feedback and stability
in [Design]
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From: Jim Thompson on 29 Dec 2009 10:50 On Mon, 28 Dec 2009 18:36:07 -0800, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >On Mon, 28 Dec 2009 18:57:06 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: > >>JosephKK wrote: >>> On Thu, 24 Dec 2009 14:52:20 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: >>> >>> >>>>Jim Thompson wrote: >> >><snip> >> >>>>> >>>>>TL431's don't SWITCH. They're more akin to an OpAmp. >>>> >>>>Strictly speaking, no, but for usage here, it's simplest to >>>>think of them switching. The switching is like what you see >>>>on a typical zener curve where V goes from sloped to flat >>>>when the zener avalanches and conducts in the reverse >>>>direction. So they can be thought of as on or off. >>>> >>> >>> >>> EUWWW. According to the datasheet is supposed to be used as a >>> linear (read non-switching) device. >> >> From the datasheet: >>"Active output circuitry provides a very sharp turn-on >>characteristic making these devices excellent replacements >>for Zener diodes..." >> >>Where do you see the datasheet indicating it "is supposed to >>be used as a linear (read non-switching) device" ? >>How do you define linear as non-switching? >> >>I'm sure you have something in mind here, but I'm not >>getting it. Vin vs Vout is anything but linear when the >>device is turned on. So you must have something different >>in mind. As to switching, the datasheet specifies a >>very sharp turn-on and refers to a zener. How does my >>description of that fail to capture the gist? >> >>Ed >> >>> >>> >>>>Ed >>>> >>>> >>>>>To do it correctly you want a FIXED FREQUENCY oscillator with variable >>>>>duty cycle. Have the TL431 vary the duty cycle. >>>>> >>>>> ...Jim Thompson > >I thought i read something like walking back and forth around the >knee of the regulation curve. Did i? What "knee"? TL431's are quite sharp. Treat it as an OpAmp... drawing showing how sometime today. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism responsibly!
From: gearhead on 29 Dec 2009 15:44 On Dec 28, 8:08 pm, ehsjr <eh...(a)nospamverizon.net> wrote: > Jim Thompson wrote: > > On Sun, 27 Dec 2009 16:47:45 -0800, > > "JosephKK"<quiettechb...(a)yahoo.com> wrote: > > >>On Thu, 24 Dec 2009 14:52:20 -0500, ehsjr <eh...(a)nospamverizon.net> wrote: > > >>>Jim Thompson wrote: > > >>>>On Wed, 23 Dec 2009 21:53:45 -0500, ehsjr <eh...(a)nospamverizon.net> > >>>>wrote: > > >>>>>Michael Robinson wrote: > > >>>>><snip> > > >>>>>>>Drawing it out will make it clear. The first schematic below > >>>>>>>is a "sanity check", just to make sure that the circuit > >>>>>>>you described is something like it: > > >>>>>>As I explained from the beginning, the votlage divider at the reference pin > >>>>>>is connected to the battery/alternator output, like this: > > >>>>>> field > >>>>>> | > >>>>>> mosfet > >>>>>> | > >>>>>> vco batt > >>>>>> | | > >>>>>> +12v---[R]---+------+ | > >>>>>> | | | > >>>>>> __|__/ [C] [R] > >>>>>> / / \ | | > >>>>>> /___\----+-------+ > >>>>>> | | > >>>>>> | [R] > >>>>>> | | > >>>>>> Gnd ----------+--------------+ > > >>>>>>Can you see how it would regulate voltage? > > >>>>>Yes. I've seen the idea all along, just not sure what > >>>>>was connected to where. Doesn't change my earlier answer. > > >>>>>>Any variation in the battery > >>>>>>voltage will cause the cathode to pull the vco up or down, changing the duty > >>>>>>cycle of the mosfet. It's a big feedback loop that includes the alternator. > >>>>>>I