LT3080 wierdness - dave's power supply (eev224) gone mad - Page 1
Forum section: electronics / projects

author: mcbmcb0
author_rank: Contributor
timestamp: March 24, 2013, 10:22:28 am
content: helloi've made a version of Dave's power supplyhttp://www.eevblog.com/2011/11/28/eevblog-221-lab-power-supply-design-part-1/and kept in pretty close to the original (circuit shown).But the LT3080 loses regulation when (1) the set voltage is above approx 2.7v and (2) the current limit kicks in at low output current or (3) i just short the collector or the transistor (shown) or set pin to ground. what happens is Vout drops to about 2v and when the I-limit releases the minimum voltage the LT3080 will adjust to is about 2.7v. I *may* re-regulate when adjusted to max volts and back down again.I've tried the LM334 sinking to -1.2v - no change, and increasing to 5ma with a 10ohm resistor on the lm334. if I'm sucking a few amps out it keeps regulated. I guess its about the required 10uA source from the set pin and this getting lost when the op amp approaches 0v. I'm using an LT1013 due to good dc specs, higher voltage and near negative rail performance. The circuit shows a pass transistor - I've removed it to no difference. I've also made 2 soldered versions and now one breadboard and all the same. Also various low ESR and tant output caps.I guess i could drive it hard with an extra buffer op amp or pull down the OA inverting input for the current limit (more poles and phase shift?) but WHY? and what to do (other than dump this reg for a lm317 (with protection diodes of course).thanks for your ideasMike

author: amspire
author_rank: Super Contributor
timestamp: March 24, 2013, 02:42:30 pm
content: You have the diode output on the current regulate amp shorted to ground, but I am sure that is just a mistake in the diagram.It is pretty easy to find where the problem is.Make sure nothing is oscillating. That would be my first step.If you have 0v from theLT3080 regulator sense pint to the output, then it is working fine.Disable the current regulator amp (eg remove the diode) and check that the voltage on the LT3080 sense pin is correct. Without the current limiter, the output of the voltage opamp is independent of the load so it is easy to debug.Then when you have that working, re-enable the current limit and get that part of the circuit working.

author: mcbmcb0
author_rank: Contributor
timestamp: March 24, 2013, 08:12:46 pm
content: thanks amspire for your reply. yeah the diode was a glitch on the diagram - now fixed. there's no oscillations - i got rid of those with a scope and hence why no 1uF cap on the max4080 output. the current limit section (which is disconnected) works fine , as does the OA voltage amp driving the set pin (0-30v).I'm testing into a 7mA load. There is 0.6-0.8v across the lt3080 when adjusting across the voltage range, not 0v as you suggest. when it fails to regulate there is 2v across the set and Vout. When i get rid of the OA and tie the set pin through a variable resistor to ground (like the pic below from the datasheet) there is still 0.7V between set and out. i can't see any reference in the datasheet saying it should be 0v...?i'm left thinking it is weirdness with the lt3080 - which i've never used before. just that the whole internet makes no reference to this problemany ideas?

author: amspire
author_rank: Super Contributor
timestamp: March 24, 2013, 08:37:42 pm
content: What it the voltage on all 3 LT3080 pins when it is not regulating properly. Just one example, along with the load resistor.If you short out the 47 ohm resistor before the LT3080, what happens with the regulation?

author: mcbmcb0
author_rank: Contributor
timestamp: March 28, 2013, 11:18:50 pm
content: thanks for you suggestion amspire - sorry for slow reply due to other commitments.1. wrt the original circuit (less the 47Ohm and pass transistor) I did some measure when (A) regulating OK, (b) when set pin shorted to ground (simulating full current lmit, (c) after this short releasedvcc=13.8v, 100R as loadtest|OA non-Inv|OA out|Vset|VoutOK0.384.534.135.07Short Vset to gnd0.3812.3501.97after.38v3.994.115.99So the voltage amp is fine (gain of 12) but the difference between Vset and Vout increases again to about 2v after Vset -> 0V. A scope shows the LT3080 is not unstable with low esr caps etc.I wondered about the decreae in the OA output voltage (and don't understand why this is so changed the LT1013 to a LM358 and got more or less the same pattern. But even weirder:2. back to the very basic circuit (attached) with a variable resistor (from the datasheet). If I put a switch across the VR to simulate full current limit i get the same thing.A) the voltage tracks fine across the range of the variable resistor, down to just below1v (min operating voltage of LM334 sinking 7mA)B) when the output voltage is below 2V and i press the switch Vout drops to just below 1VC) when the output voltage is above 2V and I press the switch then Vout drops to 2V..... WHY IS THIS?This weirdness would approximate the main circuit problem when the LT3080 Vset is suddenly pull to 0V. I am very confused about this, and can't quite believe it. I'm ready to accept any stupidity on my behalf but given I keep getting the same problem I can't yet see it.Any ideas? anyone with an LT3080 willing to replicate or disprove this?Thanks

author: xygor
author_rank: Regular Contributor
timestamp: March 29, 2013, 12:35:23 am
content: LT3080 has foldback current limiting and therefore would have two stable states, but I don't see how you could reach the foldback state with such low load current.

author: amspire
author_rank: Super Contributor
timestamp: March 29, 2013, 03:21:50 am
content: Unless the LT3080 is in current limiting, the difference between the set pin and the output pin should never be more then 6mV. So either your regulator is faulty, or there is something wrong with your wiring. You haven't wired it up reversed? The datasheet view is from the top of the device, not the heatsink (bottom) side.In the original circuit, if anything is pulling the output positive, the LT3080 cannot drag it down again. So I would disconnect the MFE15004 totally, and also whatever is connected to that wire that goes up from the output and then off from the top of the circuit to the left. A 1K load should be fine to test if it regulating.I have to admit, there is something on the LT3080 datasheet that does not seem to make sense.The absolute maximum voltage differential between the Set pin and the OUT pin is 300mV. This is the value that supposedly that never be exceeded and yet not one of the application circuits does anything to maintain this +/-0.3V maximum differential. For one thing, when ever you power up the LT3080 with a capacitor on the output, you will exceed this 0.3V until the capacitor has had time to charge. The LT3080 is rated at indefinite shorted output, but when the output is shorted, you get much more then 0.3V differential from the Set pin to the output. Even the half wave clamping circuit in the application circuits that deliberately pulls the output high for half a cycle above the regulated output uses a silicon diode to pull the SET pin up, but that will mean the SET pin is 0.6V below the OUT pin and well outside the Absolute Maximum Specs for the device. The similar LT3083 and LT3085, have a SET to OUT Maximum of +/- 10V. That is 33 times higher then the LT3080. Is it a misprint?The significance of this is that if you exceed a maximum rating, the IC can latch or fail.If anyone can explain this apparent brain fade from LT, I would be interested. If there is one spec you are not meant to stuff up in a datasheet, it is the Absolute Maximum Ratings.Richard.Edit: I have sent of an question to LT tech support to ask if it is a misprint. I did have to fill out a questionnaire form to send the question, and I may have been slightly out in some of my answers. Estimated Annual Usage: 100000000000001. Maybe one of those "1" 's is a misprint.At least I correctly answered "Yes" to Street Address. I do have a street address.

