Analog Device article low noise 5V 5 Amp base on LT6658 but ??? - Page 1
Forum section: electronics / general

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 12, 2025, 12:53:34 pm
content: HelloAs I need a reliable PSU in 5 V up to 1 Amp for my current source , I have located this circuit which looks interesting by using only 2 transistors as I don t need 5 Amp , but ?from my understanding that a voltage regulator more than other thing as I don t see how they filter something , I think the quality of the noise level is the result of the Vin voltage quality ( orange on the circuit ) , which can be a battery pack .I plan to use battery pack to limit as possible noise and pollution.RegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2025, 01:04:57 pm
content: I somewhat doubt that the shown circuit could actually deliver up to 5 A. The transistors have a current gain of maybe 50 (could be less at higher current) and thus 100 mA base current needed. This looks more like an upper limit of 0.5 A. For more current one would need higher gain transistors -likely more a darlington configuration. I am also not sure about the stability against oscillation - it may be limited to a well behaved load.

author: Andreas
author_rank: Super Contributor
timestamp: January 12, 2025, 02:43:04 pm
content: Quote from: Overspeed on January 12, 2025, 12:53:34 pmHello,Noise level is determined by- low noise voltage reference- PSRR from VIN to VOUT (so mainly the OP-AMP features)A darlington will not be possible without violating the VOUT maximum rating of 6V.It is also strange to use the 267R resistor to divide down the reference voltage to 1 V (with unknown tempco)and then amplificate back to 5V if you only need a fixed 5V voltage.The cirquit also has no current limitation.For 1 A low noise output I would use something like LT3080IT.With best regardsAndreas

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 12, 2025, 03:37:55 pm
content: Quote from: Andreas on January 12, 2025, 02:43:04 pmHelloThanks , after Kleinstein assessment , your assessment complete the concerns list.LT3080 looks easy with 40 uV of noise and if I use then in // as the linked pic I can reduce the noise ( I think ) and perhaps set a filter ? as a RC noise-reduction network ?Other advantage of the LT3080 that a TO220 component so a robust solution with possible thermal controlTPS7A85 is also a solution but I always doubts regarding the fact to draw current rom a so small IC I link the TI doc , that why I search a more ''old style circuit'' to have no concern with thermal management .ADM7151 rom Analog looks also interesting but not sure that possible to use them in //.RegardsOS

author: hli
author_rank: Frequent Contributor
timestamp: January 12, 2025, 09:16:44 pm
content: Quote from: Kleinstein on January 12, 2025, 01:04:57 pmThe LT6658 can deliver up to 150mA on that output. So it should be fine at 5AThis circuit islinkedat the official LT6658 page, and there it also discusses noise performance.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: January 13, 2025, 02:08:49 pm
content: The LT6658 was designed to deliver significant output current with low noise performance by using an existing reference coupled with low offset amplifiers (I was in those design reviews). I would agree on the question of dividing the bandgap voltage down since there is a question of tolerance and TC of resistors. The LT6658 can deliver enough current to get the 5A required, and stability should not be an issue with it configured in the emitter follower configuration, especially with the LT6658 being designed to have capacitance on the outputs (1uF to 50uF is required).The LT3080 is a good part (as are many others in the family) but not quite as low in terms of noise and is a bit worse overall for regulation (output variations with load/line). Then again, I may have some bias towards those parts.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 13, 2025, 02:55:23 pm
content: Quote from: AnalogTodd on January 13, 2025, 02:08:49 pmHelloI agree on noise level between LT3080 and the LT6658 circuit my concern is not to invest too much time in testing if there is concern in the circuit as proposed , I need 2 amp as I use only 1 amp , no need to reach 5 amp so a solution with two transistors sound realistic .Displayed noise level is interestingRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2025, 03:02:59 pm
content: Assuming a gain of 100 for the 2N3055 is rather optimistic. The specs are for a gain of 15 to 120. So one should at least check and select. There may be other power transistors with a better gain.Anyway even just 50 mA from the reference chip would cause quite some heating in the reference and this way a load effect.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 13, 2025, 04:29:27 pm
content: Quote from: Kleinstein on January 13, 2025, 03:02:59 pmHelloThanks , that point which is a concern , and I always try to never overload IC whatever the IC is so that why I plan to use a 2 amp max current for only 1 amp needed.the LT6658 is a MSOP IC so something tiny with no real surface to cooldownAs on the shelf solution without ''nothing to do'' at quite affordable price perhaps this one which can be also time saving.https://iancanada.ca/products/linearpi-mkii-ultra-low-noise-smt-linear-power-supplyMatch my need : convert 12 V dc from battery to 5 V dcRegardsOS

author: AnalogTodd
author_rank: Regular Contributor
timestamp: January 13, 2025, 08:46:09 pm
content: Depending on noise and other characteristics you are looking for, you can always parallel two LT3041, or possibly take the LT3042 and use the boosted output current circuit in the back of the data sheet.Question is how tight do you need the output to be? Taking the LT3080 and parallelling two is easy but introduces some load regulation issues and it isn't quite as low with regards to noise. It does come in TO-220 to help deal with power dissipation. The LT3042 with boosted output current will get you good regulation and you can use a big transistor mounted to a heat sink for the current boosting.Doing 12V to 5V should be easy, the critical part is what specs you need for regulation, noise, etc. for your circuit. Not to mention is this a single project sort of thing or are you looking for production worthy stuff and volumes?

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 13, 2025, 09:19:33 pm
content: Quote from: AnalogTodd on January 13, 2025, 08:46:09 pmHellothanks or your answerI don't plan to run production as I work at the laboratory level so I will built two samples + parts to repair / built a third one , electronic production under CE marking is Hell regarding costs and paper works .Purpose is to feed my current source for resistivity measurement.My initial idea was to use TI TPS7... but I don't like high current and small footprint IC with small thermal area and thin pcb tracks as hairs .I need lowest noise as possible ( the TPS7 was a nice candidate ) to provide 1 Amp with some margin to avoid overloading of the IC.Current will not need to be 24/24 hour as that 1 minute test durationI use a bank of lead batteries in 12 V so typical 13.5 V DC to eliminate AC problemLT3080 sound as an entry level solution and easy to built with strong TO220 , can used orfirst trials ( quick and dirty ) as right now I use a laboratory PSU rom Agilent but that a true heavy PSU.I have no room / enclosure limitation as that not a portable / handheld system but a bench system.the LT6658 solution was interesting ( looks to be ) as the circuit looks simple but after some review that looks not so perfect / instant solution .LT3042 boosted can be a solutionRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2025, 11:09:21 pm
content: A good current regulating circuit should not need a super stable supply. So I don't think you would need an especially low noise or low drift regulator. Most voltage reading circuits would not care much about noise above some 10 Hz.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 14, 2025, 07:30:40 am
content: HelloI agree on noise but generally noise is linked with stability and output value , in a general way I never use overkill solution or something I always prefer to balance cost / availability and first reliability .I will built some thing easy and test with my 100 mA circuit , LT3080 can be a quick solution and at realistic cost , 7 euro on Mouser website .RegardsOS