Author Topic: Importance of input and output capacitance for LDOs  (Read 7871 times)

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Offline kolonelkadat

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Importance of input and output capacitance for LDOs
« on: October 11, 2014, 08:00:40 am »
I was looking in the datasheet for the ST 1117 im using and it says that the ldo is stable with 10uF on the output. Looking at the diagram, it shows 100nF on the input and 10uF on the output.

The problem is that I dont have any 100nF caps. So I wonder, can I use one with higher capacitance? Specifically 100uF?
 

Offline Simon

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Re: Importance of input and output capacitance for LDOs
« Reply #1 on: October 11, 2014, 08:07:20 am »
yes and no, they are specifying 100nF because pretty much any 100nF capacitor will have very good ESR carachteristics which are required to keep the regulator stable. A 100uF capacitor may not have a sufficiently low ESR to be stable.

try it, it may work, but don't assume more capacitance will automatically mean more stable.
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Offline kolonelkadat

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Re: Importance of input and output capacitance for LDOs
« Reply #2 on: October 11, 2014, 09:33:34 am »
Thanks for the input. My main concern is that it would make it more unstable over time. It seems to be working at the moment, but tomorrow, who knows.
 

Offline SeanB

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Re: Importance of input and output capacitance for LDOs
« Reply #3 on: October 11, 2014, 05:55:26 pm »
Also remember that your 100nF capacitor will most likely not be 100nF at the input voltage you have there, unless you are using a disc ceramic with leads. SM capacitors do have a very large variation with voltage, which gets worse the smaller they get. You can use a few smaller ceramics in parallel on the input right next to the regulator, which will then overall have lower ESR even if the single units do not.

Often what is not disclosed on the data sheet what 100n cap they used, they might have specifies one that still is 80n at the input voltage, while your one might only be 10n at that voltage.
 

Offline AKM

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Re: Importance of input and output capacitance for LDOs
« Reply #4 on: October 11, 2014, 06:31:08 pm »
Almost any input capacitor >=100nF should work. Only the output capacitor usually shouldn`t have very low esr <0.1-0.5ohm depending on the ldo.
« Last Edit: October 11, 2014, 07:08:10 pm by AKM »
 

Offline sleemanj

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Re: Importance of input and output capacitance for LDOs
« Reply #5 on: October 11, 2014, 09:02:09 pm »
I was looking in the datasheet for the ST 1117 im using and it says that the ldo is stable with 10uF on the output. Looking at the diagram, it shows 100nF on the input and 10uF on the output.

The problem is that I dont have any 100nF caps. So I wonder, can I use one with higher capacitance? Specifically 100uF?

The input cap shouldn't be too important, it's the output cap which is more critical for stability.

The output cap needs to have a low but not very low ESR, ceramics are too low, electrolytics are too high, tantalum are about right, but people don't like tantalum for lots of good reasons.

The solution is to use a ceramic capacitor, but to put it in series with a low value resistor (say 50 to 500 milliohm) to increase the ESR into the stable range.

See this brief app note from TI on the subject:
  http://www.ti.com/lit/an/slva214a/slva214a.pdf
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Offline SeanB

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Re: Importance of input and output capacitance for LDOs
« Reply #6 on: October 11, 2014, 10:14:22 pm »
Easier than a resistor is a "zero ohm" resistor, which will be in the right range. Not zero, not as controlled as a precision milliohm resistor but will work in most cases.
 

Online Kjelt

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Re: Importance of input and output capacitance for LDOs
« Reply #7 on: October 12, 2014, 12:33:13 am »
Yes for certain LDO regulators the ESR of the output capacitor is critical BUT if so this must be mentioned in the datasheet with all specifications (for instance between 0,1 and 2 ohms).