author: mcbmcb0
author_rank: Contributor
timestamp: March 29, 2013, 08:03:26 am
content: Thanks for your thoughtful reply RichardI've stripped it all back to the basic circuit so the MJE and resistors are all well gone. Tried 3 different input power sources. Load as fixed resistors and LM334s. Your points just add to the intrigue. I have tried 3x LT3080's and got the same weirdness (tho killed one with some carelessness in all the screwing around). they are definitely wired up the right way -and regulate voltage as you'd expect until the weirdness sets in.It would be easy to conclude they are faulty - but all 3? even with the 10 billion you've been using annually that would be a little unlikely. I tend to roll my eyes at 'RMA threads'... It would be great if someone with an LT3080 in the drawer could do an exact test (test 2 in my last thread is easy) and report back.Maybe I'll roll back the years and stick in some lm317s (with a negative rail and protection diodes of course) and see if that solves the problem.I'd be very interested to hear anything you get back from LinearRegardsMike

author: Rerouter
author_rank: Super Contributor
timestamp: March 29, 2013, 08:41:05 am
content: this is a guess, but if you have some form of pre-regulator it may be the case, your current shunt amplifier has a minimum operating voltage of 4.5V, and the lt3080 has a minimum dropout voltage across it of about 1.35V max, now that would leave you with an output of about 3V, but i would hazard a guess that that shunt amplifier completly drops out at a slightly lower voltage than 4.5V which would push your 2.7V output squarely on the limits of the current sense amplifier,i would recommend disabling the output of the current limit and running your basic tests again, as if this is the case then its clear where your problem lies,

author: amspire
author_rank: Super Contributor
timestamp: March 29, 2013, 01:23:24 pm
content: Quote from: mcbmcb0 on March 29, 2013, 08:03:26 amDid it regulate at all?It is only regulating if the SET to OUT voltage differential is 6mV or less. The numbers you quoted showed bigger errors. If so, it is not regulating as it was designed to.Also since the LT3080 is a 3 terminal device and when it is regulating, the SET and OUT are the same, it cannot know when the output voltage is getting less then 2V.So I come back to the circuit. What is that extra wire going to the output from the left of your circuit?Assuming the regulators work, there is something wrong with the rest of the circuit or layout.Many regulators cannot be stable if the ESR of the output capacitor is too low. It may be oscillating, but the capacitor is hiding the oscillations. Replacing the output with a poorer electro, or adding a 0.1 to 1 ohm resistor in series with the electrolytic can test for this form of instability. How close is the output capacitor to the LT3080 - it probably should be very close.It is not impossible that all 3 LT3080s are faulty, but from my experience, this will probably come down to a head-banging realization about something you have done. What I mentioned above is a big clue - the LT3080 cannot know when the output is 2V from the output - so what does know? It can know when it it a certain voltage less then the positive supply, but if you reduce the positive supply, and the same thing happens, then you eliminate that possibility. Trust that the part will match the specifications exactly, and if it does not match a spec exactly, then you are doing something wrong.Unstable. Wrongly connected. Test circuit or physical layout has a problem. That unexplained wire to the output is causing a problem. That is about the list of options.It happens regularly to all of us.Richard.

author: mcbmcb0
author_rank: Contributor
timestamp: March 29, 2013, 11:45:03 pm
content: Richard by your definition I can't get the LT3080 to regulate - the difference between set and out is about 1v on one the them and 1.3 on the other (the 3rd I bought is now very dead). what i meant is Vout tracks Vin, but not like you say. When its going crazy the Vout-Vset is 2VI have changed the output caps (<=1cm from the LT3080) to normal electrolytics and other low esr and tant caps of varying sizes. the spec sheet does say LT3080 handles ceramic output caps so too low esr shouldn't be a problem. i don't have an esr meter tho surely all my low esr / tant caps can't be bad. If this is instability (tho nothing visible on the scope) any other thoughts on how to tame this?I didn't quite understand your bit out lowering the input voltage - tried it and the problem remains.The test circuits have changed over these posts - I'm trying both of those attached to this post.Still waiting for that jaw dropping head banging moment but I'm giving up hope of my stupidity being the problem.thanksMike

author: C
author_rank: Super Contributor
timestamp: March 30, 2013, 12:20:39 am
content: Here goes nothing,QuoteIf you look at Dave's power supply, he has this connected before the pre-regulator.If in your testing you are allowing the VControl get too low, you are entering the Dropout specifications.You might try changing you test circuit so that you always have 2 volts or more on the Vcontrol pin then your output.C

author: amspire
author_rank: Super Contributor
timestamp: March 30, 2013, 12:30:46 am
content: You need to see the regulator working, for the sake of your sanity.Build this - just floating parts soldered together.It should work. If it does, the next step is to see how this differs from the circuit you built.Richard.

author: mcbmcb0
author_rank: Contributor
timestamp: March 30, 2013, 01:28:53 am
content: what sanity?attached shows detail of free-wired circuit (just like the old days!) and larger shot of volts and current at minimum setting of variable resistor.to be precise the out cap is a 10uF tant (I've tried others), in cap 4.7uF electro.the red meter is across Vout and Vset..... =1V. its the same across the voltage sweep...!!

author: amspire
author_rank: Super Contributor
timestamp: March 30, 2013, 04:58:21 am
content: Quote from: mcbmcb0 on March 30, 2013, 01:28:53 amI need new glasses. I thought I read 1.01V on that meter showing the SET to OUT voltage. Can you reverse the leads and make sure it goes to -1.01V?Must be a dream.So to get this straight, when the SET pin is 0, you have 1V out. When the SET pin is 9V, you have 10V out?Is the middle pin a direct short to the power tab?If it is a faulty chip, I do not know what sort of fault would offset the voltage by 1v

author: mcbmcb0
author_rank: Contributor
timestamp: March 30, 2013, 05:17:10 am
content: its not your eyesight - its my nightmare:QuoteYou did. hence all the readings shown in previous posts.Quoteit doesQuoteno - the angle of the photo. i had checked. else it wouldn't regulate at all with Vcc=12vQuotethe same kind that goes even weirder and offsets by 2V under the above mentioned conditions.Perhaps its time for me to concede those LT3080s don't like me

author: amspire
author_rank: Super Contributor
timestamp: March 30, 2013, 05:42:15 am
content: Quote from: mcbmcb0 on March 30, 2013, 05:17:10 amWhat I meant is "Does it measure zero ohms from the middle pin to the tab?"The Tab is also OUT and so it should, but I am just trying everything.Is the SET to OUT a precise 1.01 for all output voltages, or does it vary?

