feedback and stability
in [Design]
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From: Jim Thompson on 29 Dec 2009 23:02 On Tue, 29 Dec 2009 22:35:56 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: >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 You are quite welcome! ...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 30 Dec 2009 10:57 On Wed, 30 Dec 2009 07:32:12 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Tue, 29 Dec 2009 14:05:33 -0700, Jim Thompson wrote: > >> Take a look at this... >> >> www.analog-innovations.com/SED/TL431_Equivalent.pdf > >Same as shown in the TI data sheet Sheeeesh! I guess that must mean I'm right ;-) ...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: Michael A. Terrell on 31 Dec 2009 13:32 Jim Thompson wrote: > > On Wed, 30 Dec 2009 07:32:12 -0800, Fred Abse > <excretatauris(a)invalid.invalid> wrote: > > >On Tue, 29 Dec 2009 14:05:33 -0700, Jim Thompson wrote: > > > >> Take a look at this... > >> > >> www.analog-innovations.com/SED/TL431_Equivalent.pdf > > > >Same as shown in the TI data sheet > > Sheeeesh! I guess that must mean I'm right ;-) It was bound to happen, some time! ;-) -- Greed is the root of all eBay.
From: JosephKK on 1 Jan 2010 17:00 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 Please reread my question. Please answer it. Did or did not someone discuss wobbling it around the corner of the knee?
From: Jim Thompson on 1 Jan 2010 18:16
On Fri, 01 Jan 2010 14:00:13 -0800, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >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 > >Please reread my question. Please answer it. Did or did not someone >discuss wobbling it around the corner of the knee? Maybe you did. I don't remember it. It's dumb. That sort of "solution" doesn't stick in my brain ;-) ...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 | No matter how bad things are Congress can always make things worse |