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Lo_Volt
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LM317 Cap On ADJ Pin

03/23/2011 5:59 PM

It's been a while since I've thrown together a linear power supply, but seeing this on a colleague's circuit raised a red flag. The cap in red will simply slow the regulator's response to high frequency changes in load. In fact at 10 uF, it will almost render the regulator useless except for very stable loads.

Am I right?
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JohnDG
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#1

Re: LM317 cap on ADJ pin

03/23/2011 6:41 PM

What persuaded your colleague to put it there? Do they have any rationale?

I haven't seen this arrangement on any datasheets.

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Lo_Volt
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#2
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Re: LM317 cap on ADJ pin

03/23/2011 7:07 PM

I haven't had the opportunity to ask him and he's gone for the night. I know this regulator was put in because the circuit it is supplying is very sensitive to noise on the power supply. I'm wondering if he was putting caps everywhere in desperation. I will ask him tomorrow.
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Tobugrynbak
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#3

Re: LM317 Cap On ADJ Pin

03/23/2011 7:33 PM

Ok, for your peace of mind.

Remove the errant 10uF cap from the adjust pin as that will cause noise.

A 1uF with a 0.1uF(as close to the LM317 as possible) on the input side. On the output have a 0.1uf (as close to the LM317 as possible) a 1uF and a 10 uF (in parallel)on the output side. Ensure that the reference resistors are close to the LM317 and that the leads are short (I'm assuming this is a breadboard/Veroboard/Plate Through Hole PCB construction) If this is a SMD design then ensure you have good ground plane coverage on the top and bottom sides of the PCB. It helps if your not drawing a heap of current out of the LM317, once over 1 Amp then things will get hot and bothered( and the noise will creep in).

Is there a reason for the use of a LM317 over a fixed 3 term regulator? Bespoke voltage perhaps, or maybe that's what was in the scrap box? Just curious is all.

I've migrated most of my designs over to switchmode regulators as they're cooler running and somewhat more efficient current wise for a given voltage. And yes you can breadboard/veroboard build them.

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JohnDG
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#4
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Re: LM317 Cap On ADJ Pin

03/23/2011 7:48 PM

GA regarding switch-mode regulators.

Replacements are available for e.g. the LM7805, pin-compatible with the TO220 version, but about 90% efficient.

Cool!

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SimpleMind
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#5

Re: LM317 Cap On ADJ Pin

03/23/2011 10:59 PM

Hi. This trick can be usefull in special cases that you want "soft starting" in a cirquit. It actually won't deteriorate the responce in fast load changes or anything similar providing you use the input and output capacitors too, and possibly a diode from adj to input (reverse) if capacitor is relativelly big ,just to protect the regulator in shutdown, it will just delay reaching final voltage (depending on capacitor value). Will start output at about 1.2V and gradually go to regulated voltage S.M.

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Dave Simpson
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#20
In reply to #5

Re: LM317 Cap On ADJ Pin

03/27/2011 11:14 AM

Exactly.

The cap between output and common (ADJ) should be as close to the IC as possible. Usually 0.1uF. That's to prevent parasitic oscillation because of resonances in the load.
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steve__y
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#6

Re: LM317 Cap On ADJ Pin

03/23/2011 11:06 PM

Yes , you are right. This cap is to smoothen the voltage changes at output, but you are using for high speed load changes and it does not respond that fast. Try with 0.1 uF or just remove it totally. My best suggestion is to read the data sheet of LM 317. I have seen 317 is used for AUDIO amplifiers output stage. I used LM317 for a TATOO machine controller and this is a fast changing load. Works fine. Same way it is used on simple two station interconnected phones for audio frequency and is OK.
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#7

Re: LM317 Cap On ADJ Pin

03/24/2011 3:38 AM

The great thing about inserting extra random parts into a design and circuitboard is you can always leave 'em out or change the value later.
I generally add 10% extra random components to allow for design cockups.
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(That's a joke...just in case anyone had their doubts about me)

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JohnDG
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#8

Re: LM317 Cap On ADJ Pin

03/24/2011 11:33 AM

You live and learn! It'll teach me to read the datasheets more carefully.