author: mcbmcb0
author_rank: Contributor
timestamp: March 30, 2013, 06:22:18 am
content: with your same free-wired circuit/100 Ohm load:1. Middle (Vout) pin to tab = 0 ohms, and 0 volts when operating2. tested now: Vcc = 13.1v.Vout-Vset = 0.98vconsistently from lowest Vout voltage (about 1v) to Vout = 11.40V, then Vout-Vset tapers off to 0V when Vout = 11.80v. Then, as i actually have a 2Meg pot, the Vout-Vset increase a little to 0.14V or something as the pot resistance rises, tho of course Vout stays the same.all i can think of is these things were tested with an input voltage of 35V - never more than the spec of 36V - could that have fried their brains?

author: mcbmcb0
author_rank: Contributor
timestamp: March 30, 2013, 07:23:49 am
content: thanks Richard for staying with me on this thread. the 2 main options appear to be the devices are defective or I killed them in an unusually strange way - important to consider for the next steps. I have a carefully re-read of the spec sheet and note:1.max ratings - SET Pin Voltage (Relative to OUT).......±0.3V – well that's clearly not the case2. from page 4 (Lt3080fc sheet):QuoteNow, I may have exceeded this on my first round of tests. Vin would have been upto 30-35v and Vout maybe 1v. Does this mean it would (a) during those conditions go into current limit at near 0mA and then recover OR (b) get damaged forever? that would certainly decrease its attractiveness as a regulator, and mean no point trying another one. On the other hand a temporary limit is no big deal as my design has a tap changer on the transformer (no noisy pre-regulators for me) so would not operate (for long) out of this spec.. as long as it doesn't kill it3. i got these (in 5 pin TO220) from elements14 that come ex Aus, whereas i think RS locally only has other packages. if i'm using the extra transistor it *may* be worth trying another.any thoughts?

author: amspire
author_rank: Super Contributor
timestamp: March 30, 2013, 10:50:59 am
content: Here is what is puzzling.It is unusual to damage a chip without seriously damaging the silicon to the point where a catastrophic failure occurs.I suppose an exception is if you have burn out a resistor on the chip. The silicon might survive that.I am pretty sure the +/-0,3V is a missprint and it should be +/-10V. Now with a 36V supply, it is easy to exceed that, but I am dubious. usually the consequences of exceeding a maximum voltage are more obvious.Do you feel like trying your other "working" LT3080 with that simple circuit?Richard

author: C
author_rank: Super Contributor
timestamp: March 30, 2013, 03:22:15 pm
content: Richard,Just an idea:From a testing point of view, there is one area where max data to see what is happening is prevented. I am talking about pin 4 and pin 5 in your drawing. Putting a resistor in line with each would allow looking at what the pass transistor is doing vs a pin that acts more as a chip supply.C

author: mcbmcb0
author_rank: Contributor
timestamp: March 31, 2013, 09:11:57 am
content: Richard - would you believe the other 'working' lt3080 Vout-Vset = 1.27VC - good point, tho i can't think how to make use of that. i guess if its not acting like its working then either way its effectively dead

author: amspire
author_rank: Super Contributor
timestamp: April 02, 2013, 11:31:58 pm
content: I got the reply back from Linear Technology asking about the 0.3V absolute maximum limit between SET and OUT pins.It seemed very odd as the LT3085 and LT3085 allowed +/- 10V, and it is near impossible to ensure the SET pin stays within 0.3V of the OUT pin. None of the LT application circuits try and achieve this.Anyway here is the answer:QuoteIf anyone can understand that, they are smarter then me.So it is OK for SET and OUT to be different by more then 0.3V, except when it is not.When you short the output, the SET pin will be more then 0.3V different from the OUT pin which exceeds the absolute maximum ratings. So even though the chip is rated at handling a continuous short, this does not apply to any typical application - only to circuits that ensure the SET pin is less then 0.3V from the OUT pin.So use the LT3083 or LT3085 but not the LT3080 - it is rubbish I gather.I will give them a link to this forum page and ask for clarification.Richard.

author: EEVblog
author_rank: Unknown
timestamp: April 02, 2013, 11:37:11 pm
content: FYI, my new proto had a blown LT3080 on it, and I've been having some weird issues that sounds quite familiar to this, but have not had time to look into it yet.I don't like the sound of this...Have not seen seen the issues on my previous protos though.

author: amspire
author_rank: Super Contributor
timestamp: April 03, 2013, 12:04:40 am
content: Quote from: EEVblog on April 02, 2013, 11:37:11 pmMy suspicion is that for the LT3080, LT3083 and LT3085, the Absolute Maximum specification should be that the SET pin must be within 0.3 V below the OUT pin and 10V above the OUT pin.That would mean that you might have to add a schottky diode and a zener between the SET and OUT to ensure this.But LT have great designers, and I cannot help thinking that maybe they have done some extra tricks to allow the SET to get to 10V below the OUT.I cannot believe the LT designers would design the chip so that the IC latches if the SET in is 0.7V above the OUT pin. To me that idea in incomprehensible.Even if it is +/-10V, that still requires extra parts to ensure this is not limit is exceeded if the supply to the LT3080 is greater then 10V.

author: C
author_rank: Super Contributor
timestamp: April 03, 2013, 12:23:11 am
content: amspireI think I under stand, but I could also be blowing smoke.I think that the problem is Looking at the circuit we are seeing one thing and it's really acting like something else.In the very quick look at the data sheet examples there is always a resistor connected to the set pin, Except the example on page 16 where they use logic to activate a fet across the set resistor and at the same time the output.If the resistor is not a resistor, what is it? Looking at the data sheet that resistor is driven with a constant current source. Constant current source + fixed resistor equals Voltage reference.I am thinking that changing the Voltage reference too fast blows the chip.Make sense?C

author: amspire
author_rank: Super Contributor
timestamp: April 03, 2013, 01:18:14 am
content: Quote from: C on April 03, 2013, 12:23:11 amIf you need to limit the current into the SET pin with a resistor, then the Absolute specs should state this instead of the +/-0.3V spec.But the data sheet does have circuits with capacitors on the set pin, and thy behave just like a low impedance voltage source under transient conditions.QuoteI would love to know from the LT designers of the chip what they intended, rather then what the datasheet writer thinks.I have written back to LT mentioning the forum, and we will see what they come up with. I did mention to the LT contact that I suspect the LT designers of the LT3080 would laugh at the +/- 0.3V maximum spec for the SET to OUT Absolute Maximum. See if he actually checks.

