2nd opinions on LT3081 based bench supply 0-24v 4ma-3a - Page 1
Forum section: electronics / projects

author: Mephitus
author_rank: Regular Contributor
timestamp: February 29, 2016, 03:13:29 pm
content: Like most newbs, my first "big" project has been a bench power supply. After struggling for the last few months trying to work out some issues with a LM723 based one, I recently ran across this design based on the LT3081. The design seems pretty sound to me, but I fully admit my lack of experience. Have any of you had any experience with this chip? Or are there any design shortcomings that stand out?Full design and spec sheet here.http://cds.linear.com/docs/en/lt-journal/LTJournal-V24N2-02-df-BenchSupply-Szolusha.pdf

author: TiN
author_rank: Super Contributor
timestamp: February 29, 2016, 03:38:12 pm
content: I had no issues with it, but my design was rather simple prototyping board bodge up just for some +9V 1A supply.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 29, 2016, 03:56:08 pm
content: There are a few limitations with this chip:The LT3081 can only dissipate a limited amount of heat, so it really needs the pre-regulator.There are no separate power and sense pins - so the output resistance can not be very low, so not a precision voltage source. The resistors used to parallel two units add to this. Current regulation is not very accurate, but still better than the LM723 internal one.I don't know how good the chip reacts to problematic loads (e.g. low ESR cap), this is a problem for many monolithic voltage regulators. Dave has a video on building a supply around the LT3080, that had some extra trouble - at least the LT3081 has internal current regulation, so not big trouble there.

author: Mephitus
author_rank: Regular Contributor
timestamp: February 29, 2016, 04:31:56 pm
content: Quote from: Kleinstein on February 29, 2016, 03:56:08 pmI just found a Spice model for the demo circuit:http://www.linear.com/solutions/5086Looks like someone was able to resolve a small problem with the design that was bringing the voltage to (in his case) -0.76:http://forums.anandtech.com/showthread.php?t=2443092If I am wrapping my head around this correctly, the best way to handle very low resistance loads would be to add a diode between the adjust and output to discharge the adjustment pin capacitance under short, or near short, conditions. This seem sound to you?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 29, 2016, 06:42:06 pm
content: The diode to discharge / reduce the set pin when the output is much lower than required is already in the plan. In addition there should be an extra diode to prevent a negative voltage at the output. This is especially important if used in series with a second supply.The minimum current requirement can in deed cause problems, and there is no simple solution around it. The added constant current like in the plan is only a partial solution: with a large capacitor (e.g. 1000 µF range) at the load and a fast current change there will be some overshoot and the capacitor will absorb all the current and the regulator does not get it's minimum current.However this problem is not unique to this type of regulator.The so called solution from the lined forum is not that good, as the current is not very stable and current reading from the shunt will not be accurate any more.

author: Mephitus
author_rank: Regular Contributor
timestamp: February 29, 2016, 07:20:49 pm
content: Good to know, I really appreciate the insight. I will play around with the spice model and maybe see if there are some better designs out there.