LT3080 Shutdown implementation. - Page 1
Forum section: electronics / beginners

author: Kareem
author_rank: Newbie
timestamp: July 01, 2016, 05:28:32 pm
content: HiI am fairly new at forum posts, so bare with me if my post is confusing or if this post is repeated.My question is related to the diode placed between the output pin and the set pin of the LT3080. I need to know its purpose and if it is possible to use the shutdown configuration without it.I ask this because I usually see a protection diode connected across the output and input pins of regulators, which is not the case here.I will be using the LT3080 in a similar manner as Dave did in his µSupply. It will be used as constant voltage and constant current source to drive a resistive load with an added feature of being controlled externally by an MCU.Thank you for your help.

author: SeanB
author_rank: Super Contributor
timestamp: July 01, 2016, 06:45:41 pm
content: Add the other diodes as well. They are added insurance as the LT3080 has some rather unfortunate habits if reverse biased in that it will blow up. Thus the extra diodes are needed. In this note they were likely left out, but do appear in the other notes so use them.

author: Signal32
author_rank: Frequent Contributor
timestamp: July 01, 2016, 07:14:05 pm
content: The diode seems unrelated to the shutdown functionality. The shutdown works by shorting the control voltage to ground, forcing 0V on the output. Furthermore it also shorts the output to ground. When shutdown is in place the two diode terminals will also be shorted together through the MOSFETs so it serves no purpose there.Indeed it is confusing that they decided to add the protection diode only when presenting shutdown functionality. So yes, you can use the shutdown functionality without it, though it's probably not the best thing to do.

author: Kareem
author_rank: Newbie
timestamp: July 01, 2016, 07:43:39 pm
content: Quote from: SeanB on July 01, 2016, 06:45:41 pmThat goes without saying, i'll definitely add the protection diodes.Quote from: Signal32 on July 01, 2016, 07:14:05 pmIt is as you say, it is probably better to trust the datasheet and place it anyways. But I will forever remain not knowing why.....

author: ZeTeX
author_rank: Frequent Contributor
timestamp: July 02, 2016, 07:14:46 pm
content: There was alot of problems with the LT3080 and building CC/CV PSU using this regulator is a very bad idea.https://www.eevblog.com/forum/projects/lt3080-wierdness-dave's-power-supply-(eev224)-gone-mad/

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 02, 2016, 09:46:14 pm
content: Quote from: Signal32 on July 01, 2016, 07:14:05 pmnot sure the actual implementation as i dont have the chip. but the diode like that probably to ensure the (probably) high impedance node of the "set input" close to the Vout rail level avoiding it wanders off randomly to 0V when Q1 is opened (normal operation) we dont like our PSU to be intermittently on and off for no reason. but as i said, i dont have the chip to attest. having said that, you probably wont need Q2 to drain the output during shutdown since the diode does the same job, its just you'll need a bigger one than 1n4148 if you have beefy inductive or capacitive load on the output.

author: C
author_rank: Super Contributor
timestamp: July 03, 2016, 01:29:56 am
content: The 1N4148 is a try at keeping the ISET pin with in theLT3080'sAbsolute Maximum RatingsSET Pin Voltage (Relative to OUT) .........................±0.3VIf you look at Diodes's 1N4148 data sheethttp://www.diodes.com/_files/datasheets/ds12019.pdfA current flow of 0.01 amps = 0.4 VoltsSo a current flow of 0.01 amps through the 1N4148 puts the LT3080's ISET out of limits.

author: Kareem
author_rank: Newbie
timestamp: July 29, 2016, 10:18:45 pm
content: My apologies for the late reply, but I'd like to thank you for your responses. As an update, I have refrained myself from using the LT3080 and have not yet settled on a component as the project is paused at the moment.