author: EEVblog
author_rank: Unknown
timestamp: April 03, 2013, 02:51:47 am
content: I'm starting to get the heebie-jeebies anyway. I'm afraid that I'll make a thousand of these things and XX% will blow the snot out of themselvesWhen my one blew, I'm pretty sure I didn't do anything wrong to do that, so that's worrying right there, as these things are supposed to be short circuit proof etc.Perhaps the old stalwart LM317 might be in order...

author: BravoV
author_rank: Super Contributor
timestamp: April 03, 2013, 02:57:47 am
content: Quote from: EEVblog on April 03, 2013, 02:51:47 amRemembered these new LDO advertising from Linear when they released them .Not your Dad's ...

author: amspire
author_rank: Super Contributor
timestamp: April 03, 2013, 03:25:03 am
content: Quote from: EEVblog on April 03, 2013, 02:51:47 amThe major fear I have is that if the difference in specs are true, then that would imply they made a mistake with the SET pin circuit in the LT3080 design that they fixed in the LT3083 and LT3085.Then for some reason, they never redesigned the LT3080.It would really help to know the full story.

author: EEVblog
author_rank: Unknown
timestamp: April 03, 2013, 03:30:36 am
content: Quote from: amspire on April 03, 2013, 03:25:03 amAnd here I am, trying to get a design finalised and tested, with potentially some big issues hanging over the whole thing.So I have the options:a) sit back and wait for an official investigation and response from LTb) sit back and wait and see if someone else sorts it outc) investigate myself, which I'm sure won't be an easy task, and I'd need stock of expensive LT3080'sd) Abandon the LT3080 now for the LM317 (with extra neg offset circuit)And options b-c will still require a response from LT before I'd be happy to trust it.I wouldn't have believed it if I hadn't killed one myself, and maybe two, I'm not sure yet.

author: amspire
author_rank: Super Contributor
timestamp: April 03, 2013, 04:30:10 am
content: Ah! the plot thickens.The LT3085 does have a modification on the SET pin that the LT3080 does not have.Take a look at Note: 7 on both datasheets:It looks like the LT3080 has the SET pin clamped to the output internally with diodes?Can anyone test this? it would be interesting to see if these are Schottky or conventional diodes, and if there are back to back diodes.The LT3085 adds 1k resistors (plural).This almost certainly means the SET pin goes to two 1k resistors - one of which goes to the opamp input, and one goes to the 10uA current source. Then both the opamp and current source circuits are individually diode clamped so they cannot go more then a diode drop negative of the OUT pin. Using two 1K resistors eliminates any loss of regulation from the addition of the resistors, and means the SET pin can now safely go higher or lower then the OUT pin by +/- 10V.It would be nice if you could add the same circuit externally to the LT3080, but you cannot as you cannot separate the current source from the opamp input.If the LT3080 does have back to back diodes internally to OUT, then the really important spec becomes the +/- 10mA limit on the SET pin current. If you have, say, 10V on the SET pin and the output gets shorted, your circuit has to ensure that the maximum current into the SET pin is less then 10mA. Same story if the load actively drags the OUT pin up to 20V. Failure to ensure this in the design could lead to chip latchup.Here is the circuit that LT meant to put into the LT3080 datasheet:(Note to LT - you have my permission to this image directly into an updated datasheet.)And the +/- 0.3V ? If they use internal schottky diodes, then I guess +/- 10mA may equal +/- 0.3 volts. Definitely if SET gets to one standard diode drop below the substrate (OUT Pin I guess) on the LT3080 and the LT3080-1, latch up could probably occur. The LT3083 and LT3085 would need the SET pin to be dragged over 10V below OUT for this to be a risk.A BIG problem with their datasheet is that they allow for a 1uF cap to be put on the SET pin to reduce noise. If the output gets shorted, there is no possible way to keep the SET pin current below 10mA with a 1uf cap directly on the SET pin. FAIL!!!!!So can someone do the test on a LT3080 to see what is between SET and OUT?Richard

author: EEVblog
author_rank: Unknown
timestamp: April 03, 2013, 04:46:14 am
content: Quote from: amspire on April 03, 2013, 04:30:10 am0.80V both ways on a Fluke 87

author: EEVblog
author_rank: Unknown
timestamp: April 03, 2013, 04:47:42 am
content: Quote from: amspire on April 03, 2013, 04:30:10 amNo kidding!I think it's a short on the output that killed mine. My design has >10uF on the set pin.Even the extra sneaky 1K series resistor might not be enough to prevent this.

author: BravoV
author_rank: Super Contributor
timestamp: April 03, 2013, 04:48:16 am
content: The forward voltage looks like silicon type instead of schottky diodes.Tested both directions between pin 2 (SET) and 3 (OUT) and both have similar 0.7xx forward voltage and tested 3 of them, brand new never used.

author: EEVblog
author_rank: Unknown
timestamp: April 03, 2013, 04:51:55 am
content: 0.82V both ways on the U1272A, for both SOT223 and TO-220 packages

author: amspire
author_rank: Super Contributor
timestamp: April 03, 2013, 04:58:18 am
content: Quote from: EEVblog on April 03, 2013, 04:47:42 amLooks like the only cure is to add external back to back schottky diodes from SET to OUT, and then see how that affects your circuit.I have passed this on to my new mate Steve at Linear Technology. See what he says.I have suggested that they need an App note on how to use the LT3080 reliably, and that the LT3080 really does need the LT3085 SET pin mode fix to partially fix the problem.

author: amspire
author_rank: Super Contributor
timestamp: April 03, 2013, 05:22:46 am
content: If anyone feels brave, the next question is what voltages and currents in or out of SET cause latchup? If you put a 10K resistor from the IN/VControl to +5V, OUT to 0V, and then feed positive and negative current into SET, you hopefully can find out when latchup occurs without damaging the IC.Can it take 10mA into the SET pin, or does latchup happen at a much lower current?

