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| Virtual ground circuit |
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| aheid:
I was looking for a way to split the PSU rails, ie introduce a virtual ground for my circuit, and stumbled over this design using an LM317 and LM337 pair: http://www.goldpt.com/virtual_ground_circuit.html I have a couple of questions regarding the circuit: 1. Why the 1 Ohm resistors on the output of the regulators? The page says these resistors can be lowered by reducing the current through the 2.5V reference, but I don't get their purpose or their connection to the voltage reference current. 2. What about the minimum load the LM317/LM337 requires to maintain regulation? If the load is asymmetric, then one of the regulators will essentially not be needed and thus, as far as I can see, the circuit is unable to ensure the minimum load current for the "idle" regulator. What will happen? I don't have any 2.5V voltage references handy, or I'd wire it up and see. For my project I was hoping to be able to use a 19-20V laptop PSU, boost to 36V, and then split to +/- 18V rails capable of handling up to 1A. If I could lower the 1 Ohm resistors and the minimum load current is not an issue (or can be handled) then this looks like it should be capable of that. If not, any tips for alternative, or better, circuits? |
| bitbanger:
This is far from a technical answer and admittedly more of a hunch, so take this for what it's worth. I suspect the 1ohms are what's sometime referred to as "ballast", or ballancing resistors. The amount of regulation error you get out of a zener is related to the ratio of current you pass through it, serially, and the current that is passed off to the 'load'. You want the serial current to be an border of magnitude larger, so that load current fluxuations don't influence the regulation. The adust pins on the LMs have a certain amount of error current passing into them, and so based on the external zener current, it may influence the "regulation" of that zener and subsequently the current into the ADJ. In any case if the ADJ currents were slightly mismatched, and the two out pins were tied directly together, the regulators would fight one another to maintain regulation. The 1ohm, especially considering the wattage spec'd, is intended to ballance any disparity at the output by dropping it across one or both of those 1ohms. Would welcome some technical insight by someone who may know a little more detail. Again, on my phone, off the cuff hunch. Best of luck. |
| spec:
Hi aheid --- Quote from: aheid on January 09, 2019, 03:43:05 am ---I was looking for a way to split the PSU rails, ie introduce a virtual ground for my circuit, and stumbled over this design using an LM317 and LM337 pair: http://www.goldpt.com/virtual_ground_circuit.html I have a couple of questions regarding the circuit: 1. Why the 1 Ohm resistors on the output of the regulators? The page says these resistors can be lowered by reducing the current through the 2.5V reference, but I don't get their purpose or their connection to the voltage reference current. 2. What about the minimum load the LM317/LM337 requires to maintain regulation? If the load is asymmetric, then one of the regulators will essentially not be needed and thus, as far as I can see, the circuit is unable to ensure the minimum load current for the "idle" regulator. What will happen? I don't have any 2.5V voltage references handy, or I'd wire it up and see. For my project I was hoping to be able to use a 19-20V laptop PSU, boost to 36V, and then split to +/- 18V rails capable of handling up to 1A. If I could lower the 1 Ohm resistors and the minimum load current is not an issue (or can be handled) then this looks like it should be capable of that. If not, any tips for alternative, or better, circuits? --- End quote --- Quite a clever rail splitter you have found. You ask what the two 1R resistors are for: here is my explanation. The rail splitter works by setting the reference input of the LM317 and LM337 to VCC/2 -1V25 and VCC/2 + 1V25 respectively. This is the essence of the design, and it is also a critical area because there is a voltage tolerance issue concerning LM137 Vref (Vref1), LM337 Vref (Vref2) and the LM336 Vref (Vref3). The circuit will only work if Vref3 is equal to or greater than Vref1 + (-1 * Vref2). If this condition is not met, there will be insufficient voltage between the LM317/LM337 output pins and reference pins, so that the LM317/LM337s will be in an indeterminate state. The first order tolerances taken from the datasheets are: Vref1 and Vref2: 1V2 to 1V3 (combined 2V4 to 2V6) Vref3: 2V39 to 2V59 So you can see that the designer has been careful to match Vref3 (LM336) to the combined voltages of the LM317/LM337 Vrefs: just 10mV off at the worst case extremes of all three voltages. But there is another effect of the above reference voltage tolerances: the LM317 and LM337 will each be trying to hold their output pins to different voltages. Taking the worst case from the first order values above, the difference between the LM317/LM337 outputs will be 2V59- 2V4 = 0.19V. In that case the current flowing through the two 1R resistors will be 0.19V/2 = 0.095A. Without the two 1R resistors the LM317 and LM337 would fight with each other, poring in more and more current, until they reached a current or thermal limit, when the output voltage would be indeterminate. So that is what the two 1R resistors are for. :) http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf http://www.ti.com/lit/ds/symlink/lm336-2.5-n.pdf |
| David Hess:
--- Quote from: aheid on January 09, 2019, 03:43:05 am ---I was looking for a way to split the PSU rails, ie introduce a virtual ground for my circuit, and stumbled over this design using an LM317 and LM337 pair: http://www.goldpt.com/virtual_ground_circuit.html --- End quote --- That is clever. DC accuracy at high currents will be poor but a shift of 1 volt/amp is acceptable for many circuits. This circuit might be thought of as a class-ab output stage using voltage regulators instead of transistors. --- Quote ---1. Why the 1 Ohm resistors on the output of the regulators? --- End quote --- The output resistors limit the current when the reference voltage does not quite match the regulator voltages. --- Quote ---2. What about the minimum load the LM317/LM337 requires to maintain regulation? --- End quote --- Without the minimum load, the output of the regulator will rise but that is not a problem here with the other regulator ready to pull it down through the output resistors. With a floating regulator like the 317/337, a resistor can always be placed between the output and adjustment pin to provide a constant minimum load current if there is no requirement for low adjustment bias. --- Quote ---For my project I was hoping to be able to use a 19-20V laptop PSU, boost to 36V, and then split to +/- 18V rails capable of handling up to 1A. If I could lower the 1 Ohm resistors and the minimum load current is not an issue (or can be handled) then this looks like it should be capable of that. If not, any tips for alternative, or better, circuits? --- End quote --- If the reference voltage could be trimmed, then the value of output resistors could be lowered. The reference could be replaced with another LM317 configured as a constant current source and an adjustable resistor but there is something good to be said about circuit which cannot be misadjusted. Perhaps better would be to add a single operational amplifier to provide feedback and hold the output exactly where desired. Then the value of the output resistors becomes less important. This circuit will be difficult to beat from a price/performance perspective at high currents. Unlike a power transistors boosted operational amplifier design, the integrated regulators provide thermal protection and current limiting. Note that integrated class-ab audio amplifiers make pretty good high current rail splitters. I am not a believer in the audio qualities of rail splitters for the simple reason that if the rail splitter matters, then the audio amplifier has something wrong with it. For the same reason, the power stages of an audio amplifier should not require a regulated low noise power supply. |
| Audioguru:
Usually the (+) input of an opamp or power amplifier is at 0VDC and draws VERY low current. Then the rail splitter does not need high power ICs, instead the rail splitter is two medium value series resistors across the supply and their junction is 0V. |
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