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Old 7th January 2013, 09:06 PM   #181
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Quote:
Originally Posted by Gopher View Post
Tony

Have you taken into account the fact that the o/p inductance of the LM317 reduces significantly as Iout increases? I think Erol Dietz's measurements of resonant frequencies were for a datasheet implementation of the LM317 circuit with 5mA or 10mA Iout, but doesn't your circuit have a larger Iout and therefore a lower o/p inductance?
Actually Dietz' paper covers this as well

Click the image to open in full size.

- so beyond about c. 50mA I'd expect very little change in the modelling ( - as I posted above)

NOTE - this graph is at 1.25v out, i.e no 'lower' R to 0v for voltage setting. That means the graph only holds if you use the optional Cadj bypass cap at higher voltage outputs (Yes, using this cap drops the effective output inductance at higher voltage output. Obvious once you think about what it does)

I have sometimes deliberately added power resistors to '317 regged supplies to push the total draw toward or well past that 30-50mA mark - on low-demand items when the raw supply / heatsinking will comfortably support it. Sometimes it seems to help by measurement, even audition.

Similar things are true of more complex regulators, too: it seems mostly to do with re' and beta in the output pass transistor IME.

Last edited by martin clark; 7th January 2013 at 09:13 PM.
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Old 14th January 2013, 04:07 PM   #182
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Dumb question: when putting this resistor in series with the output capacitor to damp any resonance, do we have to care about its wattage rating?

For example, for a 9V output voltage in series with a 1uF polyester capacitor, and a 1Ω resistor in series with it, what is the minimum wattage rating?

On DC conditions, it shall not dissipate power I suppose.
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Old 15th January 2013, 09:14 AM   #183
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Quote:
Originally Posted by audiostrat View Post
On DC conditions, it shall not dissipate power I suppose.
correct 1/4 watt should be more than enough! It is only really noise that is going to be being passed to ground via the cap, and hopefully this is already at a very low level due to the reg

I ran the sim of my reg circuit with a 600mA current source as the load and the power dissipated in the resistor (in the case of the sim a 0.5 ohm and cap was 100uF) was 270 pico watts. I believe that this is from the residual ripple still present on the output.

With a real (varying) load this may change a little but I don't think you are going to see any significant power being dissipated in this resistor. I actually used some small SMD resistors in this place.

edit: I re-ran the sim with 1uF and 1 ohm and the power dissipated in the resistor dropped to 56 fempto watts. I guess since the residual noise is 100Hz and it's harmonics that 1uF is going to struggle to do much about it.

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Last edited by wintermute; 15th January 2013 at 09:18 AM. Reason: re ran sim.
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Old 15th January 2013, 10:29 AM   #184
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Seems quite right! The 1uF cap is a bit small for anything severe to pass.
I will try a 1/4 watt metal film then!

I will also add a 100u cap on the output, in parallel with the RC series network.
I assume that this is not going to ruin the damping I will aply using the 2R7, am I right?
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Old 15th January 2013, 08:57 PM   #185
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Not exactly. If you put a low esr 100uF cap in parallel with the 1uf then it will still have a resonant peak, it will be lower in magnitude and shifted down in frequency though.

On simming the 1uF with 2.7 ohms in series even that apears to give quite a peak. 100uF with 0.7 ohms in series seems to be a much better option.

edit: that last one I forgot to put in some inductance in the 100uF cap so it does not show the rising impedance with frequency.

Tony.
Attached Images
File Type: png 1uf.png (24.4 KB, 414 views)
File Type: png 100uf.png (25.6 KB, 412 views)
File Type: png 1uf+100uf.png (26.5 KB, 409 views)
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Old 15th January 2013, 11:28 PM   #186
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Quote:
Originally Posted by wintermute View Post
Not exactly. If you put a low esr 100uF cap in parallel with the 1uf then it will still have a resonant peak, it will be lower in magnitude and shifted down in frequency though.

On simming the 1uF with 2.7 ohms in series even that apears to give quite a peak. 100uF with 0.7 ohms in series seems to be a much better option.

edit: that last one I forgot to put in some inductance in the 100uF cap so it does not show the rising impedance with frequency.

Tony.
Quite detailed. So, shouldn't I use any capacitor for feeding the stage that will be powered from the LM317? I am asking in general. Or should I not worry, counting on a possible decoupling capacitor that could be found on the input of the unit's to be fed power supply?
I get that the use of that cap "introduces" and "eliminates" at the same time possible resonances, so is that cap still the cap used for decoupling and current-feeding the stage to be powered?
Plus, I assume that your models include the varying output inductance as far as load current is concerned, posted above. In some applications where I plan to use this, the total current going through the 317 could be a quite small 15 milliamperes.
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Old 16th January 2013, 06:28 AM   #187
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good questions, and I can't give you a straight answer! I'd not thought about any capcitance on the load, and the potential effects of that. Perhaps when it is far enough away (assuming a few cm of wire between powersupply and the load) the problem is reduced.

The cap on the output is generally there to ensure stability of the regulator. The datasheet recommends using tantalum capacitors, these generally are not low esr. It is the use of low esr caps that makes addition of additional series resistance necessary.

a typical 1uF solid tantalum capacitor as specified in the datasheet probably has enough resistance to damp it to the point where it is not an issue. However if you were to use a 1uF film cap on the output it would be a very different story.

Yes the model does change with current. The resonant peak shifts when the current is varied (up to a point).

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Old 16th January 2013, 06:34 AM   #188
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Wintermute, any chance you can share you LM3xx models?

Thanks
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Old 16th January 2013, 10:13 AM   #189
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Hi Bonsai, I'll put something together Have to go to my laptop I think it has the most recent models on it.

Tony.
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Old 16th January 2013, 10:20 AM   #190
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Quote:
Originally Posted by wintermute View Post
Not exactly. If you put a low esr 100uF cap in parallel with the 1uf then it will still have a resonant peak, it will be lower in magnitude and shifted down in frequency though.

On simming the 1uF with 2.7 ohms in series even that apears to give quite a peak. 100uF with 0.7 ohms in series seems to be a much better option.

edit: that last one I forgot to put in some inductance in the 100uF cap so it does not show the rising impedance with frequency.
Interesting. In one case we can see a 7.5v resonant peak. How does that "sound" or manifests itself?

It's placed at about 35KHz, so there's no chance anyone will "hear" that, but I wonder how it does affect things, directly or indirectly.
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