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Is there anything wrong with this PS design?
Is there anything wrong with this PS design?
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Old 29th December 2017, 02:21 PM   #21
flyingfishtw is offline flyingfishtw  Taiwan
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Quote:
Originally Posted by AndrewT View Post
your layout will not perform at the best that a 317 can achieve.
How can you measure the difference? a pulse on a heavy load?
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Old 1st January 2018, 02:51 PM   #22
simonra is offline simonra  United Kingdom
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I'm getting some other PCBs made up soon so I thought I'd get some new PS ones done at the same time. I've updated the design to address several of the comments so far and decided on a fixed +-15V output.

As usual the graphics output from Eagle are a bit crappy, in places it looks like one side of the SMD pad is not fully isolated. I've checked that they are and run a DRC to be sure.

... Does this look like an improvement?
Attached Images
File Type: png Top Layer.png (71.6 KB, 130 views)
File Type: png Bottom Layer.png (63.1 KB, 126 views)
File Type: png Schematic.png (122.6 KB, 132 views)
File Type: png Layout.png (149.1 KB, 107 views)
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Old 1st January 2018, 06:07 PM   #23
martin clark is offline martin clark  Europe
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Quote:
Originally Posted by MarcelvdG View Post
Use run-of-the-mill electrolytic capacitors for C4 and C7. Some brands of LM317 become unstable when the losses of the output capacitor are too small. I once connected an LM317 to two 10 uF X5R ceramic capacitors in parallel, which turned my LM317 into a sawtooth oscillator. The capacitors themselves worked as loudspeakers, so you could hear a beeping sound coming from the board. Connecting a 47 uF cheap aluminium electrolytic capacitor in parallel solved the problem.
This!

Using 3-pin regulators off-piste: part 3

HTH.
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Old 1st January 2018, 07:30 PM   #24
MarcelvdG is offline MarcelvdG  Netherlands
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They just write about noise bumps, but my LM317 really started oscillating. It was apparently on the edge: when you started up the circuit, there was about 30 % chance that it would beep and about 70 % chance that it would not.
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Old 1st January 2018, 07:37 PM   #25
jan.didden is offline jan.didden  Europe
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I think most of these regs need a lossy cap at the output for stability. Usually the data sheet gives the minimum ESR required in the output cap.

Jan
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Old 2nd January 2018, 12:58 PM   #26
simonra is offline simonra  United Kingdom
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I'm thinking of going with something like this tantalum cap for C7 & C8 (from schematic in post 22) - it has an ESR of 6.6 ohms... do you reckon that'll do the job?
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Old 2nd January 2018, 03:56 PM   #27
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by jan.didden View Post
I think most of these regs need a lossy cap at the output for stability. Usually the data sheet gives the minimum ESR required in the output cap.

Jan
According to the TI datasheet for the LM317 an output capacitor is not needed for stability
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Old 2nd January 2018, 06:43 PM   #28
MarcelvdG is offline MarcelvdG  Netherlands
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I usually avoid tantalum capacitors because tantalum is a conflict mineral and parts manufacturers never specify where they get their tantalum from, but yes, I think they should do fine.

The application note Martin referred to shows a resonance frequency of 22 kHz at 820 nF load for a 10 V regulator without adjustment pin decoupling. That boils down to a regulator output inductance of 1/(4 pi^2 (22 kHz)^2 * 820 nF) ~= 63.824 uH. With adjustment pin decoupling, the high-frequency loop gain increases, which makes the inductance decrease: 63.824 uH * 1.25 V/10 V ~= 7.9779 uH. With a 1 uF output decoupling, the resonant frequency will be about 56 347 Hz and the reactance at resonance will be about 2.8245 ohm. With 6.6 ohm ESR, your capacitor then has a Q factor of only 0.42796, which is absolutely perfect for this application.

The only thing you have to be careful with is decoupling capacitance in the equipment you want to supply with this power supply. If it has high-Q decoupling capacitors with a total capacitance >> 1 uF, you could still get problems.
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Old 2nd January 2018, 06:50 PM   #29
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by scottjoplin View Post
According to the TI datasheet for the LM317 an output capacitor is not needed for stability
Which doesn't help much, because in most practical applications there will be some local decoupling capacitors connected to the output anyway.

I also failed to find anything about the required ESR in any of the LM317 datasheets I've seen. There are sometimes plots of the regulator output impedance (on a very course scale with no phase plot) that you can use to estimate the resonant frequency, but that's about it.
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Old 2nd January 2018, 07:16 PM   #30
simonra is offline simonra  United Kingdom
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Originally Posted by MarcelvdG View Post
I usually avoid tantalum capacitors because tantalum is a conflict mineral and parts manufacturers never specify where they get their tantalum from
That's a good reason to avoid them... I'll see if I can find something similar that will work. And thanks for doing the maths, I'm still fairly new to this and it takes a bit of time for me to absorb the information.
Quote:
Originally Posted by MarcelvdG View Post
The only thing you have to be careful with is decoupling capacitance in the equipment you want to supply with this power supply. If it has high-Q decoupling capacitors with a total capacitance >> 1 uF, you could still get problems.
I've attached a schematic of the phono stage I plan to use this PS for... I think I should be okay as the most of the decoupling happens on the output side of the DC input op amps.
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