author: EEVblog
author_rank: Unknown
timestamp: April 03, 2013, 05:40:52 am
content: Quote from: amspire on April 03, 2013, 04:58:18 amThat's still not going to fix the 10mA SET pin limit problem though

author: mcbmcb0
author_rank: Contributor
timestamp: April 03, 2013, 05:43:35 am
content: I'm glad the clever people have taken interest in this thread. Weird also how Dave gets 0.76 V Vset-Vout and I got 1.0 and 1.3V.When you say 'latch up' Richard are you referring to how when driving Vset with sudden voltage changes to 0V i then got 2V between Vset and Vout - at least until Vset was increase to rail and back down again (yikes! was that part of the problem, being >0.3V above Vout?) or switched off then on.Has anyone else out there found this, or, if agreeing to Richards challenge, willing to test it?Meanwhile I've possibly foolishly order some more of these things, tho to ensure a different batch these are SOT223 package from a different supplier that I'll only use with a current boosting transistor. And i might need to get some schottkys. I did write to element14 about the weird TO220 ones but no reply so far.looking forward to any more thoughts you all haveMike

author: amspire
author_rank: Super Contributor
timestamp: April 03, 2013, 05:57:54 am
content: Quote from: EEVblog on April 03, 2013, 05:40:52 amIt should. If the internal diodes are conventional diodes, then external back-to-back schottky's from SET to OUT will prevent the internal diodes from turning ON so the maximum SET current will be 10uA. It should make the LT3080 reliable.If the SET or OUT can ever get above the IN/Vcontrol voltage, you may need an extra Schottky from the SET and OUT back to the IN/Vcontrol.

author: airiclenz
author_rank: Contributor
timestamp: April 03, 2013, 09:25:41 am
content: Hello,I had similar issues with a LT3083. The V-Out was always way above the V-set. I needed to go up to approximately 14V with the set voltage until the V-Out "snapped in" to the V-set. After that I was able to go back down to voltages below 1V.I found the following in the datasheet (which was also mentioned by Dave in one of the videos in his PSU series):QuoteI tested to providing a path from the V-set pin (over some resistors) to GND. This fixed it. I don't see this path in your original schematic (only if Q7 is open).Could it be that?I found another problem in the original schematic from Dave. The 100 ohm resistor that sets the current source (R70 in your schematic) might burn when high voltages (>15V out) are applied. I killed 2 of them and changed the design to an op-amp comparator between V-out and V-set which toggles a power transitor to GND over a 10 ohm power-resistor.Airic

author: EEVblog
author_rank: Unknown
timestamp: April 03, 2013, 09:35:26 am
content: Quote from: amspire on April 03, 2013, 05:57:54 amOf course. Must engage brain before typing...

author: mcbmcb0
author_rank: Contributor
timestamp: April 03, 2013, 09:46:46 am
content: Airicthanks for your post.i had wondered if the Vset current was too low with the op amp near 0V and trying to sink thru 1100Ohms. I did try a resistor of some value (can't recall what) tho dumped it as i lost too many volts at Vout (as it divided the op amp out). I also thought the op amp out even at 350mV/1100Ohm = 30uA-ish, and thru the transistor 100ohms should sink some current. Tho maybe I'm mixing up sinking and sourcing - while the spec sheet says you can actively drive it, its vague on exactly how. Guess i should have tried something bigger - 1Meg-ish - what value did you use? I'll give this another try.I can't quite follow your comparator bit - any chance you can re-explain?thanksMike

author: airiclenz
author_rank: Contributor
timestamp: April 03, 2013, 10:02:57 am
content: Oh, I did the divider solution too but the used resistor was far away from mega-ohms. If I remember correctly, it was something like 100k. I am aware that this introduces other problems because v-set is not longer the voltage you really want. This could be compensated but it's not optimal though. Anyway - if this fixes your strange behavior, you have a new track to follow and finally, hopefully fix the problem. It might be it but can also be a problem with the LT3080. I thought it's worth a try.I'll post / send you a schematic of my "comparator-pull-down" later today.Edit: here it is.Airic

author: C
author_rank: Super Contributor
timestamp: April 03, 2013, 02:42:14 pm
content: I guess I am just thinking different.How would you create a cheap to use regulator for on board use for 1.567 volts?Now make it a sample manufacture step to make it work for 1.897 volts and 4.568 volts and x.xxx Volts.O by the way, It would be nice to have it simple to put many regulators all over the board to divide the load and keep the voltage spot on.-------To get to those numbers you need a cheap voltage reference.The cheapest part is a resistor.In the data sheet they talk of resistor Noise causing problems to the voltage reference and the cure a cap.They talk about a guard ring on the voltage reference as they are using such small currents to create the voltage reference.If you had one of those High dollar very accurate voltage reference chips, would you connect the OUTPUT of an op amp to the voltage reference's output?Also, Why would you connect an error feed back loop to a voltage reference?The data sheet also talks of the benefits of current sensing this voltage reference.A Current op amp has very different specifications from a Voltage op amp.Most of us have used one type of op amp for a long time. See an op amp and tack that Voltage handle on. Treating a current op amp as if it were a voltage op amp is sure to get trouble.Most of the voltage regulators are built around a Voltage op amp. What would a voltage regulator built around a Current op amp look like, something like this?C

author: SeanB
author_rank: Super Contributor
timestamp: April 03, 2013, 06:37:39 pm
content: I would guess that the 10mA max current does not cause latch up, but that at a current probably in the range of 20 to 100mA ( depends on processing as to the actual gain of the parasitic SCR in the die) it latches up, conecting input, output and set together in a SCR, and staying latched until either the die overheats, a bond wire burns out or the power is cycled. Note that in this case the overtemp protection is not operative, as the device is no longer being controlled, it is only a SCR.

author: mcbmcb0
author_rank: Contributor
timestamp: April 07, 2013, 12:04:11 am
content: richard and davesome more on this - would not adding 2x back to back schottky diodes between set and out mean a large capacitive load could dump a whole lot of current back into whatever is driving the set pin? like the op amp or current limiting transistor?i guess if they have internal diodes this could happen anyway, tho if adding schottkys perhaps they should have 1k resistors in series to current limit.i tested my 2 weird LT3080s with a semiconductor tester between set and out. the one that operated with 1V between set and out shows a diode junction of 3.3V with cathode on set. the other (which operated with 1.3V between set and out) shows as no device (and resistance of approx 2Mohm between these pins, compared with 700/1700 Ohms). Note they both still kind of regulate voltage, in between latching up. I'm now wondering if I wounded them by raising set too high above out - set was driven to near rail by the opamp (to about 35V, thru the 1100 Ohms), whereas Vout is obviously at least 1.35V lower. they must be pretty low power diodes in there.come to think of it, maybe i can bring mine weird ones back to 'normal' by external back to back schottkys??? and maybe i'll just drive it with a pot, like in the olden days.Good luck Dave if you are going to sell these as kits!

