What is "floated" linear voltage regulator?

[David Hess]

For regulators which can be configured either way, it means that the regulator's quiescent current flows into the output instead of to ground; the output becomes "common" for purposes of the circuit design.  This allows voltages in excess of the regulator's breakdown voltage to be controlled as long as the maximum input-to-output voltage is not exceeded.  The 723 application notes show examples of both.

Contrast the LM7805 which is not a floating regulator and has its quiescent current flow out through its "common" pin with the LM317 which is a floating regulator and has its quiescent current flow out through its output pin.

[Zero999]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used.

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

That's not what I said at all. I said "high resistance to ground", not impedance. I also said "to ground", not to 0 volts. The output impedance between the output terminals of a power supply should be low but we generally want the resistance to ground of a bench supply to be high or infinite. This gives us the ability to wire one supply in series with another without shorts and this is what is done with dual supplies where they can be switched for either series or parallel operation. Supplies floating with respect to ground allow us to do this or a single output supply to be wired either + or - with respect to ground.

If you look at the schematic 001 posted in post #20 to show this is what he meant. You will see this in the lower right hand corner of the schematic as shown below. The circuit has high resistance but low impedance. With a capacitor to ground the 'impedance' decreases with increasing frequency.

You appear to be confused about the context of the word "ground" in the title of the thread. For the purposes of discussing an electrical circuit, the word ground refers to 0V. Whether it is connected to the physical earth or not is immaterial. Ground is purely a reference point in circuit from where all voltages are measured from.

A floating linear regulator is a circuit who's common rail is at a different  potential to that of the rest of the circuit. The LM317 is a floating linear regulator because it has no ground pin. The adjust pin connects to the centre of a potential divider, thus it floats at a higher voltage than the circuit's ground or reference. The LM7805 is not a floating regulator because its ground pin is normally connected to the same reference as the rest of the circuit.

Yes, any bench power supply should be isolated or floating with respect to mains ground, but this has absolutely nothing to do with the definition of a floating linear regulator, such as the LM317 and is completely off-topic.

[not1xor1]

BTW probably it would be more appropriate to define it bootstrapped rather than floating.

"Bootstrapped" usually refers to a supply voltage above the maximum voltage level in the system, stored in a capacitor that gets charged while the output swings lower. It's a classic technique in some audio amplifiers, and often used in MOSFET driver circuitries. Bootstrapping won't work on a stable DC output (which is desired for a lab power supply).

In this case it is the control circuit (opamp) supply which is bootstrapped by the positive output voltage (nothing forbids to use a PNP pass transistor and bootstrap to the negative rail).
Usually that is achieved by a separate opamp supply whose 0V rail is tied to the positive output rail, but you might use an higher voltage rail and separate regulators to get 2 constant voltage positive/negative rails tied to the output rail.

addendum 
I apologize. I think you're right and although the terms "floated" and "bootstrapped" might be interchangeable in this case it might be indeed more appropriate to define the circuit as floating.

[not1xor1]

Here is a circuit that replicates LM317 topology.
This is a proof of concept, not a real circuit, so I used a voltage source rather than a real voltage reference.

Unlike what I foolishly wrote a few days ago in another thread, the adjust resistor does change the loop gain.
So the phase margin of a real LM317 is probably at its worst when the adjust pin is directly attached to the circuit ground (pls. notice the difference between ground and earth).

The effect of the noise suppressing capacitor (then one in parallel to the adjust resistor) is to keep high frequency loop gain constant (see the bottom picture), so regardless of the value of the adjust resistor the phase margin is always at its worst.



here are output voltage and adjust resistor current when stepping the resistor from 1mΩ to 1.2kΩ



here is the AC analysys. The phase margin of this circuit is just 47° when the adjust pin is connected to ground (adjust resistor 1mΩ)



I highlighted the last step (1.2kΩ resistor) where the phase margin is about 80°



This is the effect of 10µF capacitor in parallel to the adjust resistor.

[David Hess]

Unlike what I foolishly wrote a few days ago in another thread, the adjust resistor does change the loop gain.
So the phase margin of a real LM317 is probably at its worst when the adjust pin is directly attached to the circuit ground (pls. notice the difference between ground and earth).