Now lets look at the LM1117 from TI: http://www.farnell.com/datasheets/1685500.pdf
It states clearly:
Quote
The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both
minimum amount of capacitance and ESR (Equivalent Series Resistance). The minimum output capacitance
required by the LM1117 is 10uF, if a tantalum capacitor is used. Any increase of the output capacitance will
merely improve the loop stability and transient response. The ESR of the output capacitor should range between
0.3 - 22 ohms.
Now lets look at the LD1117 from ST: http://www.farnell.com/datasheets/1776449.pdf
No mention about critical output capacitors or critical ESR requirements of this capacitor.
The only mention about the output capacitor is:
Quote
"Only a very common 10 uF minimum capacitor is needed for stability".
Also the internal diagram of both LDO's differ slightly, so if you have the ST LD1117 unless I am very mistaken the output cap characteristics are not very critical, that said,
I always make a testcircuit and look at the output voltage if it is stable under couple of temperatures and loads.
« Last Edit: October 12, 2014, 12:35:44 am by Kjelt »
 

Offline LukeW

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Re: Importance of input and output capacitance for LDOs
« Reply #8 on: October 12, 2014, 11:55:08 am »
I dont have any 100nF caps.

 ???

I'm tempted to say the correct answer here is that you should get some 100nF caps. That's an extremely common value for this sort of thing, and you will want to have them next time, for your future projects, so it's worth getting some. :)

Anyway, you've got your 10uF cap on the output, which is good... but the input capacitor is not so critical. You can probably just leave it out.

Replacing a 100nF cap with a 100uF cap is a 1000x increase in the capacitance... although it probably wouldn't hurt for an LDO, as a general idea I would say that it's not an ideal replacement, in general I would try and find a closer match to the specified value in the circuit, although I understand you're only trying to use the components on hand.

Do you have any other smaller ceramic (or plastic) caps in stock, eg. 1uF, 47nF, 220nF, or anything in that ballpark between 10nF and 1uF?
 

Offline kolonelkadat

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Re: Importance of input and output capacitance for LDOs
« Reply #9 on: October 12, 2014, 12:21:31 pm »
Yes for certain LDO regulators the ESR of the output capacitor is critical BUT if so this must be mentioned in the datasheet with all specifications (for instance between 0,1 and 2 ohms).


Okay. I was worried I had either missed something or was just supposed to know. The more I looked at the datasheet, the more worried I was getting lol.  :phew:


I'm tempted to say the correct answer here is that you should get some 100nF caps. That's an extremely common value for this sort of thing, and you will want to have them next time, for your future projects, so it's worth getting some. :)

at the moment Im using up the last scraps from my last order. This was my last 10uF cap and ive got maybe 5 more 100uF ones. At some point I will be placing a new order, but at the moment its just not in the budget.
 

Offline Falcon69

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Re: Importance of input and output capacitance for LDOs
« Reply #10 on: November 12, 2016, 02:09:32 pm »
I know this is an old topic, but pertains to what I'm wanting to know.

sleemanji,

that link you posted for the TI's app notes for substituting ceramic caps for the tantalum called out for in the data sheet, i'm not sure I quite understand that.

Let's say I'm using an LM1117-5.0 fixed LDO voltage regulator.

TI calls for a 10uF tantalum on its output, and one on it's input that is optional if far from the power supply input filter. It also says in the spec sheets that acceptable ESR ratings should be between 0.3ohm and 22ohm.

Couple questions here. First off....
   
     If I'm understanding that app note correctly, I would simply use a 10uF ceramic capacitor, perhaps an X7R, (one on input rated for at LEAST my input voltage and one on the output rated for at LEAST the output voltage) and put in series a 0.3ohm - 22ohm resistor with the output and input capacitor from output of voltage regulator to ground.

    Or would simply putting a 100nF ceramic cap in parallel with a 10uF cap on both input and output be sufficient enough?  I've looked at those cheapy 5.0v regulators sold on eBay, and that's what they are simply doing. However, I want my circuit to be done the right way as it may be drawing close to the 800mA output at times.

Second...