author: amspire
author_rank: Super Contributor
timestamp: April 07, 2013, 12:30:57 am
content: Quote from: SeanB on April 03, 2013, 06:37:39 pmThat is my guess to, but allowing 10mA to flow does violate the absolute maximun SET to OUT voltage rating that LT says is correct..If the back-to-back diodes between SET and OUT are conventional diodes, and the Absolute Maximum SET to OUT voltage is +/-0.3V that implies you must never let these diodes turn on.But then they also specify a maximum SET current of +/-10mA. If you have 10mA flowing, the SET to OUT voltage is greater then 0.7V - over double the previous maximum spec.If there is a risk of latch up, it will be when SET is more negative then OUT, but probably not when it is more positive. They do not say this anywhere though.I have asked LT for clarification of their previous email that I couldn't understand, but no replies yet. In their email reply, they said the +/- 0.3V Absolute Maximum was correct, and then in the next line, they said the Ground Clamp circuits which lets SET get below OUT by one 1N4148 voltage drop is also correct.??So for now, the only safe way to stay in spec is to have two big back-to-back schottkys from SET to OUT and then see how that affects the rest of the design.I am certain that the LT designers actually considered this problem pretty carefully, but whatever safe operational rules they built into the design never made it to the datasheet. At least one of the back to back diodes from SET to OUT was deliberately added (the other diode may be the substrate diode) and so it was intended to be used. To me that implies that the SET can go to 0.7V above the OUT pin which would mean the +/- 0.3V Absolute Spec is wrong. But this is just my guess. I do not have a LT3080 myself at the moment to do some tests.To me, the safer tactic is to use either the LT3083 or LT3085 and for the moment stay away from the LT3080 totally.With the LT3083 and LT3085, at least you know you have to keep SET to OUT to less then 10V, and the datasheets do make total sense. It is pretty clear that the designers got the LT3080 SET pin design wrong and they fixed it in the LT3083 and LT3085, but at the moment, there is no indication that they ever will fix the LT3080 design. If they did, it would probably have to be assigned a new part number.

author: EEVblog
author_rank: Unknown
timestamp: April 07, 2013, 02:57:22 am
content: Quote from: amspire on April 07, 2013, 12:30:57 amYes, that appears to be the only conclusion I can come to as well.I only wish one of mine had failed way back, and I would have investigated this much earlier.But I trusted the example circuit, and it tested ok (by luck I guess), so it looked like a goer.Fo me, the LT3083 is too expensive, and the LT3085 is too low current. So it's back to the LM317 I go I guess

author: mcbmcb0
author_rank: Contributor
timestamp: April 07, 2013, 03:05:13 am
content: i've been playing similarly with the LM317 - had a hard time getting it stable when driven with an op amp (with protection diodes etc), current limiting and high current transistor. I'll be keen to see what you come up with

author: EEVblog
author_rank: Unknown
timestamp: April 07, 2013, 03:08:22 am
content: Quote from: mcbmcb0 on April 07, 2013, 03:05:13 amI was just going to drive the set pin as per my current circuit (but with a -1.25V neg rail), probably with a spare opamp as a buffer.

author: mcbmcb0
author_rank: Contributor
timestamp: April 07, 2013, 03:32:12 am
content: good. i'll be watching.i tried the LM317 without the low pass filter (1K and 100ohm /10uF - just kept the cap) so using the voltage amp to drive it instead of an extra buffer. then put the current limit op amp out thru a diode to the non-inverting input. sang like a canary (and even worse with the high current transistor).as i'm sure you know, you're apparently supposed to pull an lm317 down to ground (with a pnp) rather than drive them.

author: C
author_rank: Super Contributor
timestamp: April 07, 2013, 03:49:12 am
content: From the data sheetI think the data sheet very clearly states thatQuoteis connected to the set pinAndA resistor connected to that set pin creates a High Precision voltage reference.In normal use a High Precision voltage reference does not change.I think the LT3080's set pin is not a voltage input, from the data sheet it appears it is a very very low current input to a current operational amplifier. I have seen other current operational amplifiers have low voltage limits like this.One example that creates creates a fast change on the set pin is the shutdown circuit and there they pull both the set and output down at the same time.The examples where resistor noise creates problems for the High Precision voltage referencethey add a cap.When you think voltage change and not very very low current change, I think that is where problems start.QuoteC

author: amspire
author_rank: Super Contributor
timestamp: April 07, 2013, 04:24:14 am
content: Internal 1k series resistors in the LT3083 are enough to increase the +/-0.3V limit to +/-10V the SET to OUT differential up to +/=10V.So adding a 3k3 in series with the SET input may allow up to +/-33V safely.Adding a series 3k3 would add a 33 mV offset which could easily be fixed in calibration, and add small reduction in the regulation.If a single 3k3 turns the LT3080 from a device where the SET can safely be at any voltage possible in the supply, it is worth the 33mV offset.I think it is worth a test.A test for latch up (With the LT3080 powered from, say, 30V via a 1K resistor) would be worth while to find out if the latch up for SET lower then OUT is always over 10mA.

author: BravoV
author_rank: Super Contributor
timestamp: April 07, 2013, 04:36:19 am
content: Anyone from Oz should send Richard at least one 3080 to play with please, too bad the s/h from my place to Oz is quite high for an express shipping, other wise I would send it right away.

author: EEVblog
author_rank: Unknown
timestamp: April 09, 2013, 12:00:00 am
content: Quote from: BravoV on April 07, 2013, 04:36:19 amI would if I had any left!

author: amspire
author_rank: Super Contributor
timestamp: April 09, 2013, 12:24:47 am
content: I think it is a good exercise for anyone with (working) LT3080s.Have a look at Dave's blog on lathchup:http://www.eevblog.com/2009/07/04/eevblog-16-all-about-cmos-scr-latchup/There is a good chance that if the negative current into SET relative to OUT is high enough, the LT3080 will latch up. If you limit the current in the LT3080 with series resistance, then there is a good chance no permanent damage will occur.The question we want to know is if the LT3080 can handle +/- 10mA between SET and OUT without any sign of latch-up?If it can, then adding a series resistor to SET can offer full protection.Also if latchup can be triggered, does the internal series pass transistor also latch on, or does it continue to function normally? You would need the 5 pin package for this test - not the 3 pin SMD packages. If it continues to work normally, then adding a series resistance to the Vcontrol (or whatever it is called) may prevent any permanent damage if latchup occurs.No further word from LT yet. That could mean they do not want to answer, or it could mean that the question has been referred to someone a bit beyond the normal support staff. If they need to develop new recommendations for safe use, that is not a trivial step for an IC manufacturer. If they have made a mistake once, they do not want to compound it with further flawed advice that makes the problem worse. As I see it, a datasheet with application circuits and design recommendations (like the 1uF cap on SET) that do not meet the specified Absolute Maximum ratings means they do have a problem for sure.

