electronics Chat' started by TheLaw, Feb 29, 2012.

  1. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Hello,

    I'm trying to figure out how a shunt regulator works and more specifically the LM431. Eventually I want to use it as a low-voltage cut off for a battery powered application I have in mind, but I really want to understand the theory of operation...and right now, I really don't get it.

    Mind you, I'm 17 years old with no official education on the topic so bear with me.

    I attached one of the most basic applications of the LM431. Shunt Regulator. So it looks very much like a 3-pin voltage regulator that most of us are familiar with.

    [​IMG]

    There are 3 pins on the LM431. Cathode, Anode, and REF.

    It would make sense the cathode and anode just comprise a zener diode which keeps a constant voltage across it because it's reversed biased. But I'm confused about the REF pin. The output of a voltage divider is on that pin...so are you inputing a voltage from that divider to REF? What happens if you left REF unconnected? Does it function as a 2.5V Zener?

    If I understand this correctly, feeding a different voltage into REF will change the VZener and thus you get a different output voltage? So this is pretty much an adjustable Zener? :confused:

    But what exactly is going on here? If I provide a voltage to REF, what exactly is happening that I can get a voltage out from 2.5V to 36V?

    If you are feeling ambitious, perhaps you could help me with a more advanced circuit...

    [​IMG]

    If you want to slap me in the face, I don't blame you. I'm not exactly sure how the two of them are working together. The first 431 is similar to the shunt regulator except the output get's connected into middle of the resistor divider for the second 431. What does this accomplish? So say the scenerio is High Limit = 4V, but Vin = 5V. That produces 4V on the output? (no?) and that injected with the other voltage divider does what?

    And then I presume the other resistors and capacitors are for BJT biasing?


    Well I know I just asked a lot of questions. I know you guys aren't my personal teachers, so I hope I don't come off that way.

    If you have any contribution, I would appreciate it.

    Thanks.
     
  2. #12

    Expert

    Nov 30, 2010
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    The 431 is an adjustable zener. Whenever the voltage on Vref tries to go above 2.5V the chip conducts like a zener diode. That's it in a nutshell.

    BRB
     
    Last edited: Feb 29, 2012
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Page 9 of the datasheet shows the internal diagram of the 431.

    The main thing you need to know in order to design anything with it is that it needs a minimum cathode current in order to work properly and that if the voltage from REF to anode is higher than 2,5 V i conducts from anode to cathode. You can lookat it as an adjustable zener diode .


    If you want to use it as a battery cutoff circuit the REF voltage cannot depend on the state of the 431 so you'd have to put the voltage divider before the current limiting resistor.

    Tell us exactly what you want to do abd we can help you out.

    Battery voltage, load current, hysteresis.
     
  4. #12

    Expert

    Nov 30, 2010
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    If you left Vref unconnected, you would violate the terms of the datasheet. If you want 2.5 volts, connect Vref to the cathode.

    Now about your Voltage Protection circuit...I don't get it. This is going to take a while for me to understand it.
     
  5. #12

    Expert

    Nov 30, 2010
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    OK. The B resistors set up a condition where if there is enough voltage to cause more than 2.5 volts across R2B the Right 431 will conduct and the transistor will conduct. Lacking that much voltage (low limit), the transistor will be off.

    If you go over the high limit, the RA string will cause the left 431 to conduct which dumps the voltage on the RB string, shuts off the right 431 and the transistor goes off for overvoltage.

    The transistor is only on when the voltage is between the 2 limits.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    It's actually an integrated circuit that has similar functional characteristics to a zener. It is also notorious for oscillating. There are much better references available. I think that thing is 30 years old.
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    This equivalent circuit should help you understand how LM431 work
     
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  8. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Thank you everyone. You're contributions are priceless.

    I understand pretty much everything I was seeking.

    #12 thanks for the explanation especially.

    What other reference would anyone else suggest if the LM431 is....obsolete? Not as good?
     
  9. #12

    Expert

    Nov 30, 2010
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    I can't tell you what is a better shunt regulator but I can tell you that the circuit shown will smoke the transistor. You need a resistance in series with the base in order to keep the right 431 from trying to get the base all the way up to Vbatt.

    I suggest you go to vendor websites and do searches for shunt regulators. You'll still get partially old crap but personally, I don't know which is which. Lack of experience. You are waiting for an answer from somebody that has actually used shunt regulators.

    ps, interesting to note that in your circuit, the 431 chips are not being used as regulators. They are being used as comparators. Smart trick. Eliminates the use of a lot of resistors and op-amps.
     
  10. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Really? This is straight off just about every manufacturer's LM431 datasheet.
     
  11. #12

    Expert

    Nov 30, 2010
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    Odd as it may seem, the original poster came here to ask questions because he was having difficulty understanding the manufacturer's LM431 datasheet.
     
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