On Wed, 21 Feb 2018 04:51:28 GMT, Steve Wilson <no@spam.com> wrote:>John Larkin <jjlarkin@highlandtechnology.com> wrote: > >> On Wed, 21 Feb 2018 01:54:05 GMT, Steve Wilson <no@spam.com> wrote: >>>> Amps are amps. The load step demonstrates the ringing, and the fix for >>>> the ringing. > >>>Your simulation shows considerably different ringing between current >>>rise and fall. So the currents matter. > >> Sure, the output transistor emitter has a very different impedance >> from 30 to 200 mA. So the pole from that impedance into the ceramic >> caps is different for the two currents. > >>>You need to model the actual currents you are using. I suspect the idle >>>current may be much lower, and the actual charge current may be higher. > >>>I'd be happier to see the results with a pulsed 100ns 12 Amp load. > >I did it for you. The compensation cap is critical and very different from >your result. See below.That seems to be my sim; same currents, no ESR in the output cap, just not as pretty. Any engineering doc should have a title, author, and date. I tried adding 20 mohms ESR to the output caps in my sim. Nothing changed; it rings badly without the added compensation, and doesn't ring with the RC, or just the C, from ADJ to ground. Pulsing at 10 amps for 100 ns, the results are about the same: lots of ringing, fixed by adding the same comps. The compensation parts help.> >>>>>You may need to use a switched resistive load to provide some damping. >>>>>This may have a significant effect on the ringing. > >>>>>Note the load transient response in Figs 3 and 4 of the TI datasheet >>>>>show a considerably different response than your model. > >>>> They probably use caps with a lot of ESR. And the models differ too. > >>>You need to model the ESR.You sure like to tell me what I need to do. But I don't report to you. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
LM317 compensation
Started by ●February 15, 2018
Reply by ●February 21, 20182018-02-21
Reply by ●February 21, 20182018-02-21
John Larkin <jjlarkin@highlandtechnology.com> wrote:> On Wed, 21 Feb 2018 04:51:28 GMT, Steve Wilson <no@spam.com> wrote: > >>John Larkin <jjlarkin@highlandtechnology.com> wrote: >> >>> On Wed, 21 Feb 2018 01:54:05 GMT, Steve Wilson <no@spam.com> wrote: >>>>> Amps are amps. The load step demonstrates the ringing, and the fix >>>>> for the ringing. >> >>>>Your simulation shows considerably different ringing between current >>>>rise and fall. So the currents matter. >> >>> Sure, the output transistor emitter has a very different impedance >>> from 30 to 200 mA. So the pole from that impedance into the ceramic >>> caps is different for the two currents. >> >>>>You need to model the actual currents you are using. I suspect the >>>>idle current may be much lower, and the actual charge current may be >>>>higher. >> >>>>I'd be happier to see the results with a pulsed 100ns 12 Amp load. >> >>I did it for you. The compensation cap is critical and very different >>from your result. See below. > > That seems to be my sim; same currents, no ESR in the output cap, just > not as pretty. Any engineering doc should have a title, author, and > date.Sorry, I picked the wrong file. You can see the original by changing the current, pulse width and cap values as listed below. This is a newsgroup discussion. The title is shown at the top of the schematic. The date is the date of the post and is shown in the header. If I put my name on the document, you will get pissed.> I tried adding 20 mohms ESR to the output caps in my sim. Nothing > changed; it rings badly without the added compensation, and doesn't > ring with the RC, or just the C, from ADJ to ground.> Pulsing at 10 amps for 100 ns, the results are about the same: lots of > ringing, fixed by adding the same comps.> The compensation parts help.