Adjustable from 0V power supply ideas  - Page 1
Forum section: electronics / projects

author: ataradov
author_rank: Super Contributor
timestamp: November 01, 2023, 06:05:45 am
content: I'm working on a project and I need an adjustable power supply. Nothing special 250 mA max load (150 mA would suffice too). The input would be USB 5V. Adjustment is though a DAC from an MCU.The issue is that I need full range adjustment from 0 V to ~3.5V. This eliminates traditional adjustable LDOs without negative voltages.I'm just checking if anyone is aware of some obscure parts that would fit those requirements.If anyone has known good designs based on the adjustable LDOs, I'll take those too, although I have a general idea what is needed there.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 01, 2023, 06:40:52 am
content: There is a family of precision current based regulators, that have very low noise and naturally hit 0V at R=0.The trade off is they are not low cost.An alternative is a tracking regulator.

author: ataradov
author_rank: Super Contributor
timestamp: November 01, 2023, 06:42:46 am
content: This is for a very low volume project. If something fits the requirements and total cost is less than $15-20 - I don't care.

author: magic
author_rank: Super Contributor
timestamp: November 01, 2023, 06:43:50 am
content: Yeah, LT3080 or similar if you want a fully integrated voltage follower with protections.You can ignore the reference current source, provided you are able to sink its current.

author: ataradov
author_rank: Super Contributor
timestamp: November 01, 2023, 07:02:50 am
content: LT3080 looks interesting. I'll have a closer look at it tomorrow.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 01, 2023, 07:23:20 am
content: Quote from: ataradov on November 01, 2023, 06:42:46 amThis shows ADi's current reference regulators :https://www.analog.com/en/parametricsearch/11459#/p5417=Adjustable%20To%20Zero&sort=s3,ascThe GM1200 at lcsc is the same topology.(Addit: tho I see they have lower dropout, and spec a minimum V(Iset) > 1.5V, so maybe they chose Vcc over GND for common mode range)TI have versions called TPS7A96, TPS7A94A part like ADP1716, ADP1707 have a tracking pin that can adjust to 0V.You can get almost any regulator to go to 0V, if you inject a higher-reference-point current into the feedback node.ie Adding a 2.5V reference, can inject into a 1.25 or 0.8 or 0.6V reference feedback regulator.

author: ataradov
author_rank: Super Contributor
timestamp: November 01, 2023, 05:19:02 pm
content: ADP1716 looks pretty much perfect for what I need. I'll design around and see how it works in practice.

author: tooki
author_rank: Super Contributor
timestamp: November 01, 2023, 08:51:12 pm
content: I’ve used the LT3045, very nice part. (As is the negative counterpart, the LT3084.)

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 01, 2023, 09:36:13 pm
content: Why not generate a small negative voltage?

author: magic
author_rank: Super Contributor
timestamp: November 01, 2023, 09:54:36 pm
content: Why generate a small negative voltage if one-chip rail to rail power buffers exist?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 01, 2023, 09:57:07 pm
content: Quote from: magic on November 01, 2023, 09:54:36 pmSure, how rail-to-rail is that exactly? (Reminds me of an older thread.)

author: ataradov
author_rank: Super Contributor
timestamp: November 01, 2023, 10:07:45 pm
content: I don't want to generate negative voltages if I can avoid it. And it looks like I can avoid it.I'm not too concerned if it is a few mV away from the rails on either end. The high side would be nice to have a bit higher, but simplicity wins and ADP1716 goes on the board. The first version of the board wold be a test anyway, if it does not work as expected, I'll have to do something else.

author: magic
author_rank: Super Contributor
timestamp: November 01, 2023, 10:15:15 pm
content: ADP1716 tracking accuracy spec is a bit loose at ±100mV, the LTs are better in this regard.Let's hope it's better than that in practice

author: ataradov
author_rank: Super Contributor
timestamp: November 01, 2023, 10:30:12 pm
content: In my case it will only be used at room temperature. And I hope this is not their sneaky way to be able to specify "0V" when it is always 100 mV.The parts are on a way anyway, so I'll prototype as soon as they arrive.

