On Saturday, March 2, 2019 at 3:13:18 PM UTC-5, John Larkin wrote:> On Sat, 2 Mar 2019 11:38:17 -0800 (PST), > bloggs.fredbloggs.fred@gmail.com wrote: > > >On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, John Larkin wrote: > >> I need some programmable power supply rails, maybe -5 to +40 volts or > >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a > >> decomp, and it will be driving a big capacitive load, so there is an > >> oscillation hazard. One of my guys got the model from TI and kluged it > >> into LT Spice. The symbol is ugly but it seems to work. We don't know > >> how to make it look like a real opamp. > >> > >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 > >> > >> This looks fine, even without C1. Maybe I'll include C1 on the pcb > >> layout just to have another knob to turn. > >> > >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish > >> current limit, but I'd have to tune the pcb thermal resistance somehow > >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a > >> tweakable amount of gap-pad under the board, or above the amp? I > >> suppose I could selectively drill out vias on the rev A board, but > >> that's really ugly. > >> > >> I want a thermal conductivity trimpot. Or DAC. > > > >What's wrong with using FLAG? > > For what? We'll probably not connect it in real life. I guess it could > light up a red LED or something.Didn't you say you wanted to know when it went into thermal cutoff so you could reduce the output current? That's for what.> > > > >Is that 1p for C1 right? You have stray that's 10x that. > > That's a placeholder for "maybe we don't need a cap there." I said > that. > > Tweaking a bit more, it looks like C1 is a good idea: 1 uF.I'm not sure it will work so well since the cap shunts the feedback into the low impedance output of the opamp, when it cuts in.> > The opamp also (according to Spice) oscillates when it current limits. > That will happen when the dac voltage steps and a lot of current is > briefly forced into the load cap. That's probably not a big deal, but > we can slew-rate limit the dac output, in hardware or software. > > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
OPA552 as power regulator
Started by ●March 2, 2019
Reply by ●March 3, 20192019-03-03
Reply by ●March 3, 20192019-03-03
On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, John Larkin wrote:> I need some programmable power supply rails, maybe -5 to +40 volts or > so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a > decomp, and it will be driving a big capacitive load, so there is an > oscillation hazard. One of my guys got the model from TI and kluged it > into LT Spice. The symbol is ugly but it seems to work. We don't know > how to make it look like a real opamp. > > https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 > > This looks fine, even without C1. Maybe I'll include C1 on the pcb > layout just to have another knob to turn. > > I'd like to use the OPA552 thermal cutoff as a secondary, slowish > current limit, but I'd have to tune the pcb thermal resistance somehow > to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a > tweakable amount of gap-pad under the board, or above the amp? I > suppose I could selectively drill out vias on the rev A board, but > that's really ugly. > > I want a thermal conductivity trimpot. Or DAC.Why not the time tested complementary npn-pnp on the output with the base to emitter resistor that kicks them into conduction when the current surges above say 50mA, and the OA handles the small stuff. Then you offload the power into components that presumably are packaged to handle it. Actually the 20R you have there now is about right.> > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Reply by ●March 3, 20192019-03-03
On Sun, 3 Mar 2019 09:20:23 -0800 (PST), bloggs.fredbloggs.fred@gmail.com wrote:>On Saturday, March 2, 2019 at 3:13:18 PM UTC-5, John Larkin wrote: >> On Sat, 2 Mar 2019 11:38:17 -0800 (PST), >> bloggs.fredbloggs.fred@gmail.com wrote: >> >> >On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, John Larkin wrote: >> >> I need some programmable power supply rails, maybe -5 to +40 volts or >> >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a >> >> decomp, and it will be driving a big capacitive load, so there is an >> >> oscillation hazard. One of my guys got the model from TI and kluged it >> >> into LT Spice. The symbol is ugly but it seems to work. We don't know >> >> how to make it look like a real opamp. >> >> >> >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 >> >> >> >> This looks fine, even without C1. Maybe I'll include C1 on the pcb >> >> layout just to have another knob to turn. >> >> >> >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish >> >> current limit, but I'd have to tune the pcb thermal resistance somehow >> >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a >> >> tweakable amount of gap-pad under the board, or above the amp? I >> >> suppose I could selectively drill out vias on the rev A board, but >> >> that's really ugly. >> >> >> >> I want a thermal conductivity trimpot. Or DAC. >> > >> >What's wrong with using FLAG? >> >> For what? We'll probably not connect it in real life. I guess it could >> light up a red LED or something. > >Didn't you say you wanted to know when it went into thermal cutoff so you could reduce the output current? That's