L431

Discussion in 'General Electronics Chat' started by ronv, Aug 31, 2014.

  1. ronv

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    I'm going to use a TL431 as a comparator like the block diagram on page 6 of the data sheet - no feedback.
    Do I still need to worry about the phase margin from capacitive loading? Like fig. 18?
    http://www.ti.com/lit/ds/symlink/tlvh431.pdf
     
  2. crutschow

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    No. Normally phase-margin is not a factor when using a device as a comparator. Note that this device is compensated like an op amp so it will have a similar slow response when used as a comparator.

    Are you sure that device will operate properly as a comparator? That block diagram doesn't show it as a comparator, just the equivalent circuit of the device. It's designed to be a voltage reference.
     
  3. ScottWang

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    You can't using TL431 as a comparator, you can check the Representative Block Diagram on upper left side of page 2 from motorola TL431, the input to output is via a diode.
     
  4. Jony130

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  5. ScottWang

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    I don't think the TL431 is a comparator here, they are the zeners, you may call the whole circuit is a comparator, but not the TL431.
     
  6. kubeek

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    Yes the TL431 is acting here like an comparator, or more precisely an open-loop opamp. When voltage at R2 crosees 2.5V the voltage at R4 will suddenly drop to about 2V, since there is no negative feedback for the regulator to act like a zener.
     
  7. Jony130

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  8. MikeML

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    Tl431gain.jpg

    Think of the TL431 as a "comparator to 2.49V". The open-loop gain of a TL431 is much lower (~1V/1mV) than the typical real comparator. The gain is reduced when you put a voltage divider on the gate...

    Here is a quick LTSpice sim of what I am talking about... Note the slope of V(out).

    If you want some hysteresis, adding an NPN transistor stage is quite simple...
     
    Last edited: Aug 31, 2014
  9. Lestraveled

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    In figure 21 of the TI app note that Ronv posted, the TL431 is being used as a comparator. I have used the LM385-adj as comparator. (Same type of reference chip.)
     
  10. crutschow

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    Below is a sim of the TL431a used as a comparator with a PNP buffer to generate an output that goes between ground and the V+ supply (the Tl431a output only goes down to about 2V minimum as stated in Figure 18 of the TL431 data sheet). It trips at around 2.5V input.

    TL431 Comp..GIF
     
    Last edited: Aug 31, 2014
  11. ScottWang

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    Hi Jony:
    Maybe my brain is too hard and fixed, so what I thought about the comparator will be as LM339, 393 or using LM324 to do, they have a big range can be compare from 0 to +V, if the limited range comparator also count it then I will agree with you.

    As the zener plus bjt and some resistors also can be a voltage monitor, but I don't call it comparator, it just a voltage monitor, an voltage comparator as LM393 can be a voltage monitor.
     
  12. crutschow

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    It compares an external input voltage with the internal reverence voltage, so from that point-of-view it's a comparator.
     
  13. ScottWang

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    If focus on comparison this thing and don't care the range then that's right, two input voltages to compare with.
     
  14. ian field

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    Yes you can, because; it *IS* a comparator, the only special bit is that it has a built in 2.5V Vref.

    Using it without feedback as the OP suggests means that it will spend most of its time saturated at either one rail or the other.
     
  15. crutschow

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    With the restriction that the minimum voltage is about 2V above the lower rail.
     
  16. MikeML

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    In a way, the TL431 is more useful than a "real" comparitor. Most of the time, with a real comparator, you go to considerable effort to connect one input to a reference voltage, usually derived from a voltage divider off the supply rail. Having a built-in band-gap 2.495V reference tied to one of the comparator inputs is very handy...
     
  17. ian field

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    Yes I forgot that bit.