author: tooki
author_rank: Super Contributor
timestamp: November 02, 2023, 05:58:21 pm
content: Quote from: ataradov on November 01, 2023, 10:30:12 pmHuh? The ADP1716 expressly goes down only to 0.75V. (I do see that the text later claims it can go down to 0, but it’s very odd that they list two contradictory values.) Why would you choose this over one of the options (like the LT3045 that I mentioned) that expressly and unambiguously go down to 0V? (The 3045’s data sheet even explains how they do it.)Edit: never mind, I misread the ADP1716 data sheet.

author: magic
author_rank: Super Contributor
timestamp: November 02, 2023, 06:15:11 pm
content: No idea where you got that from. Look up "Vout to Vtrk accuracy (ADP1716)" in the specs table: it clearly states ±100mV from zero to half of the nominal output.I would be worried more about what happens above 50% of nominal...

author: ataradov
author_rank: Super Contributor
timestamp: November 02, 2023, 06:34:18 pm
content: Quote from: tooki on November 02, 2023, 05:58:21 pm0.75 V is one of the available fixed voltage options. In practice only 3.3 V devices are available anyway.

author: ataradov
author_rank: Super Contributor
timestamp: November 02, 2023, 06:36:11 pm
content: Quote from: magic on November 02, 2023, 06:15:11 pmI'd expect it to be better. It is obviously harder to track at low voltages, so they specified the worst region. But we will know soon.

author: Zero999
author_rank: Super Contributor
timestamp: November 02, 2023, 06:43:44 pm
content: Quote from: ataradov on November 01, 2023, 07:02:50 amHow does it need to be adjusted: potentiometer, DIP switch, DAC?Is there always a load present? The LT3080 has a minimum load requirement, so a constant current sink will be required, if there's no minimum load. A resistor won't do because it won't work down to 0V. Use a current mirror, or if you're cheap, a single BJT, with a high value resistor to VINi.e. omit Tr1 on the schematic shown below.

author: ataradov
author_rank: Super Contributor
timestamp: November 02, 2023, 06:48:17 pm
content: Quote from: Zero999 on November 02, 2023, 06:43:44 pmDAC from the MCU and the load is not guaranteed to be there and will be variable.I'm not using LT3080 partly because of the load requirement, partly because a dedicated enable input is a huge plus.I'm trying to use ADP1716. If that performs close to reasonable, I'll be done. Even with 100 mV offset, given that there is an enable input, I would be fine with the first step being 100 mV. Hopefully practical offset is lower, of course.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 02, 2023, 10:12:37 pm
content: Quote from: Zero999 on November 02, 2023, 06:43:44 pmA normal BJT as current mirror has some minimum voltage of some 20-50 mV (depends on the type). One can get a little better with special types or swapping emiiter and collector and thus using it in reverse.

author: tooki
author_rank: Super Contributor
timestamp: November 03, 2023, 09:49:41 am
content: Quote from: magic on November 02, 2023, 06:15:11 pmQuote from: ataradov on November 02, 2023, 06:34:18 pmOh, you guys are correct, I misread the datasheet. My bad!

author: tooki
author_rank: Super Contributor
timestamp: November 03, 2023, 10:02:17 am
content: Quote from: Zero999 on November 02, 2023, 06:43:44 pmThat’s something that’s nice about the LT3045: minimum load is only 10μA (and no minimum at all when the output is above 1V.That and the fact that setting the voltage is super easy: a single set resistor. Just take your desired output voltage and multiply it by 10000 and you have your resistor value. The current limit is set similarly with a single resistor.I’ve been meaning to build a small low-noise power supply based on it.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 03, 2023, 03:11:42 pm
content: If the load does not have to sit on USB ground the circuit in the attachment works well also from 0V.

author: JohanH
author_rank: Frequent Contributor
timestamp: November 03, 2023, 03:33:43 pm
content: The LT3080 data sheet says:QuoteOh, very good. But yes, you need:QuoteThen later in Applications Information:QuoteOh, so now it's 1 mA, not 0.5 mA?So how do you neatly create a minimum load current? Supposed you have a negative supply. Using a depletion mode Mosfet and a resistor? I'm trying to wrap my head around this, how is it supposed to be connected to the negative supply?Edit: I think I got it. So you just have to have a separate negative supply, connect it's ground to the other supply ground. Then sink 1 mA current from the regulator output to the negative supply. Even if the regulator output is at 1 pV (relative to ground), 1 mA current should still flow to the negative supply. Right?

