Experimenting with LT 3080 Low Drop Out (LDO) Voltage Regulator

RE: Experimenting with LT 3080 Low Drop Out (LDO) Voltage Regulator

jw0752 — Fri, 11 Dec 2015 17:17:50 GMT

FOLLOWUP - A couple additional Experiments.

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The first additional experiment that I wanted to do is see how well paralleling the LT3080 works. One of the limitations that must always be faced in experiments is the limitations of the test instruments of the lab. In my case I have good confidence to the level of 10 mV, 100 uA and 10 mOhm. While this is not very good for any precision decisions it is more than adequate for the level of experimentation that I am currently doing. For this experiment I mounted a second LT 3080 as I did before but I did not have an identical heat sink so the two devices are potentially not running at the same temp. I changed my extended cooling ability by putting a small 12 V fan directly on the two devices. The ballast resistors that I used for this experiment were 100 milliohm as opposed to the 10 milliohm called for in the application sheet. This was a result of the available resources and ability to easily construct the recommended values. I also wanted to read voltages across the ballast resistors to determine the individual LT 3080 currents. The two devices performed well with a combined current load of 2.15A with one device taking 56% (1.21A) of the load and the other handling 44% (0.94A). I am certain this would level out better if I was able to lower the ballast values and equally heat sink the two devices.

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For this experiment I was not able to use the electronic load as it is only available as a constant current load. Therefore I am back to using the old trusty automotive tail light. Under a this load I was able to set the current from 0 to 0.41 A using the recommended set up. The application indicated a 0 to 1.0 Amp capability but I was not able to get more than 7 volts to drive the bulb with an input of 13 volts which limited my current to the measured 0.41 A. I would then vary the load slightly by adding resistors across the light bulb and observing the change in the current output. The stability of the current was acceptable for the crudeness of the breadboard set up with all its inherent variables. Available time and interest limited my pursuit of further unanswered questions at this time.

My over all conclusion for the LT 3080 is that it is a nice flexible device that will have some applications in my future designs but it will not replace the standard 78** or the LM317 completely.

John

RE: Experimenting with LT 3080 Low Drop Out (LDO) Voltage Regulator

DAB — Sat, 05 Dec 2015 19:56:04 GMT

Excellent post John,

Your vivid descriptions of your problems and resolutions help show everyone that it takes persistence and some thinking time to resolve unknown issues.

As you pointed out, when all other issues are resolved, then the problem could indeed be the device under test.

Well done,

DAB

RE: Experimenting with LT 3080 Low Drop Out (LDO) Voltage Regulator

jc2048 — Sat, 05 Dec 2015 13:35:51 GMT

Nice example of experimenting. My vote would be for looking at the transient response next.

What shabaz writes sounds very plausible as a way the device could have been damaged, but did you think to look at the output with a scope? What you refer to as the output decoupling capacitor is actually there for stability. You have it at the end of two or three inches of wire which isn't where the chip designers intended you to place it. You might find the output is oscillating.

I've not used these, but another 'gotcha' from looking at the datasheet is the minimum current. The circuit needs some output current to function and the minimum is 1mA (below which the output "may not regulate"). At 5V you'd need a 4k7 at the output to meet that. It's looks like you've got a resistor on your breadboard, but I thought I'd flag it up for anyone else thinking of using the part.

RE: Experimenting with LT 3080 Low Drop Out (LDO) Voltage Regulator

shabaz — Sat, 05 Dec 2015 12:47:35 GMT

Hi John,

I'm glad you're experimenting with these devices! I've had my mind on these parts recently too, so it is interesting to see all your detailed findings. The paralleling capability is nice, but the metal tab is connected to Vout so it means that separated heatsinks (or insulating washers) are needed though, because they expect a small ballast resistance (e.g. wire trace) between the Vout connections.

Regarding the strange behaviour of the first part you tried, apparently it is possible to damage these parts if Vset and Vout differ by more than 0.3V. This can occur if (say) there was some rapidly changing load perhaps with charged capacitors connected to it as you mention. This failure mode may explain why the set resistance behaviour seemed different after the fault. The solution is to have a pair of schottky diodes (i.e. for low Vf) paralleled (but in opposite directions) connected between Vset and Vout, to maintain no more than the 0.3V difference at any time. However, another approach is to use the LT3081 which they introduced a couple of years after the LT3080, and it has a far higher 10V difference allowed between Vset and Vout, so these diodes are not needed.