That is right and why the output capacitor stability requirements are greater when the gain resistor is bypassed to ground for lower noise.

[spec]

Wow, a lot of posts since I was here last.

Attached is the schematic promised in reply #41 that shows simplified circuits for a 'conventional' 0V referenced PSU and a floating PSU. In both cases the feedback is shown. Notice that in both circuits the opamps non-inverting and inverting inputs are at precisely the same voltages. With precision resistors the circuits could form precision voltage references, with an output current capability of around 40ma, as subsequently mentioned in reply #57 by Silicon Wizard.

I have indicated actual components as a guide, so you can check details on the appropriate datasheets (links below). Note that the 1V25 reference diode runs at only a few micro amps. This has no significance in schematic #1, because the reference diode current just flows to 0V, but in schematic #2 the voltage reference current flows down the feedback resistor chain so it is important to keep the voltage reference current small compared to the tail current, which is a whopping 12.5mA. In some three terminal regulators the voltage reference current is made constant though.

Once you have digested these two circuits, you may like to have a look at current referenced floating supplies.

Also the opamp is a rail to rail input/output (RRIO) type, which means that its output can swing to its positive and negative rails. It also means that it can function as an opamp with an input voltage ranging from its negative rail to its positive rail. This characteristic has opened the way for a whole raft of circuit designs that could not be achieved with the older 741 type opamps, who's inputs and outputs could only get within a couple of volts of their negative and positive rails. It also explains why the opamp's rather odd-looking negative rail arrangement in schematic #2 works.

I have also indicated voltages in both circuits. If you have any questions just ask.

https://datasheets.maximintegrated.com/en/ds/MAX6006A-MAX6009B.pdf
http://www.ti.com/lit/ds/sbos620e/sbos620e.pdf

[spec]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used.

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

That's not what I said at all. I said "high resistance to ground", not impedance. I also said "to ground", not to 0 volts. The output impedance between the output terminals of a power supply should be low but we generally want the resistance to ground of a bench supply to be high or infinite. This gives us the ability to wire one supply in series with another without shorts and this is what is done with dual supplies where they can be switched for either series or parallel operation. Supplies floating with respect to ground allow us to do this or a single output supply to be wired either + or - with respect to ground.

If you look at the schematic 001 posted in post #20 to show this is what he meant. You will see this in the lower right hand corner of the schematic as shown below. The circuit has high resistance but low impedance. With a capacitor to ground the 'impedance' decreases with increasing frequency.

You appear to be confused about the context of the word "ground" in the title of the thread. For the purposes of discussing an electrical circuit, the word ground refers to 0V. Whether it is connected to the physical earth or not is immaterial. Ground is purely a reference point in circuit from where all voltages are measured from.

A floating linear regulator is a circuit who's common rail is at a different  potential to that of the rest of the circuit. The LM317 is a floating linear regulator because it has no ground pin. The adjust pin connects to the centre of a potential divider, thus it floats at a higher voltage than the circuit's ground or reference. The LM7805 is not a floating regulator because its ground pin is normally connected to the same reference as the rest of the circuit.

Yes, any bench power supply should be isolated or floating with respect to mains ground, but this has absolutely nothing to do with the definition of a floating linear regulator, such as the LM317 and is completely off-topic.

I do not understand what you are trying to say here. Who is confused about what?

What is the relevance of mentioning that a PSU can be isolated from the mains?

Please clarify so your view can be understood and discussed.

[SiliconWizard]

Attached is the schematic promised in Answer #41 that shows simplified circuits for a 'conventional' ground referenced PSU and a floating PSU. In both cases the feedback is shown.
(...)

I may have missed something, but the OPA192 has limited current output capability so, a PSU yes, but with something like 50 mA max. probably... if it's not further buffered. Just a thought!

[spec]

Attached is the schematic promised in Answer #41 that shows simplified circuits for a 'conventional' ground referenced PSU and a floating PSU. In both cases the feedback is shown.
(...)

I may have missed something, but the OPA192 has limited current output capability so, a PSU yes, but with something like 50 mA max. probably... if it's not further buffered. Just a thought!

No, you have not missed anything. Your observation about the current limitation is correct. There are missing functions too, like decoupling, compensation, and current limiting. But the purpose of the schematics is to illustrate the two feedback techniques which the OP was asking about. If I did a full design you would not be able to see the wood for the trees.