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 03, 2023, 08:07:27 pm
content: Quote from: JohanH on November 03, 2023, 03:33:43 pmHere's the thing--how low do you want to be able to go without a negative supply? Do me a favor and find a list of linear regulator ICs that could get anywhere close to zero before the LT3080 came around. No using a negative supply, that's cheating. Don't even add in there what you want for transient performance down at those levels, you have to get fairly low resistor values to discharge your output cap.The LT3080 opened the way to products like the LT3045 and a host of others that offer some incredible performance advantages. Need less than 1uVrms of noise from your LDO in the 10Hz-100kHz bandwidth? Up to 1A load or beyond? Maybe you want similar stuff on the negative side. Nobody had these products before LTC created the first ones. The first products released in a new architecture and family aren't always the best, there's always places that improvement can be made later. Sometimes you just need to get those first revolutionary devices into production and later add the iterations that improve performance.The standard linear regulator architecture of gaining up a reference voltage (giving a minimum output voltage) really didn't change for over thirty years, then came the LT3080 family going in a radically different direction. How many different companies are now putting out parts using this new architecture?

author: ataradov
author_rank: Super Contributor
timestamp: November 03, 2023, 08:11:31 pm
content: My issue with LT3080 and its statement "For true zero voltage output operation" is that they never clarify what is "true zero voltage" and how not true it would be if I don't do that.Also, if you have a negative supply, any old LDO can do 0V output. So, the distinction of what is true 0V and what is not matters here.

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2023, 08:55:38 pm
content: Quote from: Kleinstein on November 02, 2023, 10:12:37 pmI would consider 50mV to be close enough for most applications. As it's only 5V in this instance, a reversed BJT would work quite well. I wouldn't even bother with a current mirror, just connect the base to +5V, via a resistor.

author: JohanH
author_rank: Frequent Contributor
timestamp: November 03, 2023, 09:01:46 pm
content: The new LT3041 looks nice, basically similar to LT3042 and LT3045, but with 1 A output. Lowest output voltage is 0.2 V, though.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 03, 2023, 09:04:59 pm
content: Quote from: ataradov on November 03, 2023, 08:11:31 pmOnce you've been doing electronics for a number of years, you learn where the engineers talk and where the marketing people talk. When engineers talk, there is always the thought in their mind about min/max/typical values and the statistical range that is important. Tell me you want a regulator to output 3.3V and one of the first things I'll want to know is what accuracy you want. Is it important to be 3.300000V +/-1uV? Or is a 3% specification acceptable? That says you want something that will give you from 3.2V to 3.4V range. Maybe 1%? That's 3.267V to 3.333V.So my question to you will come down to this: how close to zero do you need this to be? What are you running from it? You're going to have a hard time actually turning on any active circuitry below a few hundred mV. Do you need it to be 200mV? 100mV? 50mV? 1mV? 1uV? 1pV? It's all a matter of significant digits, and the more digits you throw on there the more you should be putting this in the metrology forum instead.You're right, the true distinction of what is true zero and what is not matters here. What is YOUR definition for it versus what the marketing people made the engineers put on the LT3080 data sheet?

author: JohanH
author_rank: Frequent Contributor
timestamp: November 03, 2023, 09:08:36 pm
content: Quote from: AnalogTodd on November 03, 2023, 08:07:27 pmSure, they are very impressive regulators. I've used LT3045 before. And I'm planning other projects with the LT3080 and LT3041. I just didn't like the way it was said in the data sheet. Your insight is appreciated.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 03, 2023, 09:19:22 pm
content: Quote from: ataradov on November 03, 2023, 08:11:31 pmSince you are driving with a DAC, you do not even need a negative supply for the common VREF regulators.If I take a common 1.24V ref, like LM29152RS HTC Korea TO-252-5 ADJ and use a very simple 3 resistor T biasing,spice says for a 0-3.3V DAC swing :R1 = R3 = R3 = 10k gives 420.88403mV to 3.7199712V regulator output rangeR1 = 9.1k gives 224.69488mV to 3.8426077VR1 = 8.275k gives 9.5774748mV to 3.9784591V (that 9.5mV will depend on pulldown )

author: ataradov
author_rank: Super Contributor
timestamp: November 03, 2023, 09:26:17 pm
content: Quote from: AnalogTodd on November 03, 2023, 09:04:59 pmIt does not matter what I want, I will pick a device that meets my needs based on the datasheet. This datasheet is crap because it does not operate with numbers when it comes to "true zero".ADP1716 very clearly specifies that their tracking down to 0 V is really +/-100 mV. All other things being equal, I will at least start with a device that operates with numbers.

