Oscillating regulators with ceramic caps

On 3/28/2015 4:58 PM, Spehro Pefhany wrote:
> On Sun, 29 Mar 2015 02:45:34 +1100, the renowned Chris Jones
> <lugnut808@spam.yahoo.com> wrote:
>
>> I've just built up a batch of PCBs with several LM317 and LM337
>> regulators on them. As this thing will be in a hot environment, I wanted
>> to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the
>> inputs and outputs of the regulators. Now that I have built it, all of
>> the regulators oscillate!
>>
>> I had thought that it was only LDOs (with the drain or collector of the
>> pass device connected to the output) that would oscillate with low-ESR
>> capacitors, but I now know that this also applies to some non-LDO linear
>> regulators. Having seen the oscillation, I looked in the LM337 datasheet
>> and sure enough it says it isn't stable with low-ESR capacitors. It does
>> not even say what range of ESR it IS stable with. In the case of LDOs, I
>> already had the policy of refusing to design in the ones that require
>> some ESR without specifying what range is acceptable.
>>
>> By experiment, for the LM337LM I have found that somewhere in the range
>> of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems
>> to just barely stop it from oscillating, without any other decoupling
>> capacitors connected. I would use a somewhat higher value to get more
>> phase margin. Adding these resistors is a pain because I have a lot of
>> other MLCCs decoupling the same rails, so there is no nice place to hack
>> in the resistors, especially since the board was not routed with this in
>> mind. If I put the resistor between the regulator and the power plane,
>> then there will be some interaction between the different circuit blocks
>> as they would then share a significant common supply resistance at DC.
>> Perhaps an inductor? Meh.
>>
>> I guess there is a good reason why ADI can charge more for their AnyCap
>> LDOs. I wonder if any of them is pinout compatible with the surface
>> mount LM317MDCYRG3 and LM337LM that I have used.
>>
>> Chris
>
> Use resistors in series 1-3 ohms is more like what a tantalum or
> normal aluminum electrolytic does. You can put a 0.1uF MLCC in
> parallel.
>
> Lots of LDOs **require** this, some of the older ones neglect to
> mention it clearly on the datasheets because cheap 10uF ceramic
> capacitors were the stuff of science fiction back when the data sheets
> were written. Watch out also for the really good low-Z electrolytic
> caps.
>
> --sp

I'm usually happier putting ~100 milliohms in series with the regulator 
output, after the reference divider.  It's nice for debugging, too.

Cheers

Phil Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

On Sun, 29 Mar 2015 01:45:34 +1000, Chris Jones <lugnut808@spam.yahoo.com>  
wrote:

> I've just built up a batch of PCBs with several LM317 and LM337  
> regulators on them. As this thing will be in a hot environment, I wanted  
> to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the  
> inputs and outputs of the regulators. Now that I have built it, all of  
> the regulators oscillate!
>
> I had thought that it was only LDOs (with the drain or collector of the  
> pass device connected to the output) that would oscillate with low-ESR  
> capacitors, but I now know that this also applies to some non-LDO linear  
> regulators. Having seen the oscillation, I looked in the LM337 datasheet  
> and sure enough it says it isn't stable with low-ESR capacitors. It does  
> not even say what range of ESR it IS stable with. In the case of LDOs, I  
> already had the policy of refusing to design in the ones that require  
> some ESR without specifying what range is acceptable.
>
> By experiment, for the LM337LM I have found that somewhere in the range  
> of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems  
> to just barely stop it from oscillating, without any other decoupling  
> capacitors connected. I would use a somewhat higher value to get more  
> phase margin. Adding these resistors is a pain because I have a lot of  
> other MLCCs decoupling the same rails, so there is no nice place to hack  
> in the resistors, especially since the board was not routed with this in  
> mind. If I put the resistor between the regulator and the power plane,  
> then there will be some interaction between the different circuit blocks  
> as they would then share a significant common supply resistance at DC.  
> Perhaps an inductor? Meh.
>
> I guess there is a good reason why ADI can charge more for their AnyCap  
> LDOs. I wonder if any of them is pinout compatible with the surface  
> mount LM317MDCYRG3 and LM337LM that I have used.
>
> Chris

Thanks for the post. It's very timely. I was just about to do the exact  
same thing.