How far is FAR from the power supply filter?  I will be using something like a walwart to deliver 12 volts/3A into the regulator (and another regulator also draws from that to put out another voltage). Total trace length will probably be less than 30mm or so from the walwart input to the regulator...however, I guess you'd need to add like 6-10ft for cable length of the walwart?.
« Last Edit: November 12, 2016, 02:26:03 pm by Falcon69 »
 

Offline amitchell

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Re: Importance of input and output capacitance for LDOs
« Reply #11 on: November 12, 2016, 04:45:07 pm »
I know this is an old topic, but pertains to what I'm wanting to know.

sleemanji,

that link you posted for the TI's app notes for substituting ceramic caps for the tantalum called out for in the data sheet, i'm not sure I quite understand that.

Let's say I'm using an LM1117-5.0 fixed LDO voltage regulator.

TI calls for a 10uF tantalum on its output, and one on it's input that is optional if far from the power supply input filter. It also says in the spec sheets that acceptable ESR ratings should be between 0.3ohm and 22ohm.

Couple questions here. First off....
   
     If I'm understanding that app note correctly, I would simply use a 10uF ceramic capacitor, perhaps an X7R, (one on input rated for at LEAST my input voltage and one on the output rated for at LEAST the output voltage) and put in series a 0.3ohm - 22ohm resistor with the output and input capacitor from output of voltage regulator to ground.

    Or would simply putting a 100nF ceramic cap in parallel with a 10uF cap on both input and output be sufficient enough?  I've looked at those cheapy 5.0v regulators sold on eBay, and that's what they are simply doing. However, I want my circuit to be done the right way as it may be drawing close to the 800mA output at times.

Second...

How far is FAR from the power supply filter?  I will be using something like a walwart to deliver 12 volts/3A into the regulator (and another regulator also draws from that to put out another voltage). Total trace length will probably be less than 30mm or so from the walwart input to the regulator...however, I guess you'd need to add like 6-10ft for cable length of the walwart?.

You would certainly want local bulk capacitance on your PCB, pre LDO. I am typically using 2-3 10uf X7R 1210 or 1206 50V on my 12VIN.

I would be derating your ceramic capacitors by 3 times the voltage, the series resistor would count as ESR, but also voltage drop and heat especially at 800mA, does a Buck converter make more sense? How right is right here? how much output ripple is acceptable? Wallwart output can be pretty ugly, have you seen yours on a scope? all kinds of questions and answers.



 

Offline Falcon69

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Re: Importance of input and output capacitance for LDOs
« Reply #12 on: November 12, 2016, 04:54:32 pm »
I don't have a scope. This circuit just controls a few logic gates and some LED's. It's pretty basic. Nothing like High Frequency RF or anything.

I guess I could just do a 6.3v 22uF tantalum on the output, but I'm worried about putting a Tantalum on the input. I've heard that any type of voltage spike (like accidentally shorting supply to ground) would make the tantalum explode or catch fire.

It doesn't seem the input is that important, so maybe just keep it as a 10uF ceramic with a 100nF ceramic parallel on the input, and then just a 22uF tantalum on the output? Would that be fine?

I do have a mosFET/Zener Diode protection circuit just before the voltage regulator to help protect against reverse polarity wiring if it happens, and of course a fuse.

However, I just noticed also, that the other voltage regulator (LM2596) has a 220uF Electrolytic connected as well, that will be on that same input line going to the LM1117 and the LM2596, so I guess both regulators would see both the 220uF and the 10uF ceramic and the 100nF ceramic. Maybe too much?
 

Offline blueskull

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Re: Importance of input and output capacitance for LDOs
« Reply #13 on: November 12, 2016, 05:02:53 pm »
I do have a mosFET/Zener Diode protection circuit just before the voltage regulator to help protect against reverse polarity wiring if it happens, and of course a fuse.

That is an overvoltage protection circuit, not reverse polarity. For reverse protection, you just need a diode across input and ground, and of course a fuse.

However, I just noticed also, that the other voltage regulator (LM2596) has a 220uF Electrolytic connected as well, that will be on that same input line going to the LM1117 and the LM2596, so I guess both regulators would see both the 220uF and the 10uF ceramic and the 100nF ceramic. Maybe too much?