author: ataradov
author_rank: Super Contributor
timestamp: November 03, 2023, 09:32:18 pm
content: Quote from: PCB.Wiz on November 03, 2023, 09:19:22 pmI don't understand how that would work. Can I see that spice model?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 03, 2023, 09:33:37 pm
content: Quote from: AnalogTodd on November 03, 2023, 08:07:27 pmNot many, and none are 'low cost'.Maybe the precision current needs a lot of area and trimming ?If you wanted LDO and true 0V, the opamp would need to be rail-rail in, with low offset voltage.The TI TPS7A94 have low dropout, but lower VMAXHeadline says• Low dropout: 150 mV at 1 A• Wide input voltage range: 1.7 V to 5.7 V• Wide output voltage range: 0 V to 5.5 Vbut later they say6.3 Recommended Operating Conditions over operating junction temperature rangeVIN Input supply voltage range 1.7 ~ 5.7 VVOUT Output voltage range 0.4 ~ (VIN - VDO) VIOUT Output current 0 ~ 1Aand they spec the 150uA precision current at 1% for 1.2 V ≤ VOUT ≤ 5.1 V, & Vos typ 100uVfor 0.4 to 1.2V it is 1.5% & Vos typ 200uV, and not defined < 400mVFor DAC drive, that may not matter ?so how precise it is below 400mV is looking like it needs a bench test.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 03, 2023, 09:38:17 pm
content: Quote from: ataradov on November 03, 2023, 09:32:18 pmSure, attached, for LT spice, sim file and images.If you drive the VFB with a pullup, the LDO feedback output will drop below VREF to keep balance.Some small pulldown is needed to get to millivolts out, as some uA comes from the feedback resistor, with one end at 1.24V.eg If the regulator has bias current of 40nA, then 250k is 10mV offset error, and 250k injects ~ 5uA pulled to 0V (2k will pull that to +10mV)The precision at 0V is the difference between the 1.24V ref, and the divided DAC, so your DAC precision matters too.There are solutions for lower reference voltages too, the DAC is simply more divided.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 03, 2023, 09:44:15 pm
content: Quote from: ataradov on November 03, 2023, 09:26:17 pmIts all about physics. All "rail to rail" circuits cannot sink or source significant currents close to the rail, and regulation will be lousy or completely lost. Guess where "minimum load" requirements come from ? Its the saturation voltage of transistors trying to pull an output to ground. True "from zero" outputs simply dont work properly without (can be small) negative headroom.

author: ataradov
author_rank: Super Contributor
timestamp: November 03, 2023, 09:49:47 pm
content: Quote from: Wolfgang on November 03, 2023, 09:44:15 pmI understand that. But you can't claim "true zero" without stating the actual numbers. It is fine that device can't do real 0 V, just tell me how close to 0 V it can get and under what conditions.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 03, 2023, 10:02:44 pm
content: Just a (probably obvious) thought here (which doesn't alleviate the fact that you still need to know exactly how low your regulator can go), but if you can manage to go down to a certain voltage (say 50 mV) and you don't care about having more resolution between this voltage and 0 V, but still want a true 0 V, you could add a switching system so that it switches the output to ground with a very low impedance (very low RDSon FET) when you want to achieve a true 0 V.May sound obvious and may not address your requirements here, but something we can always consider.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 03, 2023, 10:21:40 pm
content: Quote from: Wolfgang on November 03, 2023, 09:44:15 pmNot quite.Minimum load means the regulation fails and the output drifts up.That's much worse than saturation offsets.It is possible for an emitter follower or PFET to regulate to 0V, with a pulldown resistor.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 03, 2023, 10:26:34 pm
content: Pulldown == minimum load

author: ataradov
author_rank: Super Contributor
timestamp: November 03, 2023, 10:28:10 pm
content: Quote from: SiliconWizard on November 03, 2023, 10:02:44 pmThis is part of the reason I went with ADP1716. It has dedicated enable pin, so I can control that and get the first step to be whatever the minimal natural voltage is.I will also try PCB.Wiz proposal. I'm not sure what, but something feels off here, so I'll just need to experiment with a real device.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 03, 2023, 10:29:37 pm
content: Quote from: ataradov on November 03, 2023, 09:49:47 pmAgreed. Problem is datasheet quality dominated by marketing bragging rights. Some critical numbers are often simply not there (becuase they dont look pretty). An icon of how to specify low voltage and saturation behaviour for a regulator is the old LM10 part from Bob Widlar. Never seen anything even close.

