Creating a dual supply with LM voltage regulators - Page 1
Forum section: electronics / beginners

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 08:10:38 am
content: I've been trying to create a dual voltage supply for an op amp application with the circuit attached below that is based on the use of the voltage regulators LM317 and LM337. I start from +12V DC, then stepping down to +10V, which is then split into +5V and -5V. I manage to get +10V at the first stage, but at the second one I get the output voltages just below 5V. Do you know if there is any better choice for the resistor values or maybe there is a general problem in this approach?

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 10:26:57 am
content: Replying to myself. I think it is what it is. If these 4.9V will be used as TTL logic signals, should still be well within the relevant tolerance.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 10:32:59 am
content: The idea behind creating such a dual voltage supply was to power an op amp (single supply is not an option in my application). Here comes the full circuit (please see below). The voltage on the supply rails further goes down to about +-4.5V. Any ideas on how to improve the strategy will be very much appreciated.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 24, 2023, 11:27:42 am
content: Quote from: elki on December 24, 2023, 10:32:59 amBut you're doing exactly just that here. Making a virtual ground but with overcomplicated circuit that won't even work. (but seriously, if it does work I can't see how there won't be a massive current running through the "split" 317/337 outputs)

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 11:31:02 am
content: Is there a better way to create a dual 5V supply from +12 DC?

author: RFDx
author_rank: Regular Contributor
timestamp: December 24, 2023, 11:47:41 am
content: R5 in your schematic doesn't look right at all, try 1.54 kOhm. I had to increase the input voltage to 13V to avoid unresolvable convergence problems.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 11:53:11 am
content: Thanks a lot for your suggestion and trying it out. Are there any considerations to take into account when using this supply with virtual ground to power an op amp (as mentioned above)?

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 24, 2023, 12:23:34 pm
content: Quote from: elki on December 24, 2023, 11:31:02 amHow much current do you intend to source/sink off the virtual ground?For tens of mA, the usual buffered voltage divider should work fine.Note, I pick the feedback values semi-randomly, check your margin.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 24, 2023, 12:27:35 pm
content: One should not have so many regulators in the supply path: the LM317/LM337 have a drop out of some 2 V, depending the current.The way to go is a virtual ground (e.g. some 5 V from the main ground) and than a 10 V regulator if one wants +-5 V for the OPs and not some +7 and -5 or +5 and -7. Many OPs would be OK with such a usymmetric voltage, but it depends if the output levels are OK with this and maybe CMOS switches (e.g. HC4053) may not like it.How much power is needed for the virtual ground depends on the circuit. It can be as little as just a divider and simple OP-buffer. Another option can be 2 x 5 V regulators stacked on top of each other to get the 5 and 10 V levels - if needed with an extra load for the lower 5 V regulator.

author: RFDx
author_rank: Regular Contributor
timestamp: December 24, 2023, 12:29:22 pm
content: Do you really need DC coupling in your 20dB/50 Ohm CF-amplifier? For just one Opamp, a 10V power supply with an artificial gound made of a decoupled resistive splitter would be sufficient.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 12:31:06 pm
content: Thanks. The application needs at least 500mA. I have tried the buffered options, but indeed the max current would be a few 10s of mA.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 12:38:14 pm
content: Quote from: Kleinstein on December 24, 2023, 12:27:35 pmThanks. Would the following circuit be an adequate alternative? I used there LM7805 and LM7905. The problem with this one is that I get -4.4 on the negative rail (the positive one is +5V, as expected).

author: Andy Chee
author_rank: Super Contributor
timestamp: December 24, 2023, 12:49:05 pm
content: Quote from: elki on December 24, 2023, 12:38:14 pmNope. Your circuit will not properly regulate either rail. You really need to use a second opamp to create a virtual ground for your AD8000. If you need to pull lots of current, try a power opamp e.g. LM1875

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 24, 2023, 12:50:41 pm
content: Quote from: elki on December 24, 2023, 12:31:06 pmUse discrete transistor buffer behind the opamp, or use stronger opamp (2402, 0372, BUFxxx?). Honestly at that current level you probably need to start to consider switching regulator.Quote from: elki on December 24, 2023, 12:38:14 pmThat won't work, you can't source /sink current from the "GND" at all

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 01:45:12 pm
content: Thanks a lot for all the feedback. It was extremely useful for me.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 24, 2023, 02:19:02 pm
content: Is there any reason why you can't use an AC input?