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote 
in message news:juudhatdo01q2av4kk4107mnbllql9s8eq@4ax.com...
> Took me awhile to remember how to spell Sziklai ;-)..
>
> <http://en.wikipedia.org/wiki/Sziklai_pair>
>
> In the LM337 case the PNP and NPN positions are reversed.

Yup.  And the LM1117 isn't terrifically unstable, as far as I know. 
Though it's also not a true LDO, in the sense that dropout ~= 0 for low 
currents.

Can't find one of the 337, here's the 1117:
http://s.zeptobars.ru/LM1117-HD.jpg
The PNP area seems larger compared to darlington structures (e.g., LM317, 
MC34063), which corroborates the poor hFE of most.

Was 1117 done with lateral as well, or did they have a [nearly] 
symmetrical process for that?

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com 

On 29/03/2015 12:02, Phil Hobbs wrote:
> On 3/28/2015 4:58 PM, Spehro Pefhany wrote:
>> On Sun, 29 Mar 2015 02:45:34 +1100, the renowned Chris Jones
>> <lugnut808@spam.yahoo.com> wrote:
>>
>>> I've just built up a batch of PCBs with several LM317 and LM337
>>> regulators on them. As this thing will be in a hot environment, I wanted
>>> to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the
>>> inputs and outputs of the regulators. Now that I have built it, all of
>>> the regulators oscillate!
>>>
>>> I had thought that it was only LDOs (with the drain or collector of the
>>> pass device connected to the output) that would oscillate with low-ESR
>>> capacitors, but I now know that this also applies to some non-LDO linear
>>> regulators. Having seen the oscillation, I looked in the LM337 datasheet
>>> and sure enough it says it isn't stable with low-ESR capacitors. It does
>>> not even say what range of ESR it IS stable with. In the case of LDOs, I
>>> already had the policy of refusing to design in the ones that require
>>> some ESR without specifying what range is acceptable.
>>>
>>> By experiment, for the LM337LM I have found that somewhere in the range
>>> of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems
>>> to just barely stop it from oscillating, without any other decoupling
>>> capacitors connected. I would use a somewhat higher value to get more
>>> phase margin. Adding these resistors is a pain because I have a lot of
>>> other MLCCs decoupling the same rails, so there is no nice place to hack
>>> in the resistors, especially since the board was not routed with this in
>>> mind. If I put the resistor between the regulator and the power plane,
>>> then there will be some interaction between the different circuit blocks
>>> as they would then share a significant common supply resistance at DC.
>>> Perhaps an inductor? Meh.
>>>
>>> I guess there is a good reason why ADI can charge more for their AnyCap
>>> LDOs. I wonder if any of them is pinout compatible with the surface
>>> mount LM317MDCYRG3 and LM337LM that I have used.
>>>
>>> Chris
>>
>> Use resistors in series 1-3 ohms is more like what a tantalum or
>> normal aluminum electrolytic does. You can put a 0.1uF MLCC in
>> parallel.
>>
>> Lots of LDOs **require** this, some of the older ones neglect to
>> mention it clearly on the datasheets because cheap 10uF ceramic
>> capacitors were the stuff of science fiction back when the data sheets
>> were written. Watch out also for the really good low-Z electrolytic
>> caps.
>>
>> --sp
>
> I'm usually happier putting ~100 milliohms in series with the regulator
> output, after the reference divider.  It's nice for debugging, too.

Yes, if I knew in advance then that is what I would have done. In order 
to prevent interactions between blocks (at least at DC), it is nice to 
put a separate series resistor feeding each circuit block, rather than 
one resistor common to all blocks. Sadly my existing layout is not very 
amenable to this.

