LT3081 - nice feature upgrades to LT3080 - Page 1
Forum section: electronics / projects

author: konfu
author_rank: Contributor
timestamp: July 06, 2013, 01:19:29 pm
content: Hi folks,some weeks ago I found some new product being released by Linear Technology - theLT3081.Basically it's the same like LT3080 but with more features: current control via an extra pin, current sense output and a pin to give a current equivalent to the die junction temperature. Nice features.Sounds like a great upgrade to Dave's LT3080 power supply...I now struggle a bit to understand how the current limiting function works.To set the current limit LT uses a resistor between the current-set pin and the output pin of the LT3081. So for example: 0.5A output current limit is done by 1.65k between these pins.I am now thinking whether I can control the current limit the same way we do the voltage: apply some variable voltage to the Iset pin. But I guess it's not that easy.So how does this current limit set feature works internally? It's not about voltage being applied to the Iset pin as the resistor would change it's delivered voltage if the output voltage is being changed. Same applies to the current flowing through the resistor.There is no relation to the output voltage if I read the datasheet correctly. So the Iset resistor is not delivering either a current or a voltage to the Iset pin. I think it could be like this: there might be some internal resistor setting the current limit (if Iset is being left floating, there is some internal current limit of 2A). The Iset resistor is a parallel resistor to the internal one thus changing the current set resistor value.But how would I then control the current limit with some MCU? I could of course attach some nice SPI/I2C settable potentiometer. But these potis usually only accept some smaller voltage at their resistor network pins. Since one end of the variable resistor would be attached to the Output pin of the LT3081 I would need to use something like MCP41HV51 to cope with higher voltages.Anyone any idea how this current limit function works and how to control it with a MCU?

author: firewalker
author_rank: Super Contributor
timestamp: July 06, 2013, 01:47:31 pm
content: Have they addressed the issues with LT3080?https://www.eevblog.com/forum/projects/lt3080-wierdness-dave%27s-power-supply-%28eev224%29-gone-mad/Alexander.

author: Bored@Work
author_rank: Super Contributor
timestamp: July 06, 2013, 04:45:35 pm
content: Feck, you know what they have as application example in the datasheet?Arduino Monitored Supply. Don't they know any shame at Linear?The world is going to hell in a handbasket. I smell a wave of Arduino-powered "lab" power supplies coming up.Quote from: firewalker on July 06, 2013, 01:47:31 pmFrom the datasheet:QuoteI guess "without concern" followed by "application concerns" is marketing speak forWill blow up if ±10V differential voltages or ±25mA current between SET and OUT are exceeded.And they call that a "rugged" IC.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: July 06, 2013, 08:24:23 pm
content: Am I missing something or what is LT8612 ?I can findhttp://www.linear.com/product/LT8610http://www.linear.com/product/LT8611http://www.linear.com/product/LT8614but no LT8612, are they also going to release that?http://cds.linear.com/docs/en/solutions-manual/Linear_Automotive_Brochure_web.pdfOn page 7 we have:

author: konfu
author_rank: Contributor
timestamp: July 06, 2013, 10:05:23 pm
content: The schematic shown as "typical application" is NOT an Arduino controlled but just an Arduinomonitoredsetup.And I really don't get why they use a LT3092 programmable current source. They could just feed voltage into the SET pin to control the output voltage, couldn't they? As far as I can see the SET pin just goes into a comparator + input internally. So it tries to make the SET pin voltage equal to the OUT pin voltage. No big deal - just the same like LT3080 here.But still: I don't understand how to control the current limit with a MCU.LT8612 is just some sync step down converter. It's the same technique Dave used for the "tracking pre-regulator" to get the power loss via the LT3081 down. Maybe just a typo or a new version of 8610/8611 not yet released ;-)

author: kizzap
author_rank: Unknown
timestamp: July 14, 2013, 03:14:56 pm
content: Just wondering, has anyone had a detailed look at the datasheet, and worked out if it is possible to control the maximum current output by driving the Ilim pin with an op-amp in some way? I'm trying to wrap my head around it, but I just cannot decipher how the Rlim circuitry works. Knowing my luck, it is going to be something obvious, but my lack of knowledge is really holding me back here.The thing that is confusing me, (and is probably the hint in the operation of the circuit) is that the current limit is set by a resistor, which is independent of voltage, over the entire output voltage range.If anyone is able to shed any light on it, it would be greatly appreciated.-kizzap

author: Bored@Work
author_rank: Super Contributor
timestamp: July 14, 2013, 06:53:37 pm
content: Quote from: kizzap on July 14, 2013, 03:14:56 pmI wouldn't bother, because:* The limiter varies +/-15% over temperature. I.e. it is unusable for a "lab" power supply.* The limiter cuts the output voltage off, it doesn't regulate it down. This is not what you want in a "lab" supply.* The information in the datasheet is confusing and contradicting. If Linear can't be bothered to get their shit together, why should you wade through that shit?For example, already the schematic on the first page, "Typical Application" does not make sense. The IC has an internal fixed limit at 2A, independent of ILIM. But why do they have RILIM in that schematic set to 6.04 kOhm for a 2.24A limit? The internal limit would always trigger first, the external limit does not make sense. Especially not when the circuit is supposed to show a 1.5A supply. Taking the recommended 20% variation into account, it would make sense to set RLIM for 1.8A, i.e. to 4.9 kOhm. But they show 6.04 kOhmOr take the first circuit at page 21. Two ICs paralleled, for a 3A output. A single 3k current limit resistor shared by both ICs. How on earth did they derive that value? 3k would be a 1A limit for a single IC. Depending on the magic Linear is using for the current limit setting it would make sense that a 3k resistor, shared by two ICs, sets a limit of 0.5A, 1A or 2A. But 3A?Whatever Linear is doing in that IC they really don't tell in the datasheet. I wouldn't touch that IC because of the rubbish information and because of its SET/OUT brokenness.

author: c4757p
author_rank: Super Contributor
timestamp: July 14, 2013, 06:58:29 pm
content: What a bizarre datasheet. It looks very hastily thrown together.

author: Bored@Work
author_rank: Super Contributor
timestamp: July 14, 2013, 07:11:18 pm
content: Quote from: c4757p on July 14, 2013, 06:58:29 pmYes, it is surely missing the hand of Jim Williams.

author: kizzap
author_rank: Unknown
timestamp: July 15, 2013, 01:13:16 am
content: Well, fair enough. back to the venerable lm317 then I guess...Strange how a series of chips can be so flawed to the point that no-one wants to use them.-kizzap

author: EEVblog
author_rank: Unknown
timestamp: July 15, 2013, 03:11:11 am
content: Quote from: konfu on July 06, 2013, 10:05:23 pmYou can't, it's not designed for that. Possible to use an EEPOT for example, but they don't go to high voltages.Similar to many other switching regulators that have resistor programmable current limits.There may be some tricky way to do it, but the spec is 15% tolerance over the tamp range, so it's pretty horrible. Not usable for a lab supply current adjust.

author: Tooms
author_rank: Unknown
timestamp: February 13, 2014, 07:11:40 pm
content: In the LT Journal january 2014 there is 8 pages about the LT3081 and that it is now fixed and better then the LT3080http://cds.linear.com/docs/en/lt-journal/LTJournal-V23N4-01-df-LT3081-RobertDobkin.pdf/Tooms

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 09:26:04 am
content: Quote from: Bored@Work on July 06, 2013, 04:45:35 pmThe typical LM317 and many similar 3 terminal regulators have the same output short to ground while the adjustment pin is bypassed problem. The solution there is to add a diode between the adjustment and output pins to discharge the adjustment pin capacitance if the output is shorted.