Note the comment regarding protection diodes.

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rcapper
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#9

Re: LM317 Cap On ADJ Pin

03/24/2011 10:27 PM

There are also newer regulators that specifically address the noise issue. It becomes especially important if you are doing instrumentation grade audio work. You can hear noise that is difficult or impossible to see on a scope.

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Johny451
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#10

Re: LM317 Cap On ADJ Pin

03/24/2011 11:27 PM

The resistors form a divider in order to raise the reference voltage of the regulator. They aren't there for feedback. Smoothing the reference witha cap. is fine.

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Shyam
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#11

Re: LM317 Cap On ADJ Pin

03/25/2011 1:33 AM

For LM317 regulator, the shown 10uF capacitor can kill the regulator when you switch OFF the power if parallel resistance to 10uF is too high. As a rule this regulator requires a diode between output and input to sink the output voltage on power down. In similar way it should also remove the voltage on 10uF by a diode to input if input goes below the voltage on 10uF. If you use capacitor then also use two diode. see application notes on this regulator.

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Randall
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#12

Re: LM317 Cap On ADJ Pin

03/25/2011 7:10 AM

For the record:-

Better/faster load regulation.

The regulator works by attempting to keep the voltage between the output and the adjust pin at 1.25V. With the cap there, if the load increases the voltage will reduce across those pins slightly more than without it.

As SimpleMind said it will also provide a soft start.

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Shyam
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#13
In reply to #12

Re: LM317 Cap On ADJ Pin

03/25/2011 10:24 AM

Smart one!

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#14
In reply to #12

Re: LM317 Cap On ADJ Pin

03/25/2011 1:24 PM

I assume that you took that from the NS datasheet linked by Lo_Volt in #9.

That doc is dated Feb 25, 2011. I found the same drawing (with the ref. no. TL/H/9063-7) in my battered old NS Linear Databook 1, dated 1988. Be interesting to know when the circuit first appeared.

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Randall
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#15
In reply to #14

Re: LM317 Cap On ADJ Pin

03/25/2011 1:57 PM

I think I went from Tobugrynbak's link

I suppose we are a lot better off nowadays with datasheets all over the 'net, but, it was nice when you could just reach for the right book and it would fall open at the right page. Also more and more companies seem to be deleting datasheets for obsolete components. I guess this is marketing led, but, I think we should all make more of a fuss, sometimes we need those data sheets, even if we're just trying to look for a replacement.

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Lo_Volt
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#16
In reply to #15

Re: LM317 Cap On ADJ Pin

03/25/2011 10:17 PM

I still have a Linear Applications Handbook from National, circa 1988 or so. I should have cracked it open first.
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#17

Re: LM317 Cap On ADJ Pin

03/26/2011 9:18 AM

I've done a quick spice simulation, using LTspiceIV (free download available from http://www.linear.com/designtools/software/). I don't know how good the LM317 model is; I found it on the web (here).

With reference to the circuit in #14, R1=240Ω, C1=1μF, R2=720Ω, C2=10μF.

In each case, the version without the capacitor on ADJ is shown first.

1. Turn-on (25VDC supply, 100mA load):

2. Ripple rejection (25VDC supply with superimposed sinewave 1Vp-p 100Hz, 100mA load):

3. Load variation (25VDC supply, load varying sinusoidally between 5mA and 105mA at 100Hz):

I fully accept that the simulation is just a lash-up. The input ripple would be much more likely to be a full-wave-rectified sinewave, and all the perturbations are quite extreme (to emphasise the effect).

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#18
In reply to #17

Re: LM317 Cap On ADJ Pin

03/26/2011 9:57 AM

JohnDG

Nice work on Line regulation simulation,

Now, try 0-100mA 100Hz load current change and see the regulation on simulation chart.

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#19
In reply to #18

Re: LM317 Cap On ADJ Pin

03/26/2011 10:43 AM

0-100mA is a bit unfair - the output doesn't like (near) zero load, and tries to jump up towards the supply voltage.

My example (3) shows your suggested load with a 5mA offset.

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