Chris

On 29/03/2015 08:19, Jim Thompson wrote:
> On Sat, 28 Mar 2015 16:58:33 -0400, Spehro Pefhany
> <speffSNIP@interlogDOTyou.knowwhat> wrote:
>
>> On Sun, 29 Mar 2015 02:45:34 +1100, the renowned Chris Jones
>> <lugnut808@spam.yahoo.com> wrote:
>>
>>> I've just built up a batch of PCBs with several LM317 and LM337
>>> regulators on them. As this thing will be in a hot environment, I wanted
>>> to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the
>>> inputs and outputs of the regulators. Now that I have built it, all of
>>> the regulators oscillate!
>>>
>>> I had thought that it was only LDOs (with the drain or collector of the
>>> pass device connected to the output) that would oscillate with low-ESR
>>> capacitors, but I now know that this also applies to some non-LDO linear
>>> regulators. Having seen the oscillation, I looked in the LM337 datasheet
>>> and sure enough it says it isn't stable with low-ESR capacitors. It does
>>> not even say what range of ESR it IS stable with. In the case of LDOs, I
>>> already had the policy of refusing to design in the ones that require
>>> some ESR without specifying what range is acceptable.
>>>
>>> By experiment, for the LM337LM I have found that somewhere in the range
>>> of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems
>>> to just barely stop it from oscillating, without any other decoupling
>>> capacitors connected. I would use a somewhat higher value to get more
>>> phase margin. Adding these resistors is a pain because I have a lot of
>>> other MLCCs decoupling the same rails, so there is no nice place to hack
>>> in the resistors, especially since the board was not routed with this in
>>> mind. If I put the resistor between the regulator and the power plane,
>>> then there will be some interaction between the different circuit blocks
>>> as they would then share a significant common supply resistance at DC.
>>> Perhaps an inductor? Meh.
>>>
>>> I guess there is a good reason why ADI can charge more for their AnyCap
>>> LDOs. I wonder if any of them is pinout compatible with the surface
>>> mount LM317MDCYRG3 and LM337LM that I have used.
>>>
>>> Chris
>>
>> Use resistors in series 1-3 ohms is more like what a tantalum or
>> normal aluminum electrolytic does. You can put a 0.1uF MLCC in
>> parallel.
>>
>> Lots of LDOs **require** this, some of the older ones neglect to
>> mention it clearly on the datasheets because cheap 10uF ceramic
>> capacitors were the stuff of science fiction back when the data sheets
>> were written. Watch out also for the really good low-Z electrolytic
>> caps.
>>
>> --sp
>>
>>
>> Best regards,
>> Spehro Pefhany
>
> It would be nice if some company would actually apply some
> engineering... it's not exactly rocket science to make a stable LDO
> without all these series-R shenanigans... I do it all the time
> on-chip... I even had one chip design that has +5V input, with LDO's
> at 3.3V, 2.5V and 1.8V ;-)
> 		
>                                          ...Jim Thompson
>

Exactly what I was thinking. I have designed LDOs for cellular radios, 
and one of the requirements was that it would be stable with an ideal 
capacitor or anything that approaches one. Another requirement was 
<20nV/rt-Hz noise and a startup time of a few microseconds. A few times 
I have wished I could use one of my LDOs on a circuit board but sadly 
they are only available with a cellular radio attached, they need some 
SPI commands to turn on the regulator, and the datasheet is under NDA.

At least the ADI AnyCap ones do seem stable in my experience of them.