>>>>>>You may need to use a switched resistive load to provide some >>>>>>damping. This may have a significant effect on the ringing.>>>>>>Note the load transient response in Figs 3 and 4 of the TI datasheet >>>>>>show a considerably different response than your model.>>>>> They probably use caps with a lot of ESR. And the models differ too.>>>>You need to model the ESR.> You sure like to tell me what I need to do. But I don't report to you.Sensitive? Not at all. Try the generic "you". I increased the output cap from 12 uF to 20uF to match your value, amd changed the pulse width from 100ns to 160ns to maintain the same dv. The compensation cap was very difficult to optimize. You either get underdamped with overshoot or overdamped with overshoot. But I ended up with the same value as you - 20nF. So the response is sensitive to pulse width, compensation cap and output cap values. Relatively small changes have a big effect on the response. The compensation cap ESR seems to have no effect. The output cap ESR has a very significant effect on the shape of the response. (I checked the title - this is the correct file. I need to always do that in the future.) Version 4 SHEET 1 1184 680 WIRE 64 128 -96 128 WIRE 368 128 320 128 WIRE 400 128 368 128 WIRE 512 128 480 128 WIRE 624 128 512 128 WIRE 688 128 624 128 WIRE 752 128 688 128 WIRE 368 144 368 128 WIRE -96 160 -96 128 WIRE 512 160 512 128 WIRE 624 160 624 128 WIRE 192 256 192 224 WIRE 368 256 368 224 WIRE 368 256 192 256 WIRE -96 272 -96 240 WIRE 368 272 368 256 WIRE 512 272 512 224 WIRE 624 272 624 240 WIRE 192 288 192 256 WIRE 192 368 192 352 WIRE 368 384 368 352 WIRE 192 464 192 448 FLAG -96 272 0 FLAG 368 384 0 FLAG -96 128 IN FLAG 512 272 0 FLAG 688 128 OUT FLAG 624 272 0 FLAG 192 464 0 SYMBOL voltage -96 144 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 48 SYMBOL res 352 128 R0 SYMATTR InstName R1 SYMATTR Value 200 SYMBOL res 352 256 R0 SYMATTR InstName R2 SYMATTR Value 4.53k SYMBOL LT317A 192 128 R0 SYMATTR InstName U1 SYMBOL cap 496 160 R0 SYMATTR InstName C1 SYMATTR Value 20u SYMATTR SpiceLine Rser=8.33m SYMBOL current 624 160 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName I1 SYMATTR Value PULSE(30m 12 5u 10n 10n 160n) SYMBOL cap 176 288 R0 SYMATTR InstName C2 SYMATTR Value 20n SYMBOL res 176 352 R0 SYMATTR InstName R3 SYMATTR Value 0 SYMBOL res 496 112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 1u TEXT 72 -16 Left 2 !.tran 0 200u 0 50n TEXT 80 -40 Left 2 ;'LT317A 12A Load Transient Response
Reply by ●February 21, 20182018-02-21
On 02/21/2018 04:08 AM, Steve Wilson wrote:> John Larkin <jjlarkin@highlandtechnology.com> wrote: > >> On Wed, 21 Feb 2018 04:51:28 GMT, Steve Wilson <no@spam.com> wrote: >> >>> John Larkin <jjlarkin@highlandtechnology.com> wrote: >>> >>>> On Wed, 21 Feb 2018 01:54:05 GMT, Steve Wilson <no@spam.com> wrote: >>>>>> Amps are amps. The load step demonstrates the ringing, and the fix >>>>>> for the ringing. >>> >>>>> Your simulation shows considerably different ringing between current >>>>> rise and fall. So the currents matter. >>> >>>> Sure, the output transistor emitter has a very different impedance >>>> from 30 to 200 mA. So the pole from that impedance into the ceramic >>>> caps is different for the two currents. >>> >>>>> You need to model the actual currents you are using. I suspect the >>>>> idle current may be much lower, and the actual charge current may be >>>>> higher. >>> >>>>> I'd be happier to see the results with a pulsed 100ns 12 Amp load. >>> >>> I did it for you. The compensation cap is critical and very different >> >from your result. See below. >> >> That seems to be my sim; same currents, no ESR in the output cap, just >> not as pretty. Any engineering doc should have a title, author, and >> date. > > Sorry, I picked the wrong file. You can see the original by changing the > current, pulse width and cap values as listed