author: Jeff1946
author_rank: Regular Contributor
timestamp: December 24, 2023, 04:56:41 pm
content: Here is a circuit I have used. Set first LM317 to twice single sided output voltage.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 24, 2023, 05:01:04 pm
content: i would never use an floating groud / virtual ground everthe problem is : if you have one side who short or consume more thant the other, the ground level may or will shifti you want low drop voltage use ldo ones, nomally they require around 1.5 / 2 volt more on the input vs the output to regulateand if you want to create split voltages form a single one use transformer based small dc-dc converters, don try to reinvent the wheel

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 05:14:54 pm
content: Quote from: Andy Chee on December 24, 2023, 02:19:02 pmNo other reason but the fact that I have a 12V DC adapter. It is a good point that one can switch to an AC one.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 05:17:45 pm
content: Quote from: coromonadalix on December 24, 2023, 05:01:04 pmYes, the voltage level shifting on one of the rails when increasing the load is what I was experiencing so far. Thanks for your suggestion - I will have a look at the transformer-based dc-dc converters.

author: Zero999
author_rank: Super Contributor
timestamp: December 24, 2023, 07:41:20 pm
content: Use a rail splitter, with pass transistors? The only issue is the transient response can be a little ringy. I'd build it, rather than relying on simulation.

author: wraper
author_rank: Unknown
timestamp: December 24, 2023, 08:03:53 pm
content: I rater suggest posting the circuit that should be powered so people can suggest how to avoid using split PSU. Op-amps don't even need split PSU, they do not have GND terminal. Having split PSU just makes the circuit design easier.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 08:07:53 pm
content: Quote from: wraper on December 24, 2023, 08:03:53 pmIndeed, I am now reworking my circuits to use single supplies for op amps. Makes things more robust in a long run.

author: Zero999
author_rank: Super Contributor
timestamp: December 24, 2023, 09:14:14 pm
content: Quote from: elki on December 24, 2023, 08:07:53 pmI forgot to mention that a 12V supply is a little low for an LM317, with 10V out, which is why I used 15V in my simulation.Where is the power coming from? Mains, battery?What does the op-amp circuit you're powering do?Definitely aim for a single power supply configuration if possible.Here are some circuits demonstrating how the most common op-amp topologies can be implemented as a single and dual supply. The ones with the blue text, show all signals referenced to the positive, rather than negative rail, which works better for some op-amps which have better supply rejection on the negative supply.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 09:20:07 pm
content: Thanks a lot - this is exactly what I am doing now.Since we started the discussion with LM317 and others, are there better options for regulators to go from 12V and 10V?For going from 10V to 5V I am considering LM7805.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 24, 2023, 09:28:22 pm
content: The virtual ground part is perfectly OK for a ground with OP-amp circuits. It is however tricky if the halfs are used to power chips with a reduceed voltage. Also with high ground current they are not ideal.It would really help to know the loads and more on the demands for the supply.For going from 12 V to 10 V one would need a regulator with a reltively low drop out, so not soo much voltage headroom needed. One does not yet need a true low drop regulator (e.g. LM2940 series) but an intermediate one would be sufficient (e.g.LM1117, LT1085 and similar smaller ones). These regulator may be a bit picky with the output capacitor.The LM317 would be OK for low current only.

author: Zero999
author_rank: Super Contributor
timestamp: December 24, 2023, 09:44:19 pm
content: Quote from: elki on December 24, 2023, 09:20:07 pmThe LT3080 will work but why? Most op-amps will work perfectly from a 12V supply. Quite often the power supply doesn't even need to be regulated, since the op-amp will filter out supply voltage fluctuations, if the circuit is properly designed.QuoteThat will work, but the LM7805's output can only source current, meaning it must be connected to a load, referenced to a lower voltage than 5V i.e. the load must be connected between 5V to 0V.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 09:50:11 pm
content: Quote from: Zero999 on December 24, 2023, 09:44:19 pmI was just trying to be on the safe side not to approach the absolute maximum ratings. The AD8000 has 12.6V so indeed can work well without a regulation.

author: Zero999
author_rank: Super Contributor
timestamp: December 24, 2023, 10:36:38 pm
content: Quote from: elki on December 24, 2023, 09:50:11 pmYou didn't make it clear you're using the AD8000, in this thread. It's vitally important to make everything clear at the beginning.I agree, it's good to leave a fair margin. It goes back to what you're using to power it. If it's a tightly regulated 12V power supply, which never exceeds 12 by more than a couple of hundred mV, then it's fine, but if it's a car battery, then you certainly do need a voltage regulator do drop the voltage. Another thing to note about the LM317 is, even if it doesn't regulate that well with 12V in and 10V out, it won't matter, since all it's doing is limit the voltage to well below 12V.