Chris

On 29/03/2015 03:44, John Larkin wrote:
> On Sun, 29 Mar 2015 02:45:34 +1100, Chris Jones
> <lugnut808@spam.yahoo.com> wrote:
>
>> I've just built up a batch of PCBs with several LM317 and LM337
>> regulators on them. As this thing will be in a hot environment, I wanted
>> to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the
>> inputs and outputs of the regulators. Now that I have built it, all of
>> the regulators oscillate!
>>
>> I had thought that it was only LDOs (with the drain or collector of the
>> pass device connected to the output) that would oscillate with low-ESR
>> capacitors, but I now know that this also applies to some non-LDO linear
>> regulators. Having seen the oscillation, I looked in the LM337 datasheet
>> and sure enough it says it isn't stable with low-ESR capacitors. It does
>> not even say what range of ESR it IS stable with. In the case of LDOs, I
>> already had the policy of refusing to design in the ones that require
>> some ESR without specifying what range is acceptable.
>>
>> By experiment, for the LM337LM I have found that somewhere in the range
>> of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems
>> to just barely stop it from oscillating, without any other decoupling
>> capacitors connected. I would use a somewhat higher value to get more
>> phase margin. Adding these resistors is a pain because I have a lot of
>> other MLCCs decoupling the same rails, so there is no nice place to hack
>> in the resistors, especially since the board was not routed with this in
>> mind. If I put the resistor between the regulator and the power plane,
>> then there will be some interaction between the different circuit blocks
>> as they would then share a significant common supply resistance at DC.
>> Perhaps an inductor? Meh.
>>
>> I guess there is a good reason why ADI can charge more for their AnyCap
>> LDOs. I wonder if any of them is pinout compatible with the surface
>> mount LM317MDCYRG3 and LM337LM that I have used.
>>
>> Chris
>
> I generally use tantalums, 10 to 22 uF, derated at least 2:1 on
> voltage. One big tantalum plus a bunch of smaller ceramics (bypass
> caps scattered around the board) seems to always work.
>
>

I also usually used tantalums or electrolytics. I was trying out MLCCs 
because I would prefer to avoid electrolytics at the operating 
temperature that I am expecting, and I wanted to avoid tantalums for a 
number of reasons. I did not realise that the 317 and 337 require ESR. 
Also even if I changed the bulk 10uF caps to tantalum, I suspect it 
might still be close to instability as there are a lot of other MLCCs 
distributed around the board.

Chris

On 29/03/2015 02:51, Jan Panteltje wrote:
> On a sunny day (Sun, 29 Mar 2015 02:45:34 +1100) it happened Chris Jones
> <lugnut808@spam.yahoo.com> wrote in <5516cca2$0$12771$862e30e2@ngroups.net>:
>
>> I've just built up a batch of PCBs with several LM317 and LM337
>> regulators on them. As this thing will be in a hot environment, I wanted
>> to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the
>> inputs and outputs of the regulators. Now that I have built it, all of
>> the regulators oscillate!
>>
>> I had thought that it was only LDOs (with the drain or collector of the
>> pass device connected to the output) that would oscillate with low-ESR
>> capacitors, but I now know that this also applies to some non-LDO linear
>> regulators. Having seen the oscillation, I looked in the LM337 datasheet
>> and sure enough it says it isn't stable with low-ESR capacitors. It does
>> not even say what range of ESR it IS stable with. In the case of LDOs, I
>> already had the policy of refusing to design in the ones that require
>> some ESR without specifying what range is acceptable.
>>
>> By experiment, for the LM337LM I have found that somewhere in the range
>> of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems
>> to just barely stop it from oscillating, without any other decoupling
>> capacitors connected. I would use a somewhat higher value to get more
>> phase margin. Adding these resistors is a pain because I have a lot of
>> other MLCCs decoupling the same rails, so there is no nice place to hack
>> in the resistors, especially since the board was not routed with this in
>> mind. If I put the resistor between the regulator and the power plane,
>> then there will be some interaction between the different circuit blocks
>> as they would then share a significant common supply resistance at DC.
>> Perhaps an inductor? Meh.
>>
>> I guess there is a good reason why ADI can charge more for their AnyCap
>> LDOs. I wonder if any of them is pinout compatible with the surface
>> mount LM317MDCYRG3 and LM337LM that I have used.
>>
>> Chris
>
> I have used many many LM317, and never had one oscillating.
> but I normally use tanatalum caps, say 1 to 10 uF very close.
>
> Did you try LM317 from a different source, it is not ebay fake I hope?
> LM317 is also stable with electrolytics AFAIK.
> Maybe it is simpler to replace the caps with tantalum than add series resistors.
>
> Duno about the 337.
>

The parts are from element14 and RS. I only tried one version of each 
part as these are the only ones that I have that will fit the PCB footprint.