Just for a bit of fun, I did have a go at a 1A version with current limiting and all the twiddly bits. 

[spec]

Here is a circuit that replicates LM317 topology.
This is a proof of concept, not a real circuit, so I used a voltage source rather than a real voltage reference.

Unlike what I foolishly wrote a few days ago in another thread, the adjust resistor does change the loop gain.
So the phase margin of a real LM317 is probably at its worst when the adjust pin is directly attached to the circuit ground (pls. notice the difference between ground and earth).

The effect of the noise suppressing capacitor (then one in parallel to the adjust resistor) is to keep high frequency loop gain constant (see the bottom picture), so regardless of the value of the adjust resistor the phase margin is always at its worst.



here are output voltage and adjust resistor current when stepping the resistor from 1mΩ to 1.2kΩ



here is the AC analysys. The phase margin of this circuit is just 47° when the adjust pin is connected to ground (adjust resistor 1mΩ)



I highlighted the last step (1.2kΩ resistor) where the phase margin is about 80°



This is the effect of 10µF capacitor in parallel to the adjust resistor.

Very interesting and informative. Thanks for posting

[Zero999]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used.

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

That's not what I said at all. I said "high resistance to ground", not impedance. I also said "to ground", not to 0 volts. The output impedance between the output terminals of a power supply should be low but we generally want the resistance to ground of a bench supply to be high or infinite. This gives us the ability to wire one supply in series with another without shorts and this is what is done with dual supplies where they can be switched for either series or parallel operation. Supplies floating with respect to ground allow us to do this or a single output supply to be wired either + or - with respect to ground.

If you look at the schematic 001 posted in post #20 to show this is what he meant. You will see this in the lower right hand corner of the schematic as shown below. The circuit has high resistance but low impedance. With a capacitor to ground the 'impedance' decreases with increasing frequency.

You appear to be confused about the context of the word "ground" in the title of the thread. For the purposes of discussing an electrical circuit, the word ground refers to 0V. Whether it is connected to the physical earth or not is immaterial. Ground is purely a reference point in circuit from where all voltages are measured from.

A floating linear regulator is a circuit who's common rail is at a different  potential to that of the rest of the circuit. The LM317 is a floating linear regulator because it has no ground pin. The adjust pin connects to the centre of a potential divider, thus it floats at a higher voltage than the circuit's ground or reference. The LM7805 is not a floating regulator because its ground pin is normally connected to the same reference as the rest of the circuit.

Yes, any bench power supply should be isolated or floating with respect to mains ground, but this has absolutely nothing to do with the definition of a floating linear regulator, such as the LM317 and is completely off-topic.

I do not understand what you are trying to say here. Who is confused about what?

What is the relevance of mentioning that a PSU can be isolated from the mains?

Please clarify so your view can be understood and discussed.

That was my point: the fact the PSU is isolated from the mains is irrelevant.

My reply wasn't aimed at you, but someone else who seemed to have mixed up ground as in the earth, with 0V and floating, as in not being connected to the soil, with a floating regulator, such as the LM317. I've highlighted the part where I believe the confusion occurred.

Attached is the schematic promised in Answer #41 that shows simplified circuits for a 'conventional' ground referenced PSU and a floating PSU. In both cases the feedback is shown.
(...)

I may have missed something, but the OPA192 has limited current output capability so, a PSU yes, but with something like 50 mA max. probably... if it's not further buffered. Just a thought!

No, you have not missed anything. Your observation about the current limitation is correct. There are missing functions too, like decoupling, compensation, and current limiting. But the purpose of the schematics is to illustrate the two feedback techniques which the OP was asking about. If I did a full design you would not be able to see the wood for the trees.

Just for a bit of fun, I did have a go at a 1A version with current limiting and all the twiddly bits. 

As far as doing a rough simulation of the LM317 is concerned. I prefer to use the generic op-amp model, as its parameters can be edited and is faster to simulate. Here's my basic LM317 simulation. I added in the current limiting but didn't include safe operating area protection. I agree this is only to show the basic concept and not a practical design.

[spec]

Ahh apologies H99. I did not get your point.