Not too much. A regulator doesn't care about too much input C, providing SRF and ESL as well as ESR are not compromised.
 

Offline Falcon69

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Re: Importance of input and output capacitance for LDOs
« Reply #14 on: November 12, 2016, 05:07:18 pm »
ya, thats what I meant.  Overvoltage protection. I also have the diode as you have mentioned. Tired, long day here.

Okay, I'll go ahead and design it for that then. Thank You guys.

But for future reference....If the data sheets says I need a Tantalum with an ESR value of 1.2ohms (for example), I can just use a Ceramic of same value (10uF for example) and just put a 1.2ohm resistor in series with it for that ESR value?

 

Offline blueskull

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Re: Importance of input and output capacitance for LDOs
« Reply #15 on: November 12, 2016, 05:15:42 pm »
But for future reference....If the data sheets says I need a Tantalum with an ESR value of 1.2ohms (for example), I can just use a Ceramic of same value (10uF for example) and just put a 1.2ohm resistor in series with it for that ESR value?

If they say 10uF, make sure you get 10uF or more effective capacitance. MLCCs have capacitance - bias voltage dependency. It's not uncommon to see less than 10% zero bias capacitance at rated voltage.
However, too much capacitance at lower voltage may jeopardize regulator from starting up.
Therefore, whenever possible, use recommended capacitor. If can't, get a scope and make sure it won't oscillate.
 

Offline Falcon69

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Re: Importance of input and output capacitance for LDOs
« Reply #16 on: November 12, 2016, 05:25:46 pm »
ahh, okay, understand. So I should get higher by at least 10% (if I'm using a 10% X7R capacitor)  So a 10uF, I need to actually find a 10.1uF or more.

Just looking at the buck converters. Cost is MUCH higher, and it looks like they will actually require the same amount of capacitors, but then would also require a resistor and an inductor, so more components, which would result in more footprint size as well as cost (not too mention the higher cost for the IC)

So, the buck converters are not an option for this application. It's just too cheap and simple.
 

Offline blueskull

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Re: Importance of input and output capacitance for LDOs
« Reply #17 on: November 12, 2016, 05:28:53 pm »
ahh, okay, understand. So I should get higher by at least 10% (if I'm using a 10% X7R capacitor)  So a 10uF, I need to actually find a 10.1uF or more.

10% is temperature dependency, voltage dependency can be up to -95% to +0%.
I would say 1117 should not exist in a modern design at all. There are lots of slightly more expensive but much better regulators out there. Most modern ones are designed for low ESR MLCC output.
 

Offline Falcon69

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Re: Importance of input and output capacitance for LDOs
« Reply #18 on: November 12, 2016, 05:37:05 pm »
can you possibly recommend one? I need one that outputs 5volts at 800mA-1A and another that outputs 5.4volts at 2A-2.2A.


Some of these aren't bad in price...like this one I'm finding...


http://www.mouser.com/ds/2/115/AP65201-768589.pdf

It looks like that one uses ceramic caps. 22uF on both input and output.
 

Offline blueskull

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Re: Importance of input and output capacitance for LDOs
« Reply #19 on: November 12, 2016, 05:38:46 pm »
can you possibly recommend one? I need one that outputs 5volts at 800mA-1A and another that outputs 5.4volts at 2A-2.2A.


Some of these aren't bad in price...like this one I'm finding...


http://www.mouser.com/ds/2/115/AP65201-768589.pdf

It looks like that one uses ceramic caps. 22uF on both input and output.

Vin?
 

Online mariush

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Re: Importance of input and output capacitance for LDOs
« Reply #20 on: November 12, 2016, 05:39:29 pm »
They recommend tantalum in datasheets because (at the time those datasheets were made) they usually had higher esr compared to ceramic capacitors (around 0.5-1ohm , so they fit into that 0.3 ohm - 22 ohm window).

