feedback and stability
in [Design]
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From: Hammy on 28 Dec 2009 19:29 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..." They also make good cheap comparators. I've used them for that purpose and it's also in the data sheet under application examples. >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
From: Jim Thompson on 28 Dec 2009 19:46 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 Once it starts to conduct it IS LINEAR... it's like a railed OpAmp. Believe me, I know, I was using them as OpAmps back in the late '70's. ...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 28 Dec 2009 20:08 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
From: Jim Thompson on 28 Dec 2009 20:24 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 -- | 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: JosephKK on 28 Dec 2009 21:36
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? |