want to know how the cap throws everything off. > > >>>>>Check the diagram I drew showing the 431 as an op amp > >>>>>& output xsistor, with an internal 2.5 V ref. The 431 > >>>>>works as a comparator driving a transistor, and your cap > >>>>>feeds the output back into the op amp input, causing > >>>>>oscillation. > > >>>>>Your TL431 will operate (turn on or off) in response to > >>>>>a voltage change at the battery. The current through > >>>>>R will suddenly change when the 431 switches on or off, > >>>>>changing the drop across R. *That* change will be fed > >>>>>back by the cap into the input of the 431 at the ref pin > >>>>>as a pulse, and the 431 will react to that pulse. That > >>>>>feedback path is causing your problem. You do not want > >>>>>*that* change (the change at the cathode of the TL431) to > >>>>>appear on the ref pin. > > >>>>>Ed > > >>>>TL431's don't SWITCH. They're more akin to an OpAmp. > > >>>Strictly speaking, no, but for usage here, it's simplest to > >>>think of them switching. The switching is like what you see > >>>on a typical zener curve where V goes from sloped to flat > >>>when the zener avalanches and conducts in the reverse > >>>direction. So they can be thought of as on or off. > > >>EUWWW. According to the datasheet is supposed to be used as a > >>linear (read non-switching) device. > > >>>Ed > > >>>>To do it correctly you want a FIXED FREQUENCY oscillator with variable > >>>>duty cycle. Have the TL431 vary the duty cycle. > > >>>> ...Jim Thompson > > > I hate to have to keep repeating myself... a TL431 should be > > considered as an OpAmp with VOS = VREF and with an open collector > > output. Is that too complicated for some of you to grasp ?:-) > > > ...Jim Thompson > > Yes, I think it is, for me. I think you see the implications > of that clearly, while for me it is likely less clear. > > This is what I see: > I look at the op amp input and see large gain, the output and see > large (vs op amp output) current capability with the need for a > load. I see the Vref input. In his circuit, I see the cap on > the output which can cause instability, and can only guess at the > load, but I assume it's small enough to draw a few mA. Whatever > else I should be considering, I'm missing. > > Ed- Hide quoted text - > > - Show quoted text - The load is simply a pull-up resistor. The voltage at the cathode is the control for the oscillator.
From: Jim Thompson on 29 Dec 2009 16:05 On Mon, 28 Dec 2009 18:24:29 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: >On Mon, 28 Dec 2009 20:08:49 -0500, ehsjr <ehsjr(a)nospamverizon.net> >wrote: > >>Jim Thompson wrote: >>> On Sun, 27 Dec 2009 16:47:45 -0800, >>> "JosephKK"<quiettechblue(a)yahoo.com> wrote: >>> >>> >>>>On Thu, 24 Dec 2009 14:52:20 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: >>>> >>>> >>>>>Jim Thompson wrote: >>>>> >>>>>>On Wed, 23 Dec 2009 21:53:45 -0500, ehsjr <ehsjr(a)nospamverizon.net> >>>>>>wrote: >>>>>> >>>>>> >>>>>> >>>>>>>Michael Robinson wrote: >>>>>>> >>>>>>><snip> >>>>>>> >>>>>>>>>Drawing it out will make it clear. The first schematic below >>>>>>>>>is a "sanity check", just to make sure that the circuit >>>>>>>>>you described is something like it: >>>>>>>> >>>>>>>> >>>>>>>>As I explained from the beginning, the votlage divider at the reference pin >>>>>>>>is connected to the battery/alternator output, like this: >>>>>>>> >>>>>>>> field >>>>>>>> | >>>>>>>> mosfet >>>>>>>> | >>>>>>>> vco batt >>>>>>>> | | >>>>>>>> +12v---[R]---+------+ | >>>>>>>> | | | >>>>>>>> __|__/ [C] [R] >>>>>>>> / / \ | | >>>>>>>> /___\----+-------+ >>>>>>>> | | >>>>>>>> | [R] >>>>>>>> | | >>>>>>>> Gnd ----------+--------------+ >>>>>>>> >>>>>>>>Can you see how it would regulate voltage? >>>>>>> >>>>>>>Yes. I've seen the idea all along, just not sure what >>>>>>>was connected to where. Doesn't change my earlier answer. >>>>>>> >>>>>>> >>>>>>> >>>>>>>>Any variation in the battery >>>>>>>>voltage will cause the cathode to pull the vco up or down, changing the duty >>>>>>>>cycle of the mosfet. It's a big feedback loop that includes the alternator. >>>>>>>>I want to know how the cap throws everything off. >>>>>>> >>>>>>>Check the diagram I drew showing the 431 as an op amp >>>>>>>& output xsistor, with an internal 2.5 V ref. The 431 >>>>>>>works as a comparator driving a transistor, and your cap >>>>>>>feeds the output back into the op amp input, causing >>>>>>>oscillation. >>>>>>> >>>>>>>Your TL431 will operate (turn on or off) in response to >>>>>>>a voltage change at the battery. The current through >>>>>>>R will suddenly change when the 431 switches on or off, >>>>>>>changing the drop across R. *That* change will be fed >>>>>>>back by the cap into the input of the 431 at the ref pin >>>>>>>as a pulse, and the 431 will react to that pulse. That >>>>>>>feedback path is causing your problem. You do not want >>>>>>>*that* change (the change at the cathode of the TL431) to >>>>>>>appear on the ref pin. >>>>>>> >>>>>>>Ed >>>>>>> >>>>>>> >>>>>> >>>>>> >>>>>>TL431's don't SWITCH. They're more akin to an OpAmp. >>>>> >>>>>Strictly speaking, no, but for usage here, it's simplest to >>>>>think of them switching. The switching is like what you see >>>>>on a typical zener curve where V goes from sloped to flat >>>>>when the zener avalanches and conducts in the reverse >>>>>direction. So they can be thought of as on or off. >>>>> >>>> >>>>EUWWW. According to the datasheet is supposed to be used as a >>>>linear (read non-switching) device. >>>> >>>> >>>>>Ed >>>>> >>>>> >>>>>>To do it correctly you want a FIXED FREQUENCY oscillator with variable >>>>>>duty cycle. Have the TL431 vary the duty cycle. >>>>>> >>>>>> ...Jim Thompson >>> >>> >>> I hate to have to keep repeating myself... a TL431 should be >>> considered as an OpAmp with VOS = VREF and with an open collector >>> output. Is that too complicated for some of you to grasp ?:-) >>> >>> ...Jim Thompson >> >>Yes, I think it is, for me. I think you see the implications >>of that clearly, while for me it is likely less clear. >> >>This is what I see: >>I look at the op amp input and see large gain, the output and see >>large (vs op amp output) current capability with the need for a >>load. I see the Vref input. In his circuit, I see the cap on >>the output which can cause instability, and can only guess at the >>load, but I assume it's small enough to draw a few mA. Whatever >>else I should be considering, I'm missing. >> >>Ed >> > >I'll draw up a pretty picture tomorrow... wine time right now ;-) > > ...Jim Thompson Take a look at this... www.analog-innovations.com/SED/TL431_Equivalent.pdf The snag is that the OpAmp IS compensated for unity feedback, for who-knows-what resistive and capacitive load. So some caution is advised. You can, for instance, apply the usual feed-forward/feed-around schemes to isolate the load capacitance form the feedback loop. I have, in the past, used these device as OpAmps, driving matched photo-couplers, to get a quite linear _analog_ photo-link ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism responsibly!