author: temperance
author_rank: Super Contributor
timestamp: November 03, 2023, 11:25:49 pm
content: I would use a rail to rail in-out op amp. OPAxyz, MCP6xyz and an emitter follower. It will regulate properly to 0 V and up to at least 4V, depending on the op amp output resistance and the transistor Hfe.

author: ataradov
author_rank: Super Contributor
timestamp: November 04, 2023, 06:18:00 am
content: Quote from: PCB.Wiz on November 03, 2023, 09:38:17 pmThis does seem to work a bit and I can generally see the logic, not sure why I was blocked by that before.For the tests I use the only adjustable LDO I have, which is AP2127. It uses 0.8 V reference. I used 3 x 10kOhm resistors and for some range of the DAC output, I can get output voltages from ~0V to some voltage < 3 V.So, I tried to derive some mathematical relations instead of blindly guessing the values. And I always come up with unsolvable systems. With a full 0-3.3V DAC range, I can satisfy either low end or the high end, but not both.My logic is the following. For the high end, when DAC drives 0V, we essentially have R1 as a high side resistor and R2 || R3 as a low side. So, it is easy to pick all 3 values that make necessary high output work.For the 0V output, we have a voltage divider consisting of R1||R2 and R3 on the FB input. 3.3 V from the DAC must be divided to 0.8 V by that divider. And this is where the problems start. I can't find values that would satisfy both conditions.It is getting late on Friday, so I'll double check that tomorrow with a clear head.

author: magic
author_rank: Super Contributor
timestamp: November 04, 2023, 06:30:39 am
content: Quote from: temperance on November 03, 2023, 11:25:49 pmIt's not entirely simple; you need bias on the output device to maintain some minimum frequency response for stability and this means either a minimum load current specification (most voltage regulators) or an internal current sink (LM358, R2R opamps, maybe some regulators). Alternatively, the circuit would need some means of slowing down the control loop at low load currents, or be quite slow at all load conditions. Well, maybe the latter would be acceptable, as emitter followers need only relatively little correction by the opamp. But it could still have some risk of (relatively low frequency) oscillation if the output goes down enough: 10mV, 1mV, 1μV...A "bare" rail to rail CMOS opamp could get very close to ground, particularly if the load is grounded and doesn't source current. Apparently there are some rare high current CMOS opamps; first thing that popped up in my search is OPA567. Probably overkill at 2A output, but it looks like it could work here.editOr dunno, maybe simply LM324 with all four channels paralleled with 1Ω current sharing resistors?It's not clear if it would meet the 150mA minimum load requirement, though.

author: temperance
author_rank: Super Contributor
timestamp: November 04, 2023, 04:45:41 pm
content: QuoteAn emitter follower complex? Let's see:-1 rail to rail output op amp. (input doesn't need to be rail to rail)-2 resistors to set the gain of the buffer stage-1 compensation capacitors-1 BCP56 transistor-1 base resistor, about 4.7...47 Ohm-1 minimum load resistor. 10K or so.The output doesn't have to go down to 1 mV or less.This circuit will go lower than the LDO version shown. The LDO version will not go below below (1.24V / 11K) x 1K. You can increase the feedback resistors if you like.Compensating a PNP or PMOS LDO for any type of load is more difficult.

author: tszaboo
author_rank: Super Contributor
timestamp: November 04, 2023, 06:09:43 pm
content: I use an LT1970 in an ATE project that needs to go to zero. It's not a good fit for your project because its very much not rail to rail.But the idea is the same. You create a negative bias supply, and have a power opamp as your power supply, with a small value resistor on the output.