Nowadays, electrolytic capacitors are good enough that you can find 10uF or 47uF or 100uF electrolytic capacitors with 10v-50v rating with esr below 1 ohm, in the past it was more difficult. 
Very low esr series like Panasonic FM or FR are borderline .. for example even 100uF 10v rated Panasonic FR is specified at 0.3 ohm impedance at 100kHz so maybe it wouldn't be smart to use them

Older series like Nichicon PW or Nichicon VR (meh, standard 85c only) have higher ESR values and are also cheaper, for example the 5mm x 11mm PW capacitors (10-47uF 25v rated for example, or 22-100uF 10v) have a specified ESR value of 0.6 ohm at 20c/100kHz (well, actually impedance, but at 100kHz impedance and esr are pretty much the same thing.. won't be the case with linear regulator but it's good enough indicato to use)

Other low esr series that would most likely work would be Nichicon HE (mostly smallest sized ones in series) , Nichicon HD , Rubycon PX (85c standard), YXF , ZL , United Chemi Con LXY , Panasonic FC ...

I wouldn't use tantalum capacitors in my circuits at all, if I can avoid it. I'd rather use a couple ceramic capacitors in series with a resistor if there's no vertical height for an electrolytic for example.


@Falcon69  with 12v in , 5v out , I'd worry about heat dissipation especially at high currents.  Remember, linear regulators waste the difference as heat, so at 800mA you'd have  (12v-5v) x 0.8 = 5.6 watts. You'd definitely need a heatsink for it, and a surface mount 1117 won't cut it.  I wouldn't use such a high input voltage in combination with a linear regulator for more than around 1w dissipated as heat (around 100-150mA) - 1w can be dissipated in the circuit board or in air or on a tiny heatsink
For more than 0.5A, I'd definitely consider switching regulators .. you can find very tiny ones that only need tiny components like an surface mount inductor and maybe a diode and  a couple ceramic capacitors to work, and they don't cost much.


// edit : sigh... Warning - while you were typing 7 new replies have been posted. You may wish to review your post.

guess i take a long time writing posts
 

Offline Falcon69

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Re: Importance of input and output capacitance for LDOs
« Reply #21 on: November 12, 2016, 05:45:19 pm »
okay Mariush. That makes sense about the heat. Yeah, it won't work then. I won't to try and keep everything surface mount.

Blueskull.  I think the most my circuit will ever see is 12volts for the input. Maybe 24volts, but doubtful. Depends on the walwart being used.

So, I think I'm just going to plan for the 12volt.
 

Offline salbayeng

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Re: Importance of input and output capacitance for LDOs
« Reply #22 on: November 12, 2016, 05:55:21 pm »
One good way to test if your design will be stable in the future is to put in the freezer for half an hour, and see if it's stable when you first apply power (after removing from the freezer!). LM2931 is stable with 20uF ceramics at room temp, but needs 30uF to be OK at -20C.
 

Offline Falcon69

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Re: Importance of input and output capacitance for LDOs
« Reply #23 on: November 12, 2016, 05:57:12 pm »
Ya, I read that. Electrolytic caps are bad for that. They almost triple their ESR rating when -25C.
 

Offline blueskull

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Re: Importance of input and output capacitance for LDOs
« Reply #24 on: November 12, 2016, 06:01:06 pm »
okay Mariush. That makes sense about the heat. Yeah, it won't work then. I won't to try and keep everything surface mount.

Blueskull.  I think the most my circuit will ever see is 12volts for the input. Maybe 24volts, but doubtful. Depends on the walwart being used.

So, I think I'm just going to plan for the 12volt.

3A at 7V voltage drop gives you 21W of power, which may take some serious heat sinking.
I would recommend using a buck converter to get 5.4V, followed by a low voltage dropout regulator to get 5V.
You will quickly see the saved money on using a regulator gets wasted on a bulky heat sink and reduced lifetime.

For your purpose, I will recommend TPS54541 for its thermal capability and current capability as well as high frequency, which allows you to use smaller and cheaper passives.
If you don't care about cost and just want to be simple just buy a module which has inductor integrated with it, use LMZ23605. If 24V is not required, try LMZ31704.
For the LDO, I recommend LP3855 for its low drop out at high current and thermally enhanced package.
 


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