I have built up separate breadboards with just the regulators and caps 
and some MOSFETs to turn on and off a pulsed load. The 337 oscillates 
like a banshee, the 317 just rings like a bell. So the 317 does not 
really oscillate, but it doesn't have significant ripple rejection 
around its ringing frequency. On the real board both regulators are 
powered from an isolated DC-DC, and perhaps the 317 was just resonating 
with the ripple coming from the DC-DC rather than oscillating as such. 
(240mVpp ripple at input, 170mVpp ripple at output - not much of a 
regulator)

On 29/03/2015 12:46, David Eather wrote:
> On Sun, 29 Mar 2015 01:45:34 +1000, Chris Jones
> <lugnut808@spam.yahoo.com> wrote:
>
>> I've just built up a batch of PCBs with several LM317 and LM337
>> regulators on them. As this thing will be in a hot environment, I
>> wanted to avoid electrolytics so I used 10uF MLCC ceramic capacitors
>> on the inputs and outputs of the regulators. Now that I have built it,
>> all of the regulators oscillate!
>>
>> I had thought that it was only LDOs (with the drain or collector of
>> the pass device connected to the output) that would oscillate with
>> low-ESR capacitors, but I now know that this also applies to some
>> non-LDO linear regulators. Having seen the oscillation, I looked in
>> the LM337 datasheet and sure enough it says it isn't stable with
>> low-ESR capacitors. It does not even say what range of ESR it IS
>> stable with. In the case of LDOs, I already had the policy of refusing
>> to design in the ones that require some ESR without specifying what
>> range is acceptable.
>>
>> By experiment, for the LM337LM I have found that somewhere in the
>> range of 50mOhms to 100mOhms added between the regulator and 10uF MLCC
>> seems to just barely stop it from oscillating, without any other
>> decoupling capacitors connected. I would use a somewhat higher value
>> to get more phase margin. Adding these resistors is a pain because I
>> have a lot of other MLCCs decoupling the same rails, so there is no
>> nice place to hack in the resistors, especially since the board was
>> not routed with this in mind. If I put the resistor between the
>> regulator and the power plane, then there will be some interaction
>> between the different circuit blocks as they would then share a
>> significant common supply resistance at DC. Perhaps an inductor? Meh.
>>
>> I guess there is a good reason why ADI can charge more for their
>> AnyCap LDOs. I wonder if any of them is pinout compatible with the
>> surface mount LM317MDCYRG3 and LM337LM that I have used.
>>
>> Chris
>
> Thanks for the post. It's very timely. I was just about to do the exact
> same thing.

I hope your PCB will need less bodge wires than mine.

On 29/03/2015 02:45, Chris Jones wrote:
> I've just built up a batch of PCBs with several LM317 and LM337
> regulators on them. As this thing will be in a hot environment, I wanted
> to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the
> inputs and outputs of the regulators. Now that I have built it, all of
> the regulators oscillate!
>
> I had thought that it was only LDOs (with the drain or collector of the
> pass device connected to the output) that would oscillate with low-ESR
> capacitors, but I now know that this also applies to some non-LDO linear
> regulators. Having seen the oscillation, I looked in the LM337 datasheet
> and sure enough it says it isn't stable with low-ESR capacitors. It does
> not even say what range of ESR it IS stable with. In the case of LDOs, I
> already had the policy of refusing to design in the ones that require
> some ESR without specifying what range is acceptable.
>
> By experiment, for the LM337LM I have found that somewhere in the range
> of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems
> to just barely stop it from oscillating, without any other decoupling
> capacitors connected. I would use a somewhat higher value to get more
> phase margin. Adding these resistors is a pain because I have a lot of
> other MLCCs decoupling the same rails, so there is no nice place to hack
> in the resistors, especially since the board was not routed with this in
> mind. If I put the resistor between the regulator and the power plane,
> then there will be some interaction between the different circuit blocks
> as they would then share a significant common supply resistance at DC.
> Perhaps an inductor? Meh.
>
> I guess there is a good reason why ADI can charge more for their AnyCap
> LDOs. I wonder if any of them is pinout compatible with the surface
> mount LM317MDCYRG3 and LM337LM that I have used.
>
> Chris