below. > > This is a newsgroup discussion. The title is shown at the top of the > schematic. The date is the date of the post and is shown in the header. If > I put my name on the document, you will get pissed. > >> I tried adding 20 mohms ESR to the output caps in my sim. Nothing >> changed; it rings badly without the added compensation, and doesn't >> ring with the RC, or just the C, from ADJ to ground. > >> Pulsing at 10 amps for 100 ns, the results are about the same: lots of >> ringing, fixed by adding the same comps. > >> The compensation parts help. > >>>>>>> You may need to use a switched resistive load to provide some >>>>>>> damping. This may have a significant effect on the ringing. > >>>>>>> Note the load transient response in Figs 3 and 4 of the TI datasheet >>>>>>> show a considerably different response than your model. > >>>>>> They probably use caps with a lot of ESR. And the models differ too. > >>>>> You need to model the ESR. > >> You sure like to tell me what I need to do. But I don't report to you. > > Sensitive? Not at all. Try the generic "you". > > I increased the output cap from 12 uF to 20uF to match your value, amd > changed the pulse width from 100ns to 160ns to maintain the same dv. > > The compensation cap was very difficult to optimize. You either get > underdamped with overshoot or overdamped with overshoot. But I ended up > with the same value as you - 20nF. So the response is sensitive to pulse > width, compensation cap and output cap values. Relatively small changes > have a big effect on the response. > > The compensation cap ESR seems to have no effect. The output cap ESR has a > very significant effect on the shape of the response. >At the optimal compensation cap value, the results hardly depend on pulse width at all. I ran your sim with pulse widths from 50 ns to 1 us, and with a compensation cap of 15 nF the droop is at most 70 mV over the whole range, i.e. 0.23%, which isn't bad at all. With no compensation cap, it's a mess for sure. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net https://hobbs-eo.com
Reply by ●February 21, 20182018-02-21
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> At the optimal compensation cap value, the results hardly depend on > pulse width at all.> I ran your sim with pulse widths from 50 ns to 1 us, and with a > compensation cap of 15 nF the droop is at most 70 mV over the whole > range, i.e. 0.23%, which isn't bad at all.> With no compensation cap, it's a mess for sure.> Cheers> Phil HobbsI tried to keep the drop under 0.1V during the high current pulse. With 1us pulse width, the drop calculates to i = c dv/dt i dt = c dv dv = i * dt / c = 12 * 1e-6 / 20e-6 = 0.6V 0.6 / 28 = 2.142e-2 = 2.142% I measure 0.637V. This is 0.637 / 28 = 0.02275 = 2.275% I don't know where you are measuring the drop. With 15nF compensation, the peak current from the LT317A is 1.7A, a bit over the spec of 1.5A. The recovery from the pulse is slow and draggy but probably acceptable at low duty cycle. Here's the file: Version 4 SHEET 1 1184 680 WIRE 64 128 -96 128 WIRE 368 128 320 128 WIRE 400 128 368 128 WIRE 512 128 480 128 WIRE 624 128 512 128 WIRE 688 128 624 128 WIRE 752 128 688 128 WIRE 368 144 368 128 WIRE -96 160 -96 128 WIRE 512 160 512 128 WIRE 624 160 624 128 WIRE 192 256 192 224 WIRE 368 256 368 224 WIRE 368 256 192 256 WIRE -96 272 -96 240 WIRE 368 272 368 256 WIRE 512 272 512 224 WIRE 624 272 624 240 WIRE 192 288 192 256 WIRE 192 368 192 352 WIRE 368 384 368 352 WIRE 192 464 192 448 FLAG -96 272 0 FLAG 368 384 0 FLAG -96 128 IN FLAG 512 272 0 FLAG 688 128 OUT FLAG 624 272 0 FLAG 192 464 0 SYMBOL voltage -96 144 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 48 SYMBOL res 352 128 R0 SYMATTR InstName R1 SYMATTR Value 200 SYMBOL res 352 256 R0 SYMATTR InstName R2 SYMATTR Value 4.53k SYMBOL LT317A 192 128 R0 SYMATTR InstName U1 SYMBOL cap 496 160 R0 SYMATTR InstName C1 SYMATTR Value 20u SYMATTR SpiceLine Rser=8.33m SYMBOL current 624 160 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName I1 SYMATTR Value PULSE(30m 12 5u 10n 10n 1u) SYMBOL cap 176 288 R0 SYMATTR InstName C2 SYMATTR Value 15n SYMBOL res 176 352 R0 SYMATTR InstName R3 SYMATTR Value 0 SYMBOL res 496 112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 1u TEXT 72 -16 Left 2 !.tran 0 200u 0 50n TEXT 80 -40 Left 2 ;'LT317A 12A Load Transient Response
