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LM317 load capacitance
LM317 load capacitance
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Old 19th October 2017, 11:43 AM   #21
greierasul is offline greierasul  Romania
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Capacitance multiplier before LM317. It is very important how constant is that 500mA current. If you have a small variation and low frequency, you can use a reasonable C-ferrite-C filter after LM317.

Last edited by greierasul; 19th October 2017 at 12:01 PM.
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Old 19th October 2017, 05:42 PM   #22
mchambin is offline mchambin  France
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Originally Posted by dotneck335 View Post
On the LM317 application notes, they show a 1uF capacitor on the output. Is that what you are recommending?
yes
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Old 19th October 2017, 08:33 PM   #23
Xoc1 is offline Xoc1  United Kingdom
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I built some 317 337 regulators recently and found this article very useful

Using 3-pin regulators off-piste: part 1

I built 2 stage regulators with diode protection - Used zeners and LEDs to set the voltages instead of the lower resistor in the voltage setting resistive divider, which gives better performance at lower frequencies, and used a cheap 10uf hign esr capacitor on the output local to the regulator.
If you follow the article the circuit I used was similar to the last circuit used in part 4.
I have not measured what I built but I am very happy with the results when used in my DAC project.
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Old 20th October 2017, 03:06 AM   #24
dotneck335 is offline dotneck335  United States
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Originally Posted by abraxalito View Post
THANK YOU!! That is exactly the sort of information I was seeking! Certainly not some snide remark about lists.
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Old 20th October 2017, 04:08 AM   #25
infinia is online now infinia  United States
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Originally Posted by dotneck335 View Post
THANK YOU!! That is exactly the sort of information I was seeking! .
Linear Tech. recommends 1uF solid tantalum as an output bypass cap because of its high frequency esr performance. This is useful b/c the V regulators loop gain tapers off. Really big output caps cannot offer any performance compared to the devices negative feedback, plus they fail to deliver at extended frequencies when its needed most! Depending your application Re. wiring/ PCBs you might need more bypassing further away from the 3T Vreg.
That guys blog echoes the LM317 data sheet, not any new information. My National Semi. Linear data book had the best data sheet since they invented it. Not every LM317 is created equal .E.g. internal band gap designs with nearby pass transistors and semi-processes matter. I reckon Linear Tech's is better than most esp. since everybody and their brother offers them E.g. its a jellybean part. The data sheet numbers are not hard to meet, its the typical numbers and how tight your processes are.
BTW If you need better ripple rejection (unlikely) it's reasonable to cascade regulator devices. Perhaps a series plus a shunt part, rather than more low pass filtering using BFcaps!
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Last edited by infinia; 20th October 2017 at 04:36 AM.
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Old 20th October 2017, 04:44 AM   #26
gmphadte is offline gmphadte  India
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Capacitance multiplier is another option.

Gajanan Phadte
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Old 20th October 2017, 04:55 AM   #27
flyingfishtw is offline flyingfishtw  Taiwan
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I'm curious, If I want to lower high frequency noise, is it helpful to use 10R +470uf on the output near op amp side?

So the HF noise I have to reduce will be emi,distortion from main, and LM317's thermal noise? Another question, if a op amp is 30Mhz, set gain as 10, so BW is 3Mhz, what should the power supply bandwidth be?
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Old 20th October 2017, 07:28 AM   #28
dotneck335 is offline dotneck335  United States
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Originally Posted by traderbam View Post
I'd tend to agree with Wintermute to get rid of it upstream of the regulator. It depends how far you want to go...how small you want the ripple to be. Here's a LCR solution (an inductor is your friend) .
However, the ripple isn't pure 120Hz and contains a lot of HF hash. The LCR would definitely crush this.
Yes,getting rid of as much unwanted ripple and noise before the regulator is a good idea. I get how the capacitor(s) and inductor do just this. But how does the resistor help in this case? Seems to just eat heat and regulator headroom.
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Old 20th October 2017, 07:52 AM   #29
traderbam is offline traderbam  Europe
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Old 20th October 2017, 08:01 AM   #30
Chris888 is offline Chris888  United Kingdom
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Originally Posted by dotneck335 View Post
Yes,getting rid of as much unwanted ripple and noise before the regulator is a good idea. I get how the capacitor(s) and inductor do just this. But how does the resistor help in this case? Seems to just eat heat and regulator headroom.
The R in post 19?
Without it, the two C's and the L form a resonant circuit.
It also limits the inrush current to the second C
That kind of thing can bite you if the voltage regulated load is subject to steps in current drawn.
The optimum pre-filter in one application may be an own goal in another.

You make a fair point about regulator headroom. Sometimes you have to think about keeping everything working when the mains dips. Heat dissipated by the R was going to be dissipated by the reg anyway, for a given transformer voltage.
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