author: mcbmcb0
author_rank: Contributor
timestamp: April 09, 2013, 03:11:43 am
content: also no word from element14 yet (sent to NZ office, which is likely a branch of Aus) on my query about the strange behaviour of the devices they supplied and whether they were aware of related reports. doesn't encourage repeat purchases.

author: firewalker
author_rank: Super Contributor
timestamp: April 10, 2013, 04:48:23 pm
content: This story deserves a long video...Alexander.

author: amspire
author_rank: Super Contributor
timestamp: April 11, 2013, 12:15:05 am
content: An update.Steve at LT has passed the request for the specification and datasheet clarifications on the the design group, so hopefully we will get some good information.I have referred to this thread and have said that if LT wanted to respond directly, we would be grateful.It is pretty impressive that someone is responding and it is not trivial for a sales/support guy to escalate a query to the design department. Lets see what happens.Richard.

author: ftransform
author_rank: Frequent Contributor
timestamp: April 25, 2013, 04:41:12 pm
content: I am sure glad that I did not go with the lt3080 in my design!

author: amspire
author_rank: Super Contributor
timestamp: April 25, 2013, 11:02:47 pm
content: Just prompted LT again. They have gone very quite about the LT3080.

author: mcbmcb0
author_rank: Contributor
timestamp: April 25, 2013, 11:24:34 pm
content: i think i'd pause on anything LT3080. i foolishly got 2 more to try - this time sot223 package.1. with a 1M pot btwn set and ground, a current boost transistor as per the spec sheet and and a current limit circuit (2n3904 or something) pulling set down - i managed to maim one in about 5 mins. Vin was about 16volts. now its got 2V between set and out (where this thread started).2. the other- with a 1M pot, no current boost transistor and no current limit, driving a 7mA load (LM334...) survives with virtually 0V difference between set and out... YAY! but what's the point?these would have to be the least durable voltage regulator I've ever met. of course no word back from element14 either - they're off my christmas card list too.also - as per the protection diode approach - i stuck some back to back schottkys around one of my maimed to220 version's vset to vout . problem is when set gets near 0V and Vout is higher (as Vout to low for min required load) it puts high current thru the device holding set near 0V... = near death for that too.

author: Bloch
author_rank: Unknown
timestamp: June 02, 2013, 01:06:24 pm
content: Quote from: amspire on April 25, 2013, 11:02:47 pmHave you heard anything about the LT3080. Or are LT hiding ?

author: Yaksaredabomb
author_rank: Regular Contributor
timestamp: June 16, 2013, 09:53:43 am
content: Quote from: Bloch on June 02, 2013, 01:06:24 pmHow about now?Always good to have an example every now and then of how the flaw with a circuit's operation might not be a mistake with how you designed or assembled the circuit. The apparent or actual flaw could be from how you're measuring it or from a problem with the components themselves not behaving as expected! I'm curious what more will come out of LT on this, if anything.

author: parbro
author_rank: Contributor
timestamp: June 26, 2013, 11:23:49 pm
content: I noticed the LT3080 datasheet states "Capacitors up to 1uF can be used.", regarding putting a cap on the set pin for noise reduction. Maybe using a 10uF or higher cap on the set pin is causing too much transient current to flow into the set pin such as when a short occurs on the output. Do you really need greater than 1uF for noise suppression especially if you are not using a resistor but actively driving it?

author: tld
author_rank: Regular Contributor
timestamp: July 22, 2013, 09:49:58 pm
content: Any thoughts on LT3081 for the designs previously intending to use an LT3080?More expensive, but same ballpark, and seems to have a focus on ruggedness.tld

author: Bored@Work
author_rank: Super Contributor
timestamp: July 22, 2013, 09:58:19 pm
content: Quote from: tld on July 22, 2013, 09:49:58 pmCrap.https://www.eevblog.com/forum/projects/lt3081-nice-feature-upgrades-to-lt3080/msg258008/#msg258008

author: wigman27
author_rank: Regular Contributor
timestamp: August 11, 2013, 10:48:11 am
content: Hey All!This is more of a curiosity thing, I know Dave has mentioned using the LM317 in place of the LT 3080 and he would create a negative voltage rail to take it down to 0V. I know all the stuff about using LM317s in power supplies and so on but its peaked my curiosity on the best way to generate a negative voltage from a single supply like this, then how to still control it using a micro.Its not something I have done before and have been searching madly on how to do it from the battery. From what I can find, the best way to generate the negative supply initially is to use a charge pump DC-DC converter. A couple of reasonably price ones I have found are:MAX1720TC7662LM2611From what I understand, the last 2 actually regulate the output voltage much better, however the first one does do the quite well considering the load current is very low. What do you think?drive these directly from the 3.3V regulator to acheive a -3.3V output. Then, to achieve the -1.2V offset voltage, use an LM385 or similar in series with the set pin. Then would you use a summing amplifier to add this -Vref to the set voltage?Is that a good way of doing it?ThanksLee

author: JJalling
author_rank: Unknown
timestamp: August 11, 2013, 10:50:26 am
content: Lee,take a look here:BR Jonas

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 13, 2013, 07:52:08 am
content: Quote from: wigman27 on August 11, 2013, 10:48:11 amI think that once you have a negative supply rail you wont need the LM385 or the summing amplifier. You can just rely on the op amp connected to the adjust pin to pull it below ground for output voltages less than 1.25V.I haven't tried it (other than in the simulator) but something like the attached should work.Edit: Fixed bug in schematic re: compensation cap.

author: mnmlistmktg
author_rank: Newbie
timestamp: August 27, 2013, 11:16:56 pm
content: Unfortunately, I read the forum AFTER buying the parts and putting everything together (analog-style, exactly as demonstrated in the third video). I went ahead and swapped the 3080 for the 3083 and everything is working perfectly.... until I connect the dummy load (also built to spec from Dave's video).With Vout set at 11V, load regulation is dropping an average of .07mV per 50mA of load. Doesn't actually sound that bad until you start using different Vout settings; over a number of ranges 3.3, 5, 9, the average voltage drop per 50mA of load is actually .3mV - so starting with 5v, you are at 2.6V with a 400mA load. I'm about as new as you can get to this stuff, but that doesn't seem like a good thing.The 3083 datasheet doesn't have much to say about load regulation, other than to keep the wires thick and short. Is there anything that can be modified to this circuit to improve load regulation? (schematic is the same as the others I've seen posted in this thread). I need to check the change in offset voltage under load, but don't think that's it.Any ideas would be appreciated.