From: Jim Thompson on 29 Dec 2009 16:05 On Tue, 29 Dec 2009 08:50:37 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: >On Mon, 28 Dec 2009 18:36:07 -0800, >"JosephKK"<quiettechblue(a)yahoo.com> wrote: > >>On Mon, 28 Dec 2009 18:57:06 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: >> >>>JosephKK wrote: >>>> On Thu, 24 Dec 2009 14:52:20 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: >>>> >>>> >>>>>Jim Thompson wrote: >>> >>><snip> >>> >>>>>> >>>>>>TL431's don't SWITCH. They're more akin to an OpAmp. >>>>> >>>>>Strictly speaking, no, but for usage here, it's simplest to >>>>>think of them switching. The switching is like what you see >>>>>on a typical zener curve where V goes from sloped to flat >>>>>when the zener avalanches and conducts in the reverse >>>>>direction. So they can be thought of as on or off. >>>>> >>>> >>>> >>>> EUWWW. According to the datasheet is supposed to be used as a >>>> linear (read non-switching) device. >>> >>> From the datasheet: >>>"Active output circuitry provides a very sharp turn-on >>>characteristic making these devices excellent replacements >>>for Zener diodes..." >>> >>>Where do you see the datasheet indicating it "is supposed to >>>be used as a linear (read non-switching) device" ? >>>How do you define linear as non-switching? >>> >>>I'm sure you have something in mind here, but I'm not >>>getting it. Vin vs Vout is anything but linear when the >>>device is turned on. So you must have something different >>>in mind. As to switching, the datasheet specifies a >>>very sharp turn-on and refers to a zener. How does my >>>description of that fail to capture the gist? >>> >>>Ed >>> >>>> >>>> >>>>>Ed >>>>> >>>>> >>>>>>To do it correctly you want a FIXED FREQUENCY oscillator with variable >>>>>>duty cycle. Have the TL431 vary the duty cycle. >>>>>> >>>>>> ...Jim Thompson >> >>I thought i read something like walking back and forth around the >>knee of the regulation curve. Did i? > >What "knee"? TL431's are quite sharp. Treat it as an OpAmp... >drawing showing how sometime today. > > ...Jim Thompson Take a look at this... www.analog-innovations.com/SED/TL431_Equivalent.pdf The snag is that the OpAmp IS compensated for unity feedback, for who-knows-what resistive and capacitive load. So some caution is advised. You can, for instance, apply the usual feed-forward/feed-around schemes to isolate the load capacitance form the feedback loop. I have, in the past, used these device as OpAmps, driving matched photo-couplers, to get a quite linear _analog_ photo-link ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism responsibly!
From: ehsjr on 29 Dec 2009 22:35
Jim Thompson wrote: > On Mon, 28 Dec 2009 18:24:29 -0700, Jim Thompson > <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: > > >>On Mon, 28 Dec 2009 20:08:49 -0500, ehsjr <ehsjr(a)nospamverizon.net> >>wrote: >> >> >>>Jim Thompson wrote: >>> >>>>On Sun, 27 Dec 2009 16:47:45 -0800, >>>>"JosephKK"<quiettechblue(a)yahoo.com> wrote: >>>> >>>> >>>> >>>>>On Thu, 24 Dec 2009 14:52:20 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: >>>>> >>>>> >>>>> >>>>>>Jim Thompson wrote: >>>>>> >>>>>> >>>>>>>On Wed, 23 Dec 2009 21:53:45 -0500, ehsjr <ehsjr(a)nospamverizon.net> >>>>>>>wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>>>Michael Robinson wrote: >>>>>>>> >>>>>>>><snip> >>>>>>>> >>>>>>>>>>Drawing it out will make it clear. The first schematic below >>>>>>>>>>is a "sanity check", just to make sure that the circuit >>>>>>>>>>you described is something like it: >>>>>>>>> >>>>>>>>> >>>>>>>>>As I explained from the beginning, the votlage divider at the reference pin >>>>>>>>>is connected to the battery/alternator output, like this: >>>>>>>>> >>>>>>>>> field >>>>>>>>> | >>>>>>>>> mosfet >>>>>>>>> | >>>>>>>>> vco