author: ataradov
author_rank: Super Contributor
timestamp: November 04, 2023, 06:53:21 pm
content: Looking at it on a fresh head definitely helped. After some simple derivation, Vout = Vfb * (1 + R1/R2 + R1/R3) + Vdac * R1/R3. R1 - high side of the divider, R2 - low side, R3 - DAC.After substituting desired voltages (3.5 V max output and 3.3 V max DAC voltage) and letting Wolfram Alpha do the math, we get R2=264/611 * R1 and R3 = 33/35 * R1.With R1 = 22 kOhm, I've got R2 = 9506 Ohm, R3 = 20743 Ohm. Substituting with the closest values I could get out of two parallel standard resistors for weird values, I've got full range output from ~10 mV to 3.5 V with DAC varying from 3.3 V to 0V.So, this is a workable solution, but it involves really weird resistor values, there is no way to round to the standard values, since it is very sensitive to the values.I also managed to kill one waveform output channel on my AD2, so now I need to get AD8067 as well.

author: David Hess
author_rank: Super Contributor
timestamp: November 05, 2023, 02:54:54 am
content: Quote from: AnalogTodd on November 03, 2023, 09:04:59 pmOn my old Tektronix power supplies, the control loop is slightly biased to produce a very small negative output, and the output is pulled below zero, so the actual control range always crosses zero even if the operational amplifier offsets go the other way. The attention to detail by the designer surprised me.Quote from: magic on November 04, 2023, 06:30:39 amIt is not that difficult, even with a common emitter output stage, but it does require attention. Usually increasing the output capacitance for dominant pole compensation is enough. This also works with many operational amplifiers.An optocoupler or single transistor can be used to generate a small negative current, but in most applications just getting within 10s of millivolts of zero is close enough.I would be very tempted to use an operational amplifier and transistor rather than a regulator for this application, but it does mean more complexity. Those Linear Technology emitter follower regulators are very useful.

author: temperance
author_rank: Super Contributor
timestamp: November 05, 2023, 05:13:30 am
content: QuoteWhy would a negative current be required for the emitter follower stage to regulate 1 mV or lower if the op amp is a rail to rail in and output op amp?

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2023, 11:27:56 am
content: A rail-to-rail output op-amp + a pass transistor. The problem might be capacitive loads, which would require some frequency compensation.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 05, 2023, 03:24:23 pm
content: Quote from: ataradov on November 03, 2023, 09:26:17 pmSome of the numbers you can (and should!) figure out yourself. How low can the SET pin of the LT3080 go? Well, stick a zero ohm resistor in there and it goes to zero. Now, what does the output do when the SET pin is at zero? Well, there's the minimum load of 1mA needed on the thing. So, if you have a 100 ohm load resistor, that means it will go down to 100mV on the output. Of course, that's based on the worst-case minimum load required by the part. Typical minimum load is actually under 0.5mA based on the data sheet. So how is the writer of the data sheet going to quote numbers when they cannot predict what someone may use on the output?Before you label something as 'crap' maybe you should step back and look at the problem differently and try to understand what you are asking for. The overwhelming percentage of linear regulators out there are single-quadrant devices. That means that a positive linear regulator will only source current. Usually, a minimum load is needed to keep them in regulation, especially when someone is asking for the minimum complexity available (not needed as much when you have a minimum output voltage and the part can include that in its design).The ADP1716 is a "tracking" regulator, which means you need to supply it with a voltage that it follows up to the nominal output voltage. Funny enough, I would be wary of using it because there are no numbers in the data sheet to say how much current you need to source or sink on the TRK pin. So now, how are you driving the TRK pin without knowing this?Quote from: David Hess on November 05, 2023, 02:54:54 amHere's the thing that everybody seems to want to ignore. You need that negative voltage available to get down to 'true zero' if you want to do things entirely in a linear fashion off of a single input rail. If you add an extra winding or tap on your transformer from the wall, or drop in a switcher, there's no problem. Do you want simple? Or are you willing to go more complex?If I want to make a power supply that goes to zero, I'm going to design it with a negative internal rail to ensure I can get things all the way to zero. It's not going to be simple, with one or two ICs on it but instead be more complex to ensure that I can cover the operational corners all the way. There's a positive/negative supply that I like on p. 20 of the LT3094 data sheet that uses a switcher coupled to the final output voltages to give the best efficiency across all voltages and loads while still being nice and clean on the output.As @magic pointed out, a simple, single IC design of a power supply that can go to zero needs bias to maintain some frequency response. A common emitter stage sounds great, but frequency response at zero load is horrible as you basically now have to charge the base of the device up one Vbe just to start delivering current to the load (base capacitance is not zero). Once you have a minimum load going, frequency response is much better since you get logarithmic response from the device.Once somebody has actually taken the time to design one of these things, they will appreciate that things are never as simple as they want.Quote from: Zero999 on November 05, 2023, 11:27:56 amAgain, things like this look great until you build it. In order for this one to work well, you need to make sure you keep the pass device on, which means you have to sink current in the op amp and can't get to 'true zero'. Power supplies really want capacitance to handle transient loads, otherwise you get large excursions on the output until the feedback loop can compensate, so a capacitive load is pretty much guaranteed.