Reply by ●February 21, 20182018-02-21
On Wed, 21 Feb 2018 09:08:45 GMT, Steve Wilson <no@spam.com> wrote:>John Larkin <jjlarkin@highlandtechnology.com> wrote: > >> On Wed, 21 Feb 2018 04:51:28 GMT, Steve Wilson <no@spam.com> wrote: >> >>>John Larkin <jjlarkin@highlandtechnology.com> wrote: >>> >>>> On Wed, 21 Feb 2018 01:54:05 GMT, Steve Wilson <no@spam.com> wrote: >>>>>> Amps are amps. The load step demonstrates the ringing, and the fix >>>>>> for the ringing. >>> >>>>>Your simulation shows considerably different ringing between current >>>>>rise and fall. So the currents matter. >>> >>>> Sure, the output transistor emitter has a very different impedance >>>> from 30 to 200 mA. So the pole from that impedance into the ceramic >>>> caps is different for the two currents. >>> >>>>>You need to model the actual currents you are using. I suspect the >>>>>idle current may be much lower, and the actual charge current may be >>>>>higher. >>> >>>>>I'd be happier to see the results with a pulsed 100ns 12 Amp load. >>> >>>I did it for you. The compensation cap is critical and very different >>>from your result. See below. >> >> That seems to be my sim; same currents, no ESR in the output cap, just >> not as pretty. Any engineering doc should have a title, author, and >> date. > >Sorry, I picked the wrong file. You can see the original by changing the >current, pulse width and cap values as listed below. > >This is a newsgroup discussion. The title is shown at the top of the >schematic. The date is the date of the post and is shown in the header. If >I put my name on the document, you will get pissed.If I modify someone else's sim or schematic, I show both names.> >> I tried adding 20 mohms ESR to the output caps in my sim. Nothing >> changed; it rings badly without the added compensation, and doesn't >> ring with the RC, or just the C, from ADJ to ground. > >> Pulsing at 10 amps for 100 ns, the results are about the same: lots of >> ringing, fixed by adding the same comps. > >> The compensation parts help. > >>>>>>>You may need to use a switched resistive load to provide some >>>>>>>damping. This may have a significant effect on the ringing. > >>>>>>>Note the load transient response in Figs 3 and 4 of the TI datasheet >>>>>>>show a considerably different response than your model. > >>>>>> They probably use caps with a lot of ESR. And the models differ too. > >>>>>You need to model the ESR. > >> You sure like to tell me what I need to do. But I don't report to you. > >Sensitive? Not at all. Try the generic "you". > >I increased the output cap from 12 uF to 20uF to match your value, amd >changed the pulse width from 100ns to 160ns to maintain the same dv. > >The compensation cap was very difficult to optimize. You either get >underdamped with overshoot or overdamped with overshoot. But I ended up >with the same value as you - 20nF. So the response is sensitive to pulse >width, compensation cap and output cap values. Relatively small changes >have a big effect on the response. > >The compensation cap ESR seems to have no effect. The output cap ESR has a >very significant effect on the shape of the response.If I put that extra 0805 cap on my PC board, I can elect to leave it out, or to change its value if that improves transient response. But just changing the ring Q from 20 to 2 is a big improvement. What I don't want is the trigger rate (controlled by my cutomer) to push the ringing into bad places. This is, to me, a new way of using the LM317. I thought I'd share it. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●February 22, 20182018-02-22
