LT3080 coming out of regulation (followup to old thread) - Page 1
Forum section: electronics / projects

author: RustyShackleford
author_rank: Regular Contributor
timestamp: October 24, 2017, 07:23:02 pm
content: Hi, new member here. I googled across an old thread here, discussing the LT3080 falling out of regulation:https://www.eevblog.com/forum/projects/lt3080-wierdness-dave's-power-supply-(eev224)-gone-mad/In that thread, user 'mcbmcb0' (apparently no longer here) posted regarding problems with an LT3080 (in a larger circuit). Basically, the thing was loosing regulation, with the voltage between SET and OUT diverging from zero.I'm experiencing something that seems somewhat similar, though in a much simpler circuit. My circuit is a low-dropout adjustable current source for driving series strings of LEDs. It's pretty much identical to the circuit "Low Dropout Voltage LED Driver" on p.17 of the LT3080 datasheet (see image below); except I use a 100K pot in place of the resistor from SET to GND, I use a 3 ohm sense resistor (from OUT to GND), and I drive strings of up to 10 LEDs up to 300ma or so.I'm using an even simpler test circuit to investigate this problem. The LEDs are replaced by a 32-ohm resistor, and the pot is set at 85K. The failure occurs as I change Vin (the supply voltage). At 9v or so, it seems to regulate ok, with v(SET) at 850mv (85K * 10ua) and v(OUT) at 800mv or so (still not that close) . As I crank Vin up to 14v or so, v(SET) drops to 450mv and v(OUT) to 700mv. Something very wrong is going on.However, if I drive a voltage directly into SET, it works perfectly. This is the approach that user EEvblog described using in his powersupply in that old thread. With this approach, if I change the OUT resistor to 1-ohm so I never drive more than 300mv into SET to get my 300ma maximum current output, then I can mostly guarantee V(SET,OUT) < +/-0.3v (even if OUT gets shorted to GND). And since I'm driving SET from a voltage divider with Kohm resistors and a 1K pot, I limit the current into SET to well under 10ma.So I'm just wondering if people have further thoughts on this, these years later ? With the approach of driving voltage directly into SET ? Further insight on the problem with using a resistor instead ? I'd actually rather use a digital pot (instead of driving a voltage) if I could figure out how to make it work.Looks like LT3081 fixes the problem, but I'd rather have the lower dropout voltage (with dual supplies on VCTRL and IN) of the LT3080. And the LT3083 can not handle the voltages I need (I'd like to have Vin = 30v, so I can drive 10-LED strings). The LT3092 looked promising as well, except it doesn't come in a TO-220 package, and I need to be able to dissipate some fairly large amounts of heat (if LED strings are significantly shorter than the design maximum).Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: October 25, 2017, 04:22:33 am
content: Quote from: RustyShackleford on October 24, 2017, 07:23:02 pmSo since Vout should track Vset within 6 millivolts or so, something has already gone wrong under these conditions.QuoteThe LT3081 uses a 50uA Vset current instead of the 10uA set current of the LT3080. That results in raising the impedance at the Vset pin to produce the same output voltage and suggests a measurement problem; how are you measuring the voltage at the Vset pin?To accurately measure the voltage at the Vset pin, I would temporarily add a single supply operational amplifier configured as a follower to drive the voltmeter.To lower the impedance at the Vset pin, I would either decouple it with a capacitor (which does nothing for DC impedance) or use the configuration shown on the lowest schematic on page 17 of the LT3080 datasheet to raise the effective Vset current.The change in behavior between 9 volts and 14 volts suggest that either the LT3080 is protecting itself against excessive power dissipation or stability was always marginal and a change in operating point is causing oscillation. This should be tested using an oscilloscope to monitor at least the collector and emitter of the output transistor and maybe by adding decoupling at those points to see how it affects operation.

author: RustyShackleford
author_rank: Regular Contributor
timestamp: October 25, 2017, 05:52:35 am
content: Thanks much for your response ...Quote from: David Hess on October 25, 2017, 04:22:33 amI'm not quite following: I didn't mean to say that I'd tried the LT3081 yet - it was just suggested to be less problematic, in that earlier thread I linked. Or are you just saying that's why the 3081 works better ?QuoteMy Fluke 87 DVM claims to have 10Meg input impedance, wouldn't that allow it to measure the SET pin with sufficient accuracy ?QuoteMy circuit was working so well, driving a voltage into SET, that I haven't even looked into the various decoupling and compensation schemes yet (the LT3081 datasheet has a nice discussion).At any rate, directly driving SET appears more robust. (Getting astray of my original question, I want to control the current from an Arduino, and have the setting persist on power cycling. There's no appropriate DAC with non-volatile memory. I could substitute a digital pot into the voltage divider I'm using to generate the SET voltage, and digital pots with non-volatile memory for the tap register are not hard to find).

