LT3083 operation - Page 1
Forum section: electronics / general

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 02, 2012, 04:48:16 pm
content: Hi,I've been reading the datasheet (and the info available on Linear's site) of the LT3083 and I am a little confused about it.If I understand correctly the LT3083 is a floating device and the voltages specified are relative to Vout.So "Wide Input Voltage Range: 1.2V to 23V" would mean that, even if the input is, say, 30V, as long as the input is at max 23V above Vout, the device should operate properly without harm.Is this correct?Next I did some simulations using LTSpice and I got even more confusedI attached the simulation screenshots.In Image 1:- Vin=30V- Vout(set) = 28V- Rload=14 ohm => load current is 2ASo it seems to be working properly.But in Image 2 it seems that it is no longer working correctly.Image 2:- Vin=30V- Vout(set)=20V- Rload=10 ohm => the output current should be 2ABut Vout seems to drop to about 6.87 V instead of 20V and the output current is limited to about 700mA even if according to the datasheet at a Vin-Vout of 10V the current should be limited to about 2.8A. What is going on?It seems that I am missing something but I am not sure whatIf I remove the "startup" directive from the simulation then it seems to be working properly. I am not sure what the "startup" directive does so I can't tell which one of the simulations is more close to the reality: the one with the startup directive or the one without.InImage 3:we have the same setup as in Image 2 but with the startup directive removed.I am confused about the operation of the LT3038. So any clarifications are more than welcomedThank youImage 1:Image 2:Image 3:

author: Bored@Work
author_rank: Super Contributor
timestamp: September 02, 2012, 05:35:37 pm
content: In general, it there is no point in trying to evaluate the limits of a component using SPICE. This is because SPICE component models are typically not build to show real-world behaviour outside their specified operating area. You would have to carefully study the model (impossible with the encrypted LTSpice models) or get reliable information from the model producer to get confirmation that the particular model is an exception.Regarding the floating, well, LT apparently defines this as the device has no ground pin. But that definition is not very helpful here. First, Voltages are are always relative, that is the definition of Voltage - the electrical potential difference. So no surprise here. Second, you reference the output to ground, because you connect any load between Vout and ground. With the output referenced to ground non of the voltages specified really float. Vout is referenced to ground, and most other voltages are referenced to Vout. Therefore, for a particular Vout all the other voltages are also referenced to ground and do not float. You can see that as an application of Kirchhoff's second law.

author: hans
author_rank: Super Contributor
timestamp: September 02, 2012, 05:39:45 pm
content: You say the supply must be relative - yes that is correct. But you're exceeding the specifications here with 30V input.Why? Because at startup the output voltage is 0V, and so if you apply 30V the output-to-input voltage differential is 30V. This exceeds the device specification briefly, whichmayresult in failures.So that's why also the simulation may act weird in that case. Try decreasing the input voltage to 20V and play around.The lt3080 can go to 36V. You can also parallel these for more current too, see the applications in the datasheet.. It operates the same way; the SET in is basically the output voltage, so set it to any level you need, as long as it's within the absolute maximum specifications. So watch out for the maximum output to input voltages etc. - it's indeed all relative.

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 02, 2012, 06:06:09 pm
content: @Bored@Work:Yes, that makes perfect sense. However:QuoteDoes this mean that I understand correctly what the datasheet says: that Vin must not exceed 23V relative to Vout, and not relative to GND? So basically a Vin of 30V would not violate this as long as Vout is higher than 7V. Is this correct?@hans:That makes sens too.But I tried the simulation with two LT3083s in parallel and in that case it worked fine without acting weird even if at startup the input-to-output voltage was 30V.Thank you

author: Bored@Work
author_rank: Super Contributor
timestamp: September 02, 2012, 07:25:33 pm
content: Quote from: NewBeginner on September 02, 2012, 06:06:09 pmThat is indeed what the datasheet says:QuoteBut, there are many buts:These are the absolute maximum ratings. You should never come close to these. Do not max out the maximum ratings.Also, note that the 23 V are even provided with a condition. No overload, no short-circuit. Can you guarantee these, or is there a chance the output will be overloaded or even shorted?And the next "but"QuoteOnly if you can guarantee that Vout is always higher than 7V. Which might not be the case during startup, e.g. when you have a big capacitance load. Or when you have a short.

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 02, 2012, 09:54:47 pm
content: Thank you