John Larkin <jjlarkin@highlandtechnology.com> wrote:> This is, to me, a new way of using the LM317. I thought I'd share it.To me, it is amazing that a device can take such a horrible whacking and recover so gracefully. This is in a device that has zero phase shift at DC and a constant voltage difference between the output and the feedback pin. Before we terrorize users with the response to transient pulses, the behaviour is much more docile with softer loads. Here's the result with a sine wave load. The behavior is very much the same with huge variations in load and adj cap values and load current. It is very well behaved. Version 4 SHEET 1 1184 680 WIRE 64 128 -96 128 WIRE 368 128 320 128 WIRE 400 128 368 128 WIRE 512 128 480 128 WIRE 624 128 512 128 WIRE 688 128 624 128 WIRE 752 128 688 128 WIRE 368 144 368 128 WIRE -96 160 -96 128 WIRE 512 160 512 128 WIRE 624 160 624 128 WIRE -96 272 -96 240 WIRE 512 272 512 224 WIRE 624 304 624 240 WIRE 192 336 192 224 WIRE 336 336 192 336 WIRE 368 336 368 224 WIRE 368 336 336 336 WIRE 368 352 368 336 WIRE 192 368 192 336 WIRE 624 400 624 384 WIRE 192 448 192 432 WIRE 368 464 368 432 WIRE 192 544 192 528 FLAG -96 272 0 FLAG 368 464 0 FLAG -96 128 IN FLAG 512 272 0 FLAG 688 128 OUT FLAG 192 544 0 FLAG 336 336 R1R2 FLAG 368 128 R1R4 FLAG 624 400 0 SYMBOL voltage -96 144 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL res 352 128 R0 SYMATTR InstName R1 SYMATTR Value 470 SYMBOL res 352 336 R0 SYMATTR InstName R2 SYMATTR Value 3242 SYMBOL LT317A 192 128 R0 SYMATTR InstName U1 SYMBOL cap 496 160 R0 SYMATTR InstName C1 SYMATTR Value 1u SYMBOL cap 176 368 R0 SYMATTR InstName C2 SYMATTR Value 10n SYMBOL res 176 432 R0 SYMATTR InstName R3 SYMATTR Value 20m SYMBOL res 496 112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 1u SYMBOL res 608 144 R0 SYMATTR InstName R5 SYMATTR Value 100 SYMBOL voltage 624 288 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value SINE(0 1 1e3) TEXT 72 -16 Left 2 !.tran 4m TEXT 80 -40 Left 2 ;'LT317A Sine Wave Load
Reply by ●February 22, 20182018-02-22
Steve Wilson <no@spam.com> wrote:> John Larkin <jjlarkin@highlandtechnology.com> wrote:>> This is, to me, a new way of using the LM317. I thought I'd share it.> To me, it is amazing that a device can take such a horrible whacking and > recover so gracefully. This is in a device that has zero phase shift at > DC and a constant voltage difference between the output and the feedback > pin.> Before we terrorize users with the response to transient pulses, the > behaviour is much more docile with softer loads. Here's the result with > a sine wave load. The behavior is very much the same with huge > variations in load and adj cap values and load current. It is very well > behaved.Another aspect of op amps and linear regulators is the ouput impedance. It often rises with frequency, giving an inductive characteristic. There is a paper by Erroil H. Dietz, Senior Technician, National Semiconductor titled "Understanding and Reducing Noise Voltage on 3-Terminal Voltage Regulators" that describes this. The article is very hard to find. Some pdf links have it inverted which is a pain to have to rotate twice to read it. The article is also on p204 in Appendix C of Bob Pease's book titled "Troubleshooting Analog Circuits". There is a a copy online. Scroll down to page 204 to read it: https://www.slideshare.net/jrbb2000/105768251-troubleshootinganalogcircuits There is a series of Bob's articles at http://www.introni.it/riviste_bob_pease.html There is a possibility that the Dietz paper may be included in them, but it will take a while to look for it.
