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
Oscillating regulators with ceramic caps
Started by ●March 28, 2015
Reply by ●March 28, 20152015-03-28
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. > >ChrisI 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.
Reply by ●March 28, 20152015-03-28
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. > >ChrisI 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. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 28, 20152015-03-28
LM337 is a typical LDO configuration, so is sensitive to load much the same as a LM1117 or etc. That was a limitation of the technology back then. LM317 should be OK, but maybe your layout has other problems (chains of LC resonators?). It's generally unwise to use just ceramic caps (or aluminum polymers). The ESR is much lower than sqrt(Lstray/C), so you get resonant peaks everywhere. You always want a bulk R+C somewhere to dampen the lumped equivalent transmission line that is the supply network. Use a cap value several times the total capacitance on the rail, with ESR (MLCC or Al Poly + external resistor; or tantalum or electrolytic of known ESR) equal to sqrt(Lstray/Cbyp). If Lstray is the average inductance between bypass caps, then use Cbyp = value of those individual caps. Or use Lstray = total supply inductance and Cbyp = total bypass. Same thing. Preferably this should be placed at the start and end of a chained (linear, a>b>c>...) network, or at the end of each leaf in a branched network (a>b(>d>e)>c>...), since it serves as a termination resistor for the lumped equivalent transmission line. Note that ferrite beads and filter chokes change the values of this damper substantially; design accordingly. Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com "Chris Jones" <lugnut808@spam.yahoo.com> wrote in message news: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
Reply by ●March 28, 20152015-03-28
On Sat, 28 Mar 2015 12:31:04 -0500, "Tim Williams" <tiwill@seventransistorlabs.com> wrote:>LM337 is a typical LDO configuration, so is sensitive to load much the >same as a LM1117 or etc. That was a limitation of the technology back >then.Not quite. While the output primary current carrier is NPN, the configuration is actually PNP follower, except that the PNP is "synthetic", a weenie PNP drives a NPN Darlington (the LM317 is NPN follower). So the output impedance is low. The trouble with that configuration is that the PNP is a lateral structure... poor fT _plus_ a transit time excess phase issue... that's where the potential instability comes from.> >LM317 should be OK, but maybe your layout has other problems (chains of LC >resonators?). > >It's generally unwise to use just ceramic caps (or aluminum polymers). >The ESR is much lower than sqrt(Lstray/C), so you get resonant peaks >everywhere. You always want a bulk R+C somewhere to dampen the lumped >equivalent transmission line that is the supply network. Use a cap value >several times the total capacitance on the rail, with ESR (MLCC or Al Poly >+ external resistor; or tantalum or electrolytic of known ESR) equal to >sqrt(Lstray/Cbyp). If Lstray is the average inductance between bypass >caps, then use Cbyp = value of those individual caps. Or use Lstray = >total supply inductance and Cbyp = total bypass. Same thing. > >Preferably this should be placed at the start and end of a chained >(linear, a>b>c>...) network, or at the end of each leaf in a branched >network (a>b(>d>e)>c>...), since it serves as a termination resistor for >the lumped equivalent transmission line. > >Note that ferrite beads and filter chokes change the values of this damper >substantially; design accordingly. > >Tim...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●March 28, 20152015-03-28
On Sat, 28 Mar 2015 11:48:12 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On Sat, 28 Mar 2015 12:31:04 -0500, "Tim Williams" ><tiwill@seventransistorlabs.com> wrote: > >>LM337 is a typical LDO configuration, so is sensitive to load much the >>same as a LM1117 or etc. That was a limitation of the technology back >>then. > >Not quite. While the output primary current carrier is NPN, the >configuration is actually PNP follower, except that the PNP is >"synthetic", a weenie PNP drives a NPN Darlington (the LM317 is NPN >follower). So the output impedance is low. The trouble with that >configuration is that the PNP is a lateral structure... poor fT _plus_ >a transit time excess phase issue... that's where the potential >instability comes from. >[snip] 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. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●March 28, 20152015-03-28
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. > >ChrisUse 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's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●March 28, 20152015-03-28
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 PefhanyIt 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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●March 28, 20152015-03-28
On Sat, 28 Mar 2015 14:19:29 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> 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 ThompsonThe LM8261 (c-load opamp) architecture does it right. The dominant pole isn't buried inside, it's hung on the output pin. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 28, 20152015-03-28
On Sat, 28 Mar 2015 14:32:26 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:>On Sat, 28 Mar 2015 14:19:29 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> 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 > >The LM8261 (c-load opamp) architecture does it right. The dominant >pole isn't buried inside, it's hung on the output pin.In CMOS it's almost impossible not to gain a pole inside the regulator as well as the one due to the load... solved by magical application of a load-current-sensitive zero in the regulator structure ;-) ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