author: David Hess
author_rank: Super Contributor
timestamp: October 25, 2017, 12:22:02 pm
content: Quote from: RustyShackleford on October 25, 2017, 05:52:35 amI am suggesting that the lower Vset resistance of the 3081 makes it easier to use. I have not played with either of these parts.QuoteWell, your measurements show that something is not right. It would take 1.35M to make the 50mV difference you measured with an 85k Vset resistance and some multimeters are that low.QuoteWhich is what makes me think there is a problem with Vset being such a high impedance. There should not be except for making measurements but something is going on.How is your test circuit constructed? Is the test circuit oscillating?Page 12 of the LT3081 datasheet discusses the high impedance problem at the Vset pin which will be 5 times worse with the LT3080. The application examples about using a lower value of Vset resistance show a way to help with this.

author: RustyShackleford
author_rank: Regular Contributor
timestamp: October 25, 2017, 11:37:05 pm
content: I think you're right about the SET pin impedance. As you point out, the admonitions in the 3081 datasheet (even with its 50ua SET current) concerning board leakage and guard rings, bring this home. It seems to me that directly driving the SET pin with a relatively low impedance voltage source (a voltage divider with a 1K pot to GND and many 10s of K to Vin) neatly sidesteps this problem. Do you see any downside to doing this (other than that I'd be ignoring the issue that there's something going on with the circuit that I do not understand) ?The 3080 circuit I've been testing is on a little prototype PCB (from one of those Chinese places) that I fab'ed for an earlier attempt at this problem. So there are loose wires. In fact, I designed it for a 3-pin TO-220 (power mosfet) so the 3080 is actually not soldered to the baord, but rather connected by some wires 3" long or so. And as I said, I haven't added a single capacitor to the circuit as yet, I don't know if it's oscillating; I don't have a scope, but can probably borrow or buy one from a buddy who's all into hamfests.

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2017, 11:59:22 pm
content: Quote from: RustyShackleford on October 24, 2017, 07:23:02 pmI simulated your LT3080 with the 32R, 3R and 85k.At 9V there's not enough voltage across the 32R, 3R and output transistor, for it to regulate with Vout=Vset. Once Vin reaches 10.1V Vset=Vout with 280mA through the 32R right up to Vin 30V before it starts to reduce the 280mA current.QuoteYep, I wouldn't test one of these without 2 back to back diodes between Set and Out, I think they're still a bit delicate.

author: David Hess
author_rank: Super Contributor
timestamp: October 26, 2017, 03:29:24 am
content: Quote from: RustyShackleford on October 25, 2017, 11:37:05 pmI took a quick look at this after you mentioned using a non-volatile potentiometer DAC and I think it can be made to work fine without an external reference or operational amplifier.QuoteThat is better than I was expecting but 3 inch wires might be a little long unless decoupling is added to the right spots; these regulators are pretty fast to allow good transient response and low noise.A simple AC coupled two diode RF probe will reveal oscillation if it is large enough and an AC voltmeter can reveal it if the frequency is low enough.Maybe your ham buddy can help you find an inexpensive but working or repairable used oscilloscope.

author: RustyShackleford
author_rank: Regular Contributor
timestamp: October 26, 2017, 06:07:24 pm
content: Quote from: StillTrying on October 25, 2017, 11:59:22 pmMy dummy load might be a little less than 32 ohms, but yes, the thing starts to dropout around 9v. But the main point is, when Vin is raised to where it should easily be able to regulate, it does even worse.QuoteGreat idea, I'm going to start doing that.

author: RustyShackleford
author_rank: Regular Contributor
timestamp: October 26, 2017, 06:26:20 pm
content: Quote from: David Hess on October 26, 2017, 03:29:24 amOnly needing to sink 10ua, opamp voltage follower from the DAC to SET pin shouldn't be necessary; it might be a different story if I use the 3081 (needing to sink 50ua), but the resistor scheme should work with that part (when I try it).My main desire for adjustability is that the LEDs dim smoothly, I don't care about the exact max, so that's all I'd need voltage reference for. I would need a 3-5v supply for the digital pot chip.QuoteFor sure I need to add some decoupling, and of course I'll tighten things up at some point (it's only about $20 to get a 10-piece run of PCBs up to 4"-square).QuoteTwo diode ? Can you describe more ? This is the one I've seen (see picture below).

author: RustyShackleford
author_rank: Regular Contributor
timestamp: October 29, 2017, 07:00:47 pm
content: As a followup, I've tested the LT3081, and it works fine just using a resistor to GND to program the SET pin. I think it's undoubtably a more robust part (than the LT3080). I just wish they had brought out separate power supplies for the control circuitry and the power transistor, as is done on the LT3080. There must be a good reason, which I can't discern, for doing this; but it costs in dropout voltage (being about 1.25v instead of a fraction of a volt).