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2023, 04:45:12 pm
content: Quote from: AnalogTodd on November 05, 2023, 03:24:23 pmIt should be able to get as darn near to 0V, as required for most applications.You're right that the transient response isn't great.Why do you need to keep the pass device on? If the load is very light, the op-amp will simply power it directly and Q1 won't be needed.

author: temperance
author_rank: Super Contributor
timestamp: November 05, 2023, 06:15:33 pm
content: According to ataradov 100 mV is good enough.QuoteThe circuit posted by Zero999 with included short circuit protection or an emitter follower buffered op amp will do much better than that with a 14 R load (Requested: 3.5 V 250 mA). This equates into 70 µA @ 1 mV, god enough to bias any transistor into it's active region.For anything else with no load attached you will indeed need some way to keep all things biased which is not the case here.

author: ataradov
author_rank: Super Contributor
timestamp: November 05, 2023, 06:40:36 pm
content: There is already a working solution a regular adjustable LDO. I would still need to evaluate it with a range of loads, but it looks like it will be fine.ADP1716 arrives tomorrow and we'll see how well its tracking works. If it works reasonably well, I'll use that as it allows me to avoid a lot of strange valued resistors too.And I'm 1000% sure I don't want to invent voltage regulators with discrete components.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2023, 09:16:32 am
content: Quote from: temperance on November 05, 2023, 05:13:30 amI mean in general if the output needs to sink any current, which will usually not be the case with a power supply, but adding a current sink to the output is still useful to keep the output transistor out of cutoff. Many power supplies also include a current sink on the output so that the output can follow the control despite the output capacitance.As AnalogTodd points out, keeping the output transistor away from cutoff has advantages for frequency compensation.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 06, 2023, 04:04:11 pm
content: ... not just for frequency compensation, also for a load->no load transient response.If you dont have something to suck your output cap empty, a large and long spike occurs.

author: ataradov
author_rank: Super Contributor
timestamp: November 07, 2023, 01:30:55 am
content: Here are ADP1716 results. Blue - tracking voltage, Yellow - output, Red - math Y-B.So, close to 0V, the difference is 50 mV, which gradually gets better going to 0 when approaching the target voltage.The test is done with 330 Ohm load, without that, output capacitor takes forever to discharge and it does not "track" on the way down.This is some sort of a result, for sure.It is small and does not take up a lot of space, so I'll just place both LDO with a bunch of resistors and ADP1716 on the first version of the board and we'll see how they perform in real conditions.

author: ataradov
author_rank: Super Contributor
timestamp: November 07, 2023, 02:35:54 am
content: It is extremely unlikely that something from the 80s would be useful for me. I'm looking for a single IC with a minimum number of passives around it. This power supply is a tiny part of a big project and not a standalone supply.

author: magic
author_rank: Super Contributor
timestamp: November 07, 2023, 04:09:55 pm
content: Quote from: ataradov on November 07, 2023, 01:30:55 amActually, it's worse above 3V than it is near zero.

author: ataradov
author_rank: Super Contributor
timestamp: November 07, 2023, 04:21:37 pm
content: Yes, that's true. If I don't have > 3.3 tracking voltage, it looks like it won't get to 3.3 V on the output.So, may be 100 mV at < 1/2 VIN was the best case scenario.Well, LDO with a bunch of resistors it is then.

author: magic
author_rank: Super Contributor
timestamp: November 07, 2023, 04:46:04 pm
content: Either that or a power opamp, if you don't need to drive weird capacitive loads that could take even more passives to stabilize.