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LM317DCYR (and others) miscategorized as LDO on website?

Other Parts Discussed in Thread: LM317, TLV1117

I'm having a bit of a fight with Digi-Key over the description on their website, which describes the part as "IC REG LDO ADJ 1.5A SOT223", LDO being the problematic bit. Throughout industry and TI's own documentation, LDO is defined as "low dropout":

http://www.ti.com/lit/ml/slup239/slup239.pdf

http://www.ti.com/lit/an/sbva026e/sbva026e.pdf

http://www.ti.com/lit/sg/slyt228l/slyt228l.pdf

https://en.wikipedia.org/Low-dropout_regulator

Yet, Digi-Key is stuck on the fact that, for some unfortunate reason, the LM317 product page at TI places it and many others in the category "Linear Regulator (LDO) > Single Channel LDO". Thus they are erroneously adding LDO to the description.

It looks to my like all linear regulators on the TI site are put into this "Linear Regulator (LDO)" category, whether the LDO tag applies to them or not. As stated by one TI employee, https://e2e.ti.com/support/power_management/linear_regulators/f/321/t/248252, the LM317 is not an LDO.

It's sort of like a typo, but some poor sap that rushes into using the LM317 thinking that he has an LDO without properly interpreting the specification could get bitten, don't you think?

  • Hello Ryan,

    I completely understand your concern. As I support the LM317 device, I have certainly thought about this myself.

    I have a few points to share. Some are facts, and some are just my opinion. Feel free to let me know what you think.

    1. The first question to ask is, what exactly defines a "low-dropout regulator" vs. a "linear regulator"? Without getting too much into the philosophical, obviously there are certain structures that allow lower dropout voltages. For example the LP2951-50, clearly a lower dropout, while LM317 is clearly not a "low dropout". Even still, where is the cutoff? 500mV, 1V? I am not sure to be honest. The TLV1117 is a device considered to be a low dropout regulator, but the dropout voltage is typically 1.1V .

    I see the same thing with Rail to Rail input or output amplifiers. How close to the rail can we get? I see some that are <10mV and I see some that are >100mV. So when I hear the terminology, I really see it more as a general term, but not something you can always take to the bank for meeting a specified threshold. 

    All of this above really to say, at some point in time certain devices were considered low dropout or rail to rail, but as time has passed we have improved even upon what met these original standards, and therefore the standard for what people expect changes even though what we call it doesn't. While certain devices aren't "low dropout", what can we really take away from the fact that they are called LDOs? If I need something with a 500mV dropout, I can't necessarily just pick any device that says "LDO." The answer really is that if you have a need for a specific dropout, then you really have to look at the dropout specification in the datasheet.

    2. Linear Regulator vs Low Dropout Regulator Terminology.

    In my opinion, over time, the term LDO has become more popular in the Electrical Engineering community. Why exactly? I am not sure, but I do know that LDO is easier to say than Linear Regulator. I also know that Linear Regulator is a relatively vague term. What kind of Linear Regulator are we talking about here? A voltage reference linear regulator? An LM317 type linear regulator? What do I call the LM317 type Linear Regulator? It has the same structure as what we call LDOs, so maybe we just starting using the word (if you can call it that) LDO to help differentiate an LM317 type regulator from other types of linear regulators.

    At this point it seems that Linear Regulator and LDO have become somewhat interchangeable. Even in one of the documents you pointed to, the title says linear regulator and the app note goes on to talk about LDOs. http://www.ti.com/lit/an/slyt201/slyt201.pdf  

    Wait are we talking about LDOs or Linear Regulators here? Do you see my point?

    3. As for the listing in Digikey. This type of listing for LDO is not limited to just Texas Instruments devices. I see it across many manufacturers in the Digikey listing. I also see similar naming structures on other manufacturers websites as well (not all, but some).

    Hopefully you are not posting this because you ran into this problem, but I do understand your concerns. Ultimately, I think the two terms have become interchangeable to the point that people search more frequently for LDOs than they do Fixed Regulators. Also there is nothing you can absolutely be sure of when hearing about a part description. Whether it be LDO or Rail to Rail, nothing is a guarantee except the datasheet specification, that is a guarantee.

    Final Point:
    Do not let the terminology mislead you. Check the datasheet.

    As I mentioned before, some of what I posted is fact, some is opinion. Feel free to let me know how you feel about this. Maybe others will see and it will generate an interesting discussion. 

    Best,

    Michael

  • I guess I concede your point that LDO may have become synonymous with Linear Regulator, but I don't like it. So many app notes from TI and literature around the world defines LDO as "Low drop out" that it really bothers me that a whole bunch of lazy and/or ignorant engineers have twisted the meaning of a useful moniker so that it is now useless for its original purpose. Not that that would be a first - the English language has been doing it for as long as it has existed.

    I agree that at the end of the day the datasheet needs to guide your decisions, not the part description. Still it's a shame. Incidentally, I would define 'low drop out' as 1 volt or lower. 3 V, as you say, is clearly not low drop out. The core of the issue is the perversion of the LDO acronym to the point that I have a Digi-Key tech telling me "'LDO as a part category means 'linear drop out'", which is nonsense historically, but may have become de facto truth today.

    I never referred to the app note you credit me with referring to, by the way ( www.ti.com/.../slyt201.pdf ). I haven't run into an issue with this per se, as I did read the datasheet and see that the LM317 wasn't the part for me. Still, I never would have even started looking at the part if the 'LDO' in the description hadn't caught my eye, so it does represent a waste of my valuable time.

    I'll mark this as answered because I think you make a good argument for why things are the way they are, or at least THAT they are the way they are. I don't have to like it to accept it. If nothing else, I'll never take the LDO tag for granted so even if I don't like it, it doesn't have to negatively affect me.
  • As another data point, TI app note AN-1148 http://www.ti.com/lit/an/snva020b/snva020b.pdf explicitly defines 'LDO' as 'low-dropout': 

    "In the low-dropout (LDO) regulator, the pass transistor is a single PNP transistor (Figure 2). The big advantage of the LDO is that the PNP pass transistor can maintain output regulation with very little voltage drop across it: VDROP = VSAT (LDO REGULATOR) (2) Full-load dropout voltages < 500 mV are typical. At light loads, dropout voltages can fall as low as 10 to 20 mV."

    It gives a "typical" dropout of < 500 mV. Still, it doesn't fix the problem of mistakenly referring to any linear regulator as an LDO.

  • Ryan,

    I appreciate the discussion. 

    I don't know that I necessarily disagree with the points that you are making, but as you mentioned, I was simply making an argument for why things are the way they are, and the key is always to look at the datasheet because a general descriptor doesn't guarantee a specification. Whether it be LDO or Rail to Rail Input or Rail to Rail Output.

    Best,
    Michael