On Sun, 29 Mar 2015 21:00:28 +1000, Chris Jones <lugnut808@spam.yahoo.com>  
wrote:

> 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

That does sound an awful lot like what happens with LDO's - there is a  
range of output cap that will oscillate. Never heard of it for a 317  
though. A re-spin or an LDO in drag?

No, the resistor goes between the regulator and the cap, but the voltage divider is on the regulator side. Otherwise the output voltage would go nuts. 

Cheers

Phil Hobbs

>
> 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
>
>

like that low ohmic resistor R for debugging and measuring current too, but
as i understand the R is after the cap, so the mlcc problem for LDOs still
exists.

I like to put a current measure R between the cap and the feedback voltage
divider, so the V drop is regulated out.
Disconnecting the load for debugging is done by putting the feedback divider
on another pad connected directly on the cap output, additionally put on the
layout.

MIKE

-- 
www.oho-elektronik.de
OHO-Elektronik
Michael Randelzhofer
FPGA und CPLD Mini Module

<krw@zzz.com> wrote in message 
news:ql3ghap3vo2e589mn1lh3spa5hn168f355@4ax.com...
> What temperature?  Note that ceramics have a limited temperature
> range, too.  X5Rs, for example are only rated to 85C.  X7R = 125C.

They'll be better off than the regs -- 100C-ish cutoff, and that's Tj on 
top of whatever power they're dissipating, so I hope this thing isn't 
actually needing an ampere of capacity and/or an ambient over 60C.

Tim

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

On Sun, 29 Mar 2015 10:41:19 -0400, krw@zzz.com wrote:

>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.

The ESR of wet aluminum electrolytics skyrockets below 0C. Using them
is a good way to build a temperature-activated oscillator.

Polymer alums are good over temperature, but their ESR may be too low
for some regulators. Tantalums are just right for 1117 type regs.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

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.

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 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 02:51, 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 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