author: elki
author_rank: Regular Contributor
timestamp: December 24, 2023, 10:40:37 pm
content: Quote from: Zero999 on December 24, 2023, 10:36:38 pmIndeed, sorry for not making it clear from the beginning. The voltage is supplied by a standard 12V DC adapter, then I step down it to 10V (for op amps) and further down to 5V for other modules with bit logic communication.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: December 25, 2023, 12:19:16 am
content: The LM317 requires at least 3V headroom. It will not operate in regulation from 12V. There are modern LDOs that cam make safely 10V from 12V input, like LM1086, LT1086.

author: David Hess
author_rank: Super Contributor
timestamp: December 25, 2023, 03:06:06 am
content: Power rail splitters are difficult. The best option may be a class-ab emitter follower like that which would be found in an audio output stage. Last time I checked you could still get linear power operational amplifiers for audio at a reasonable price. A two transistor current boosted operational amplifier might be able to do it, but I have not tried it; at some point I just gave up on power rail splitters because they always seem to cause more problems than they solve.I like the idea of using a positive and negative voltage regulator as shown with 2.5 volts of bias. Add a trim to the bias voltage and lower or remove the output resistors for lower output impedance. Maybe it will work but check that it does not thermally runaway over the operating temperature range.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 25, 2023, 03:29:25 am
content: Quote from: David Hess on December 25, 2023, 03:06:06 amThe "problem" with that approach is that it relies on the three voltage references being in sync. LM317/337 are traditionally specced for 1.2-1.3 Vref. Having the Vref on the lower half will gets you shoot through current between the two regulators. Theoretically about 50mA with 2 ohm total resistance on the output. This might actually beneficial for load regulation, kinda similar to class AB operation. On the other hand if your regulator references are on the high side you'll get a dead band on the middle rail for ~100 mV (at worst) where you can't source or sink any current.Of course the counter is that on practice modern LM317/337 Vref tolerances are actually much better than the traditional spec.

author: David Hess
author_rank: Super Contributor
timestamp: December 25, 2023, 03:49:42 am
content: Quote from: ArdWar on December 25, 2023, 03:29:25 amThat is why I said to add an adjustment trimmer to the bias voltage. Adjust the nominal 2.5 volt reference to the combined outputs of the 317/337. But depending on the temperature tracking, this may not work.When Linear Technology released the LT1086 series, they made of big point of their tighter internal reference allowing operation in parallel with much lower values of ballast resistance; a printed circuit board track can be enough.Adding a couple of operational amplifiers can remove the variation of the 317/337 references for perfect tracking, but with that much complexity there are probably better ways.

author: elki
author_rank: Regular Contributor
timestamp: December 25, 2023, 09:57:25 am
content: The LM saga continues. I've been trying to create a basic 12V to 10V converter in LTspice using the official PSpice TI trans model for the adjustable version of LM1117. I wonder if this model is incompatible with LTspice because it gives me something around 1V at the output. When I use the built-in LT1117, the output voltage is at 10V, as expected. Do you have any experience with making the LM voltage regulators working in LTspice?

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2023, 11:11:08 am
content: Quote from: dobsonr741 on December 25, 2023, 12:19:16 amThat's not strictly true. The LM317 will work with a less than 3V of headroom. It's just most of the characteristics on the data sheet are specified with an input-output differential of greater than 3V. It will work at lower voltage differentials, but not so well.The actual drop-out voltage depends on the current and temperature. The original poster is only using it at 500mA, so it will be under 2V, except at extremely low temperatures. It probably won't meet the other characteristics i.e. load regulation, but that doesn't matter, since the op-amp's supply rejection will handle it. Heck I doubt it matters even if it goes into drop-out: the op-amp will still workhttps://www.onsemi.com/download/data-sheet/pdf/lm317-d.pdfQuote from: elki on December 25, 2023, 09:57:25 amI would take simulation results with a large grain of salt. Try building the LM317 version first. Connect a 20 Ohm load resistor to it and if the voltage is steady, even if it drops a bit, then it's good enough.

author: jonpaul
author_rank: Super Contributor
timestamp: December 25, 2023, 12:34:08 pm
content: Use LDO: Low dropout regulators.Use a positive and negative reg pair.See the many app notes from National Semi, Linear Technology, TI on theui topicj