batt >>>>>>>>> | | >>>>>>>>> +12v---[R]---+------+ | >>>>>>>>> | | | >>>>>>>>> __|__/ [C] [R] >>>>>>>>> / / \ | | >>>>>>>>> /___\----+-------+ >>>>>>>>> | | >>>>>>>>> | [R] >>>>>>>>> | | >>>>>>>>>Gnd ----------+--------------+ >>>>>>>>> >>>>>>>>>Can you see how it would regulate voltage? >>>>>>>> >>>>>>>>Yes. I've seen the idea all along, just not sure what >>>>>>>>was connected to where. Doesn't change my earlier answer. >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>>Any variation in the battery >>>>>>>>>voltage will cause the cathode to pull the vco up or down, changing the duty >>>>>>>>>cycle of the mosfet. It's a big feedback loop that includes the alternator. >>>>>>>>>I want to know how the cap throws everything off. >>>>>>>> >>>>>>>>Check the diagram I drew showing the 431 as an op amp >>>>>>>>& output xsistor, with an internal 2.5 V ref. The 431 >>>>>>>>works as a comparator driving a transistor, and your cap >>>>>>>>feeds the output back into the op amp input, causing >>>>>>>>oscillation. >>>>>>>> >>>>>>>>Your TL431 will operate (turn on or off) in response to >>>>>>>>a voltage change at the battery. The current through >>>>>>>>R will suddenly change when the 431 switches on or off, >>>>>>>>changing the drop across R. *That* change will be fed >>>>>>>>back by the cap into the input of the 431 at the ref pin >>>>>>>>as a pulse, and the 431 will react to that pulse. That >>>>>>>>feedback path is causing your problem. You do not want >>>>>>>>*that* change (the change at the cathode of the TL431) to >>>>>>>>appear on the ref pin. >>>>>>>> >>>>>>>>Ed >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> >>>>>>>TL431's don't SWITCH. They're more akin to an OpAmp. >>>>>> >>>>>>Strictly speaking, no, but for usage here, it's simplest to >>>>>>think of them switching. The switching is like what you see >>>>>>on a typical zener curve where V goes from sloped to flat >>>>>>when the zener avalanches and conducts in the reverse >>>>>>direction. So they can be thought of as on or off. >>>>>> >>>>> >>>>>EUWWW. According to the datasheet is supposed to be used as a >>>>>linear (read non-switching) device. >>>>> >>>>> >>>>> >>>>>>Ed >>>>>> >>>>>> >>>>>> >>>>>>>To do it correctly you want a FIXED FREQUENCY oscillator with variable >>>>>>>duty cycle. Have the TL431 vary the duty cycle. >>>>>>> >>>>>>> ...Jim Thompson >>>> >>>> >>>>I hate to have to keep repeating myself... a TL431 should be >>>>considered as an OpAmp with VOS = VREF and with an open collector >>>>output. Is that too complicated for some of you to grasp ?:-) >>>> >>>> ...Jim Thompson >>> >>>Yes, I think it is, for me. I think you see the implications >>>of that clearly, while for me it is likely less clear. >>> >>>This is what I see: >>>I look at the op amp input and see large gain, the output and see >>>large (vs op amp output) current capability with the need for a >>>load. I see the Vref input. In his circuit, I see the cap on >>>the output which can cause instability, and can only guess at the >>>load, but I assume it's small enough to draw a few mA. Whatever >>>else I should be considering, I'm missing. >>> >>>Ed >>> >> >>I'll draw up a pretty picture tomorrow... wine time right now ;-) >> >> ...Jim Thompson > > > Take a look at this... > > www.analog-innovations.com/SED/TL431_Equivalent.pdf > > The snag is that the OpAmp IS compensated for unity feedback, for > who-knows-what resistive and capacitive load. > > So some caution is advised. You can, for instance, apply the usual > feed-forward/feed-around schemes to isolate the load capacitance form > the feedback loop. > > I have, in the past, used these device as OpAmps, driving matched > photo-couplers, to get a quite linear _analog_ photo-link ;-) > > ...Jim Thompson Thanks! Ed |