Some more experimental results:

If I leave the 10uF MLCC directly connected to the regulator output, but 
also connect in parallel an additional 100uF tantalum with measured ESR 
of 0.17 Ohms, this makes both the LM317M and LM337LM show very 
stable-looking response to a load transient (current step) with no 
ringing. Similarly, a 220uF aluminium electrolytic with 0.14 Ohms ESR 
produced a very nice load transient response.

Adding these capacitors in parallel with the 10uF MLCC rather than 
adding resistors in series with the LDO output does give better load 
regulation and transient response, but of course the large value 
capacitor is undesirable due to size and long-term reliability, or cost 
(if it is made of MLCCs with series resistors added).

Chris


Chris

On Sun, 29 Mar 2015 15:15:54 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>On 29/03/2015 03:44, John Larkin wrote:
>> On Sun, 29 Mar 2015 02:45:34 +1100, Chris Jones
>> <lugnut808@spam.yahoo.com> wrote:
>>
>>> I've just built up a batch of PCBs with several LM317 and LM337
>>> regulators on them. As this thing will be in a hot environment, I wanted
>>> to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the
>>> inputs and outputs of the regulators. Now that I have built it, all of
>>> the regulators oscillate!
>>>
>>> I had thought that it was only LDOs (with the drain or collector of the
>>> pass device connected to the output) that would oscillate with low-ESR
>>> capacitors, but I now know that this also applies to some non-LDO linear
>>> regulators. Having seen the oscillation, I looked in the LM337 datasheet
>>> and sure enough it says it isn't stable with low-ESR capacitors. It does
>>> not even say what range of ESR it IS stable with. In the case of LDOs, I
>>> already had the policy of refusing to design in the ones that require
>>> some ESR without specifying what range is acceptable.
>>>
>>> By experiment, for the LM337LM I have found that somewhere in the range
>>> of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems
>>> to just barely stop it from oscillating, without any other decoupling
>>> capacitors connected. I would use a somewhat higher value to get more
>>> phase margin. Adding these resistors is a pain because I have a lot of
>>> other MLCCs decoupling the same rails, so there is no nice place to hack
>>> in the resistors, especially since the board was not routed with this in
>>> mind. If I put the resistor between the regulator and the power plane,
>>> then there will be some interaction between the different circuit blocks
>>> as they would then share a significant common supply resistance at DC.
>>> Perhaps an inductor? Meh.
>>>
>>> I guess there is a good reason why ADI can charge more for their AnyCap
>>> LDOs. I wonder if any of them is pinout compatible with the surface
>>> mount LM317MDCYRG3 and LM337LM that I have used.
>>>
>>> Chris
>>
>> I generally use tantalums, 10 to 22 uF, derated at least 2:1 on
>> voltage. One big tantalum plus a bunch of smaller ceramics (bypass
>> caps scattered around the board) seems to always work.
>>
>>
>
>I also usually used tantalums or electrolytics. I was trying out MLCCs 
>because I would prefer to avoid electrolytics at the operating 
>temperature that I am expecting, and I wanted to avoid tantalums for a 
>number of reasons. I did not realise that the 317 and 337 require ESR. 
>Also even if I changed the bulk 10uF caps to tantalum, I suspect it 
>might still be close to instability as there are a lot of other MLCCs 
>distributed around the board.
>
What temperature?  Note that ceramics have a limited temperature
range, too.  X5Rs, for example are only rated to 85C.  X7R = 125C.