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At a high level both a linear and LDO regulators appear to be the same. Most block diagrams appear to be the same too. Most of the device packages are three terminal packages. What are the differences between linear and LDO regulators?

Regulator

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The difference is due to the strongest force in the universe: Marketing.

Both devices are linear regulators. This means they take a high and varying voltage in, and produce a low and steady ("regulated") voltage out. These type of regulators are totally powered from the input voltage, and adjust how much current they allow to pass from the input to the output. The concept is the same as a mechanical pressure regulator. In that case there is a real valve inside that is opened more or closed more so that the output pressure is maintained. Electronic regulators use transistors for much the same purpose.

One parameter of a linear regulator is how low the input voltage can go while it can still control the output voltage within spec. This parameter is often referred to as the headroom or dropout voltage. Old regulators needed several volts. The famous LM7805 needs about 2½ volts. Since it's output is 5 V, that means its input needs to be at least 7.5 V else the output being 5 V is not guaranteed.

Back in neolithic times, cavemen would make a power supply by using a big-iron transformer to drop the line voltage, 4 diodes to take the absolute value, and a cap to smooth it out. To get a regulated supply, they'd follow that with a linear regulator. It didn't matter much that the linear regulator needed over 2 V headroom since there had to be significant slop built into the raw unregulated supply for various other reasons anyway.

Today there is much more pressure to make power supplies small, light, cheap, and sometimes efficient. The strive for efficiency is actually not so much to not waste the power, but to not have to get rid of the heat from wasting the power. Electronic components have advanced considerably since the caveman days, so today it's generally cheaper and smaller to make the power supply more efficient than to add the ability to dissipate a lot of heat. Over 80% efficient is "easy", 90% efficient reasonably attainable in a lot of mainstream circumstances, and close to 95% attainable when need be and efficiency is a prime priority.

Now consider a 7805 in this context. Even at only 500 mA output (half its maximum capability), it would dissipate 1.3 W at minimimum. That can be a lot of heat to get rid of in a small and sealed plastic package. You can't just plunk a surface mount part on a board if it's going to dissipate that much power.

Manufacturers of linear regulator ICs started paying attention to the dropout voltage. A whole new class of linear regulators emerged that needed less than a volt, sometimes just a couple 100 mV, of headroom. Of course these technological advances were accompanied by advances in marketing. It was no longer good enough to simply publish a lower headroom spec. This new class of regulator was to be brought forth as a new device altogether, called the [insert trumpet blasts, picture of clouds parting, deep booming voice] Low-Dropout Regulator, henceforth to be know as a LDO.

Of course, being a marketing term, nobody bothered to specify how low exactly "low" is. Marketing has advanced even more since the dawn of the LDO, so nowadays pretty much every regulator everyone makes except anything called a 78xx or 79xx is called a "LDO". That's become the TLA for linear regulators to the point that them new kids (those that never had to walk to school barefoot in the snow uphill both ways, what's this world coming to?!) don't even know the difference anymore.

If I had to pick a number, I'd say anything that can work with less than 1 V headroom can call itself a LDO, preferably 500 mV, but as always, you have to read the datasheet.

Power supplies are largely not made with big-iron transformers anymore where you start with a raw unregulated voltage and linearly regulate it down to the voltage you actually want. Today this would be done with a switcher all in one. However, no matter how you make a regulated voltage, you still have a distribution problem if the device draws significant current. There is also the ripple caused by the switching pulses which can cause trouble with some kinds of analog circuits.

One solution is to have the switcher make a few 100 mV more than you want, then use LDOs at multiple points of use. Let's say you're making local 3.3 V from a 3.8 V pre-regulated bus coming directly from a switcher. At 100 mA, the LDO only dissipates 50 mW, which even a SOT-23 package can do easily. This method not only guarantees that the voltage will be the regulated value despite some drop in the pre-regulated voltage by the time it gets to the point of use, but also produces a nice and clean supply since the local regulator will greatly attenuate the ripple from the pre-regulated supply.

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  • $\begingroup$ "adjust how much current they allow to pass" - no they don't do that. They adjust resistance to match the desired output voltage. Current depends entirely on the load. $\endgroup$ – rustyx Apr 4 at 7:03
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At a very high level linear low-dropout regulators have a pass transistor, an operational amplifier, a resistor divider circuit feeding the operational amplifier and VREF. The operational amplifier is an error amplifier. In short the pass transistor acts as a variable resistor which controls the current flow.

Regulator Block Diagram

Block diagram for linear & low dropout regulator

Dropout voltage = $V_{in}$ - $V_{out}$


Linear Regulator: The pass transistor is a Darlington transistor. The dropout voltage is around 1.5V–2.5V.

Linear Regulator

Basic schematic for linear regulator

Below is dropout voltage specification for LM7805 linear regulator

LM7805 Droput Voltage


Low Dropout Regulator: The pass transistor is a PNP transistor. The dropout voltage is very low, approximately 0.7 V or even lower.

LDO Regulator

Basic schematic for low dropout regulator

Below is dropout voltage specification for LT1965 low dropout regulator

LT1965 low dropout voltage


References:

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    $\begingroup$ You should clarify that a LDO is a linear regulator. Your writeup gives the impression the two are exclusive. Also, using a darlington or a single PNP transistor is not the definition of either regulator. They are only reasons for why some regulators have the dropout voltage they do. Many LDOs use a FET as the pass element, for example. $\endgroup$ – Olin Lathrop Sep 6 '16 at 11:59
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    $\begingroup$ Another good reference: "AN32 - High Efficiency Linear Regulators" by Jim Williams, see Appendix A page 10 "Achieving Low Dropout". It includes an interesting discussion on the pros and cons of the various pass elements. $\endgroup$ – Tut Sep 8 '16 at 14:36
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    $\begingroup$ @OlinLathrop is there a word for "non-low-dropout regulator"? "High dropout regulator? $\endgroup$ – endolith Aug 13 '18 at 16:15
  • $\begingroup$ @end: No. There is no upside for marketeers pointing out their product is not as good as others. A high drop out regulator is any where the mention of low drop out voltage is conspicuously absent. Even when drop out voltage is mentioned, you need to check the actual spec, since there is no formal definition of how low "low" is. $\endgroup$ – Olin Lathrop Aug 14 '18 at 1:14
  • $\begingroup$ @OlinLathrop I don't mean for marketers, I mean for us $\endgroup$ – endolith Aug 14 '18 at 20:26

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