Differentiating between a Voltage Regulator and LDO

Discussion in 'Power Electronics' started by Byron.govender1, Aug 29, 2017.

  1. Byron.govender1

    Thread Starter New Member

    Aug 6, 2017
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    Good day

    I would just like someone to clear up something for me, according to my research when I use an 7812 Voltage Regulator and my input is 13.6v my output is still 13.6v. Is this because the 7812 requires a minimum of 14.5v to give me the desired 12v output?

    Also if I had to rather use an LM2940CT-12 Voltage Regulator(LDO) for my function would this be a better option? My input is 13.6v and the load should be approximately 0.5A. I need to get it down to 12v and 0.5A.
     
  2. jayanthd

    Active Member

    Jul 4, 2015
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    1. Yes. It is better to have 12V (Vout) + 3.0V = 15.0V at the input of 7812.

    2. Power supply should be able to provide current required by the load. If load requires say 3A then it is better to use a 5A power supply.

    So, In your case if load requires max 500 mA then you can go with 750 mA or 1A power supply.

    See if you can use LM2756-12 or is it (LM2576). 12V Buck Regulator which is a better option. I have not checked its datasheet recently but see if it gives 12V out for 13.6V in.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    How better?
    I see no advantage and several disadvantages to using a switching regulator instead of an LM2940CT-12 LDO for a 12V output with a 13.6V input.
    The switching regulator is more complicated with more parts and has a much noisier output.
    And the LDO would have an efficiency of 12/13.6 = 88% which is likely not much worse than the switcher.
     
  4. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Perhaps I've misunderstood how dropout voltage specs work, but I don't think that's correct.

    My understanding was that there's a specific dropout voltage for any given amount of current through a linear regulator (each regulator has a different current vs. dropout voltage curve.) For whatever current you're drawing through the regulator, if your input voltage is higher than the target voltage plus the dropout voltage, you'll get a regulated output at the target voltage. However, if your input voltage is lower than that, your output voltage will be unregulated, and it will be the input voltage minus the dropout voltage.

    In other words, the voltage at the output will always be lower than the voltage at the input, by at least the dropout voltage. So in your example, if you supply 13.6V and your load draws current corresponding to a 2.5V drop, you'd expect to see 11.1V output, not 13.6. For what it's worth, it looks like the dropout voltage is a little lower than that I'm most cases - I just used 2.5V because that number was implied in your 14.5V to get 12V example.

    Hopefully the experts will come along and correct me if I've described this incorrectly.
     
  5. RichardO

    Senior Member

    May 4, 2013
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    This is not quite right. The 7812 requires a minimum of about 2.5 volts from its input to output to work properly. This is called dropout voltage. So, if you input 13.6 volts the best you can expect from the regulator output is 13.6 volts - 2.5 volts = 11.1 volts.

    With 14.5 volts the 7805 might just barely regulate the voltage to 12 volts: 14.5 volts -2.5 volts =12.0 volts. The problem here is that at extreme conditions of temperature and over time the regulator will drift and not be able to supply the 12 volts.

    edit: Drat. @ebeowulf typed faster and has a better expanation.
     
    Last edited: Aug 29, 2017
  6. Byron.govender1

    Thread Starter New Member

    Aug 6, 2017
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    Thanks for the response guys but what I've noticed in testing is that the 7812 doesn't regulate when the input voltage is too low so basically the LM2940CT-12 will function even when there's a tenth increase in the input supply to regulate it to 12v
     
  7. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    I'm not sure I understand what you mean here. A tenth increase from what to what? Are you saying that below 12V, the output is the nearly same as the input, and that any input above 12.1V results in a regulated output? If so, that might make perfect sense, depending on your load:
    IMG_2385.PNG
    At 100mA of current, you'd expect to see 0.1V dropout voltage.

    The example voltage drops I described earlier were based on the other regulator. With this one, depending on your current needs, you could be dealing with only 0.1V dropout.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    Per the thread title, the difference between a voltage regulator (such as the LM7812) and an LDO regulator (such as the LM2940-12) is the dropout voltage.

    When the 7812 and other integrated voltage regulators appeared, the circuits they were based on required enough overhead voltage (the difference between the input voltage and the output voltage) in order to regulate the output to 12 V adequately (meaning, to be within spec). This is just the way they were and all regulators were about the same and so it was just understood that you needed a minimum of a few volts (3 V was commonly used for design purposes and most specs called for a minimum of 2 V to 2.5 V) for the device to work properly.

    Along came regulators that could get by on significantly less overhead, around 0.5 V. Thus was a significant change from the perspective of what it allowed the designer to accomplish, and so the manufacturers needed a name the emphasize that they were not the same as the regulators currently available. They didn't want to call them something other than a "voltage regulator", so they opted to add a qualifier and chose "low drop out", or LDO. The term immediately stuck (since it was a good and useful choice, unlike some terms that manufactures come up with).

    That's the principle difference.

    Now, I don't understand your claim that, according to my research when I use an 7812 Voltage Regulator and my input is 13.6v my output is still 13.6v." Are you saying that you've observed this with physical hardware, or that it is your understanding based upon what you've read. I'm guessing the latter, which is perfectly fine. But once your input voltage falls below the regulated output voltage plus the dropout voltage, the output voltage should "dropout" of regulation and fall. So I would expect it to be BELOW 12 V, probably somewhere in the 11 V to 11.5 V range. I would be surprised if it were to rise above 12 V and very surprised if it were to go all the way up to the input voltage, even under no load.

    And, yes, if your input voltage is going to be in the 13.6 V range and you want 12 V out, then you want either an LDO regulator or a switcher (which may or may not be a reasonable option depending on your requirements, particularly in terms of noise). At 0.5 A a linear regulator (which an LDO regular is) will need to dissipate a good fraction of a watt of heat. This is manageable and you can probably live without a heatsink, but you might consider putting one on if it's feasible to do so.
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    A standard, old-school linear regulator like the 7812 needs a supply voltage at least 2.5V above the desired regulated voltage.

    A low drop-out voltage Linear regulator like the LM2940CT works the same way as the LM7812. However, a careful (and better) use of transistors will allow a supply voltage much closer to the desired output (maybe a few tenths of a volt).

    Look at the data sheet for the internal schematic of each and you'll see slightly different output transistors. The low-drop out uses a PNP as the high-side output control, whereas the old-school 7812 uses a darlington non high side and, therefore needs more headroom (has high "drop-out"). Where drop-out is defined as "extra" input voltage to insure a regulated output.
     
  10. Byron.govender1

    Thread Starter New Member

    Aug 6, 2017
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    Good day guys

    I managed to get my hands on a few LM2940CT-12 regulators and once added to my circuit It's as if my circuit is null and void. I added it to the input as I wanted only 12v to run through my circuit .
     
  11. Byron.govender1

    Thread Starter New Member

    Aug 6, 2017
    9
    0
    Good day guys

    So I've added the LM2940CT-12 voltage regulator to my circuit. I added it into the circuit so I would get the desired 12v running through my circuit but now it seems as if my circuit doesn't even exist because I don't get the desired output from the circuit... my circuit is supposed to have a time delay but after adding in the LDO Regulator I have no delay at all. Can someone please assist.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Post your circuit's schematic, input voltage and what you expected from your circuit.
     
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