Reply by ●February 22, 20182018-02-22
Steve Wilson <no@spam.com> wrote:> Steve Wilson <no@spam.com> wrote: > Another aspect of op amps and linear regulators is the ouput impedance. > It often rises with frequency, giving an inductive characteristic. There > is a paper by Erroil H. Dietz, Senior Technician, National Semiconductor > titled "Understanding and Reducing Noise Voltage on 3-Terminal Voltage > Regulators" that describes this.> The article is very hard to find. Some pdf links have it inverted which > is a pain to have to rotate twice to read it.> The article is also on p204 in Appendix C of Bob Pease's book titled > "Troubleshooting Analog Circuits". There is a a copy online. Scroll down > to page 204 to read it:> https://www.slideshare.net/jrbb2000/105768251-troubleshootinganalogcircui > ts> There is a series of Bob's articles at> http://www.introni.it/riviste_bob_pease.html> There is a possibility that the Dietz paper may be included in them, but > it will take a while to look for it.Ha! There is a better source at http://b-ok.org/book/767820/de94cb It is on page 201. You need WinDjView to read it, which you can get at https://windjview.sourceforge.io/
Reply by ●February 22, 20182018-02-22
On 02/22/2018 01:54 AM, Steve Wilson wrote:> Steve Wilson <no@spam.com> wrote: > >> Steve Wilson <no@spam.com> wrote: >> Another aspect of op amps and linear regulators is the ouput impedance. >> It often rises with frequency, giving an inductive characteristic. There >> is a paper by Erroil H. Dietz, Senior Technician, National Semiconductor >> titled "Understanding and Reducing Noise Voltage on 3-Terminal Voltage >> Regulators" that describes this. > >> The article is very hard to find. Some pdf links have it inverted which >> is a pain to have to rotate twice to read it.I posted it upthread. It's on my website. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net https://hobbs-eo.com
Reply by ●February 22, 20182018-02-22
On Sun, 18 Feb 2018 13:12:50 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 02/18/2018 08:14 AM, Chris Jones wrote: >> On 17/02/2018 07:18, bloggs.fredbloggs.fred@gmail.com wrote: >>> On Friday, February 16, 2018 at 12:51:24 PM UTC-5, John Larkin wrote: >>>> On Fri, 16 Feb 2018 08:57:05 -0800 (PST), >>>> bloggs.fredbloggs.fred@gmail.com wrote: >>>> >>>>> On Thursday, February 15, 2018 at 8:32:35 PM UTC-5, John Larkin wrote: >>>>>> LM317's like some ESR in their output capacitors. I don't want any >>>>>> electrolytic or tantalum caps in my new thing, just ceramics, and the >>>>>> sim sure rings: >>>>>> >>>>>> https://www.dropbox.com/s/11b3w42nsvpliki/317_nocomp.jpg?raw=1 >>>>>> >>>>>> But this fixes it: >>>>>> >>>>>> https://www.dropbox.com/s/9q80heyfbwh5frp/317_comp.jpg?raw=1 >>>>>> >>>>>> This ain't rocket science, but I haven't seen it done before. >>>>> >>>>> 317 needs no such ESR compensation. >>>> >>>> The data sheet says it does. >>>> >>>>> The ringing looks suspiciously like excitation of the SRF of an >>>>> output capacitor. >>>> >>>> The frequency is low, and is different on the rising and falling edges >>>> of the load current pulse. It's the chip pseudo-inductance resonating, >>>> not the cap's ESL. If the ringing were local to the caps, my damping >>>> on ADJ wouldn't fix that. >>>> >>>> Cap series L makes a different waveform than paralleled inductance. >>>> >>>> >>>>> Did your model give it any ESL? And your solution merely reduces the >>>>> shunt resistance by a factor of 20x which probably has more to do >>>>> with damping than anything else. >>>> >>>> With a big cap from ADJ to ground, it rings badly, too. It has to be >>>> the *right* capacitor to damp the ringing. >>>> >>>> I tried this with two different LM317 models; the ringing is somewhat >>>> different (the LT317 is better), but the damping idea is the same. >>>> >>>> It's amazing that LT ever made a 317. I think they did that early on, >>>> when they needed some revenue. They want $4 for it! I'm paying less >>>> than a tenth of that for TI. >>> >>> I doubt you're going to see this energetic resonance on anything other >>> than the LT part. >>> >> >> I doubted it too, but found out the hard way when: >> my 337's all oscillated, and >> the 317s rang so badly that the oscillation ripple on the positive rails >> was even bigger than on the negative rails. >> >> The 317s wouldn't oscillate by themselves, but they would ring like a >> bell even after I cured the 337's of oscillation. >> >> I had to scratch off a lot of solder mask and tack on many tantalums to >> cure my boards. Quite embarassing. >> >> >Check out the Erroll Dietz article I posted upthread.He used three values of Cadj, 0, 10u, and 1000u. He didn't try anything like 22nF. I'm sort of surprised that nobody seems to have tried that, or at least publicized it. -- John Larkin Highland Technology, Inc lunatic fringe electronics