author: mcbmcb0
author_rank: Contributor
timestamp: August 28, 2013, 08:27:56 am
content: well, i've never met a lt3083 but that sounds truly weird, and a different problem from the Vset/Vout problem that this thread started with on the 3080. i guess you've done the obvious checking your made it right etc. you'd need pretty skinny or resistive wires to account for that voltage drop. assuming you followed dave's original circuit (or variant) i'd be wondering:1. have you got sufficient load on the 3083 when its at rest? the lm334 bit. tho that wouldn't really account for a steady drop of Vout by current.2. does your power supply offer sufficient input voltage at higher current? is that rolling off? if you had a scope you could check its not loosing half the wave due to faulty or small input smoothing caps. (are they big enough?)3. is the voltage at V-set (from the op amp) stable or does that too drop with current. that would be odd and suggest a supply problem.4.again with a scope are you sure you haven't got any oscillation with the current limit - disconnect that pull down transistor til you get this sorted.5. if V-in is stable and the V-set/V-out increases (it shouldn't) with current then perhaps you've discovered an entirely new form of LT weirdness... who knows whats possible with these things? (LT certainly isn't going to tell us)good luckmb

author: Strada916
author_rank: Frequent Contributor
timestamp: August 28, 2013, 09:39:49 am
content: You also need to remember that the lt3083 has a problem where it shuts down when shorted. and needs the input set to zero to reset it. see data sheet.looks lm series regulars are looking better and better.

author: mnmlistmktg
author_rank: Newbie
timestamp: August 29, 2013, 12:33:21 am
content: Quote from: mcbmcb0 on August 28, 2013, 08:27:56 amYes, LM334 in place and functioning as expected... also, these issues only happen under load - same with either a power resistor or the EEV dummy loadQuoteI believe so... I've been using a power adapter (12V 2A) for the time being, not going to set up the transformer / rectifier until I have the rest of the circuit worked out. I have tried other adapters and even an ATX PSU, all with same results.QuoteThe more I've investigated, the more odd things I've found.... can't recall if v-set has been stable, but am documenting everything and will post full report when done.QuoteI've found some sporadic oscillations, but I can never duplicate the conditions. I'm going to start breaking the circuit down and do tests section by section as I build it back up.QuoteNo, adjusting the current limit doesn't impact anything but the current limit, so at least that is working!Thanks for the input, I'm starting to think I'll have to figure something out with the 317, but I want to try and nail down what's going on with the 3083 first.

author: mcbmcb0
author_rank: Contributor
timestamp: August 29, 2013, 08:36:43 am
content: for now i'd still remove the pull down current limit transistor. that's potential trouble you don't need.re your power pack... has it got a smoothing capacitor? most wall warts don't. could you stick a 4700 or 10,000uF cap across it and test again?still may not explain the weirdness at lower Vout, but worth a try

author: mj21
author_rank: Regular Contributor
timestamp: August 29, 2013, 09:17:01 pm
content: i had the same issues with lt3080 and even lm 317 seemed bit temperamental, so i decided to built my own regulator. after searching for few minutes and reading many posts i found this. i have tested this design and i must say it is very stable. i have already made 10 prototype with different op amps and they are all stable. give it a try. i am using dual rail supply. have not tested it with single supply as all my op-amps are dual rail supply ones.all you have to do is remove the lm329dz and connect the adjust output of the daves circuit. it goes all the way to zero volt as well.

author: mnmlistmktg
author_rank: Newbie
timestamp: August 30, 2013, 01:00:58 am
content: Quote from: mcbmcb0 on August 29, 2013, 08:36:43 amPulled out the transistor... didn't change anything, but I think I'll leave it out anyway until I figure this out.QuoteAdded a 2200u on the input and tested in/Vctrl as well as Vout over loads... both were dropping as load increased, which leaves me to believe that load regulation isn't the issue, but rather line regulation. I was a bit surprised that the 2200u input cap didn't seem to make a big difference, I'll need to order a bigger cap to see if that helps. What I really don't understand is that the line regulation dropped even when I started loading at 5v out. It's a 12v supply, so there should be plenty "in reserve"

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 22, 2014, 06:35:19 pm
content: The conclusion here, for anyone not wanting to read the entire thread, is that LT never gave a final response. From the lack of a response, (which really is a response) we can conclude that LT3080 is a problem.Is it a documentation problem or a hardware/design problem?From reading this thread, I think it's a hardware/design problem and LT is never going to publicly admit that it's a problem. But they have admitted so by providing no guidance.Am I wrong? If so how so?

author: AscendedDaniel
author_rank: Newbie
timestamp: September 22, 2014, 06:37:27 pm
content: I am also designing a power supply, and I found this thread. I decided to try out some of the test circuits on a breadboard, but I have been unable to replicate the failures mentioned here. Attached is my test circuit. (I least I hope it's attached. It's my first time on the forums.)I set the output voltage to around 5 V with the pot. Then, I tried grounding test point T1 with a jumper wire to simulate entering constant current mode. That should put Vset below Vout. I also tried grounding Vout, to get Vout below Vset. After multiple attempts Vout-Vset was <3mV, within spec for the TO-220 package, according to the datasheet.Am I not using the same circuit as in the posts before? The other possibilities I can think of are that LT silently fixed the issue in the LT3080 or there was a bad lot that eluded testing at the factory and I got a newer lot. The only codes I see on the chip are 1153860 and e3, but I'm not sure how to convert that into a date. 53rd week of 2011? I have a few LT3080s in TO-220 to mess around with. Let me know if you have any other ideas to try.

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 09:42:18 am
content: Quote from: amspire on April 03, 2013, 04:30:10 amLatchup also depends also depends both on the semiconductor process used and some aspects of the layout. One of the big advantages of dielectrically isolated processes as oppose to junction isolated processes is their immunity to latchup.QuoteTheir simplified schematic was a little too simplified. :/Texas Instruments tends now to not include a schematic at all and has been removing schematics as they rewrite data sheets.

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 09:57:41 am
content: Quote from: mcbmcb0 on April 07, 2013, 03:05:13 amI have never had a problem driving an LM317 (or 7805) with an operational amplifier. Depending on some details like the operational amplifier bandwidth, you may need to add frequency compensation and this will become more critical if an external PNP pass transistor is added in the typical configuration.One of my favorite circuits is similar to this with a LT1007 or equivalent, precision shunt regulator, a lead network and frequency compensation around the amplifier, and a 2N3906 replacing the JFET if a 7805 or similar is used:The last time I did a few of these, the line and load regulation was practically unmeasurable.