0-20V 0-3A  LT3080 Design Sanity Check  - Page 1
Forum section: electronics / projects

author: tijeff
author_rank: Newbie
timestamp: January 09, 2023, 03:38:30 am
content: I was reading true the LT3080 Datasheethttps://www.analog.com/media/en/technical-documentation/data-sheets/3080fc.pdfI was wondering if merging the Higher Output Current reference design with the LabSuply Reference design could be simply merge together to builda simple 0-18V 0-3A Bench power supply?From a 24V - 3A DC source-Both MJ4502 Will need a substantial heat sink , (24v*3A)= 72W in worst case-The 1ohm should be able to dissipate (24v*3A) = 72W in worst case-Would this design allow me to use a more thermally modest MSOP-8 package with no issue ?Is This design ok ? (botom of the picture)

author: coppercone2
author_rank: Super Contributor
timestamp: January 09, 2023, 03:51:39 am
content: well one think to keep in mind is you lose out on the thermal protection built into the 3080 when you put a external thing on it.

author: tijeff
author_rank: Newbie
timestamp: January 09, 2023, 04:29:49 pm
content: you are right, For shure the LT3080 will not be aware of the heat generated on the external transistors/resistors,But it still should be protected from its own overheat?If I understand it corectly, the MJ4502's will start conducting when the 50ohm get in the 0.6-1v range, so when the LT3080 use about 1V/50ohm =20mA. So, even if it sink the whole 24V, the LT3080 should never dissipate more than 20mA * 24V=0.48W. This could be too much , but event then it should then turn off the output to protect itselfIs my logic right ?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 09, 2023, 04:54:02 pm
content: The current regulation can be a bit tricky. Not sure it would be stable (not oscillating under all circumstances) and low noise. In addition to the 20 mA for the 50 ohm resistor the LT3080, but also the base current. This may be another 20 mA, or maybe more depending on the power transistor. With enough cooling this should still be OK, but the chip would get a bit warm. The chip in internal protection would protect the chip, but not the main transistors. The MSOP case may be a bit on the small side.When the LT3080 gets hot there will be some drift of the chip internal reference. So the output voltage is not super accurate and may drift a little.

author: Marco
author_rank: Super Contributor
timestamp: January 09, 2023, 05:06:20 pm
content: Why not justclone DC2132Aand not worry about heat?

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2023, 02:05:32 am
content: Quote from: coppercone2 on January 09, 2023, 03:51:39 amMount them to the same heat sink and increase the thermal resistance of the 3080.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 11, 2023, 08:21:53 pm
content: I'd say that any circuit for this IC is insanity. The IC is far too expensive for what it does. I'd rather use a few LM317's in parallel and an ICL7660 to get rid of the reference voltage than use the lt3080, even if the end result has a highe BOM cost and board area.But wit a bit of ingenuity it is not that difficult to use an opamp and a power transistor to make a power supply.As far as I'm concerned use of an LT3080 only means one thing: Lazy designer.

author: coppercone2
author_rank: Super Contributor
timestamp: January 11, 2023, 09:26:33 pm
content: as soon as I read even if "more board area" and "higher cost"

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 12, 2023, 01:23:45 am
content: Quote from: coppercone2 on January 11, 2023, 09:26:33 pmThat was a bit over the top. But after I wrote that, I had a look at a local shop, and a LM317 costs 45ct (in TO220, 25+) while the lt3080 is over EUR6. That still means you have to have a darn good reason to use the lt3080 instead of some standard part.And for me it is not only the total BOM cost that counts. I have a thorough dislike for artificial price inflation, and I do not want to sponsor or encourage that.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 12, 2023, 09:20:08 am
content: A practical lab supply has a bunch of other design concerns. Without any particular order:Output capacitance. 100uF seems too much when current limits expected to be enforced.https://www.eevblog.com/forum/testgear/output-capacitance-of-higher-end-bench-power-supplies/Transient protection scenarios. When the mains power turned on/off with the load connected. As a minimum, a discharge diode from the output to the DC in. A more defensive approach calls for generic overvoltage protection, as a crowbar or other transient suppression circuits. If the supply has an output off switch/function, output cap discharge is needed to quickly bring power down.Short circuit protection. At any current or voltage setting, the supply expected to tolerate an indefinite duration short circuit.All the above is to protect the circuit the lab supply is about to power up. There’s no point to cut cost and corners designing (or buying) a cheap lab power supply and later kill expensive or unreplacable parts when used on the bench.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2023, 10:22:06 am
content: There are a few tricky parts in designing a lab power supply:1) The stability (not too much overshoot and no oscillation) even with a difficult load like a low loss capacitance combined with an ideal current sink.2) A good cross over between voltage and current regulation, so that the current does not overshoot very much on a sudden short and the votlage does not overshoot much on removing a large load.3) Stability of the output voltage with load.Using the LT3080 is not solve any of these points. As a voltage regulator it is made for relatively well behaved load - though the series resistors for load sharing in some designs may safe the day in this respect. However the series resistor ruins any chance for good load regulation. In addition there is the general problem of having the power stage and reference / regulation in the same case. This causes naturally drift from self heating of the chip. Even with a low TC a chip that gets hot (e.g. 50-100K temperture rise) the change gets quite large. Having the voltage sensing directly at the IC pin and not the terminal or even extra sense inputs also limits the possibly accuracy of the regulation.Adding current limiting / current regulation to a regulator that is build for voltage regulation only is a tricky point. Chances are high the cross over gets horrible and it can effect the stabilty that often not good to start with.When used with external power transistors the LT3080 is not more than glorified OP-amps with a reference and sense connection in the wrong place.The the better alternative is not so much an LM317, but more an LM358 and TL431.A good lab supply should have a way to turn of the output and some overvoltage protection (e.g. with a crow bar). At least the later could be added in most cases. Adding an extra output enable to the LT3080 based circuit could be a bit tricky.

author: Marco
author_rank: Super Contributor
timestamp: January 12, 2023, 11:32:25 am
content: In all those respects the regulator one number up is a lot better suited. The LT3081 could be seen as worth the money. Not an LDO and far more stable down to zero output capacitance, near instantaneous current limiting (not ultra precise without an outer control loop, but if you're just setting things with pots it doesn't matter).So as I said, just clone theDC2132A.

author: tijeff
author_rank: Newbie
timestamp: January 13, 2023, 04:01:16 pm
content: Thanks alot for all your precious advices !

author: nachus001
author_rank: Contributor
timestamp: January 13, 2023, 08:26:14 pm
content: HeyAdditionally, for any high current linear lab design that you may build, take into account to add a voltage conditioner to the PSU input rectifier and bulk filterin order to mantain a input/output voltage the smallest possible for the output supply range.This needs a couple of comparators and a relay (or optocouplers and triacs) that will switch between transformer taps depending on the input output voltage difference, ensuringthe lowest possible dissipation in the pass device.If you use a multivoltage switching power supply as input then you can use MOSFETS to switch between rails. And if you have a fixedcontinous voltage (no taps or rails), you can add a switching pre-regulator in order to switch between two voltages.regardsNachus

author: coppercone2
author_rank: Super Contributor
timestamp: January 13, 2023, 09:07:32 pm
content: I think it might be hard to find a transformer like that. I guess you would look for one with alot of voltage rails.