for what. > >> >> > >> >Is that 1p for C1 right? You have stray that's 10x that. >> >> That's a placeholder for "maybe we don't need a cap there." I said >> that. >> >> Tweaking a bit more, it looks like C1 is a good idea: 1 uF. > >I'm not sure it will work so well since the cap shunts the feedback into the low impedance output of the opamp, when it cuts in. >The cap closes the feedback loop local to the opamp, at high frequencies, so the extra pole of the big output cap doen't make the closed loop unstable. That's pretty common. Win discusses that config in AoE3 p 264. I tweaked that concept a bit to let me use the undercomp amp that we have in stock; it could oscillate if the HF feedback had unity gain. It simulates nicely. Spice sims are usually spot on for simple stuff like this, but I didn't expect the oscillation in slewing current limit. Maybe it's a limit-cycle thing. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●March 3, 20192019-03-03
On Sunday, March 3, 2019 at 1:42:31 PM UTC-5, John Larkin wrote:> On Sun, 3 Mar 2019 09:20:23 -0800 (PST), > bloggs.fredbloggs.fred@gmail.com wrote: > > >On Saturday, March 2, 2019 at 3:13:18 PM UTC-5, John Larkin wrote: > >> On Sat, 2 Mar 2019 11:38:17 -0800 (PST), > >> bloggs.fredbloggs.fred@gmail.com wrote: > >> > >> >On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, John Larkin wrote: > >> >> I need some programmable power supply rails, maybe -5 to +40 volts or > >> >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a > >> >> decomp, and it will be driving a big capacitive load, so there is an > >> >> oscillation hazard. One of my guys got the model from TI and kluged it > >> >> into LT Spice. The symbol is ugly but it seems to work. We don't know > >> >> how to make it look like a real opamp. > >> >> > >> >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 > >> >> > >> >> This looks fine, even without C1. Maybe I'll include C1 on the pcb > >> >> layout just to have another knob to turn. > >> >> > >> >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish > >> >> current limit, but I'd have to tune the pcb thermal resistance somehow > >> >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a > >> >> tweakable amount of gap-pad under the board, or above the amp? I > >> >> suppose I could selectively drill out vias on the rev A board, but > >> >> that's really ugly. > >> >> > >> >> I want a thermal conductivity trimpot. Or DAC. > >> > > >> >What's wrong with using FLAG? > >> > >> For what? We'll probably not connect it in real life. I guess it could > >> light up a red LED or something. > > > >Didn't you say you wanted to know when it went into thermal cutoff so you could reduce the output current? That's for what. > > > >> > >> > > >> >Is that 1p for C1 right? You have stray that's 10x that. > >> > >> That's a placeholder for "maybe we don't need a cap there." I said > >> that. > >> > >> Tweaking a bit more, it looks like C1 is a good idea: 1 uF. > > > >I'm not sure it will work so well since the cap shunts the feedback into the low impedance output of the opamp, when it cuts in. > > > > The cap closes the feedback loop local to the opamp, at high > frequencies, so the extra pole of the big output cap doen't make the > closed loop unstable. That's pretty common. Win discusses that config > in AoE3 p 264.He does? The feedback resistor from the output usually joins to the OA IN(-) and not the inner feedback R+C zero. And you're not compensating for the OA since your gain is large enough. You're compensating for that dominant pole on the output.> I tweaked that concept a bit to let me use the > undercomp amp that we have in stock; it could oscillate if the HF > feedback had unity gain. > > It simulates nicely. Spice sims are usually spot on for simple stuff > like this, but I didn't expect the oscillation in slewing current > limit. Maybe it's a limit-cycle thing.I wouldn't believe the sim. Are you using the TI model of the 552?> > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Reply by ●March 3, 20192019-03-03
On Sun, 3 Mar 2019 11:46:45 -0800 (PST), bloggs.fredbloggs.fred@gmail.com wrote:>On Sunday, March 3, 2019 at 1:42:31 PM UTC-5, John Larkin wrote: >> On Sun, 3 Mar 2019 09:20:23 -0800 (PST), >> bloggs.fredbloggs.fred@gmail.com wrote: >> >> >On Saturday, March 2, 2019 at 3:13:18 PM UTC-5, John Larkin wrote: >> >> On Sat, 2 Mar 2019 11:38:17 -0800 (PST), >> >> bloggs.fredbloggs.fred@gmail.com wrote: >> >> >> >> >On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, John Larkin wrote: >> >> >> I need some programmable power supply rails, maybe -5 to +40 volts or >> >> >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a >> >> >> decomp, and it will be driving a big capacitive load, so there is an >> >> >> oscillation hazard. One of my guys got the model from TI and kluged it >> >> >> into LT Spice. The symbol is ugly but it seems to work. We don't know >> >> >> how to make it look like a real opamp. >> >> >> >> >> >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 >> >> >> >> >> >> This looks fine, even without C1. Maybe I'll include C1 on the pcb >> >> >> layout just to have another knob to turn. >> >> >> >> >> >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish >> >> >> current limit, but I'd have to tune the pcb thermal resistance somehow >> >> >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a >> >> >> tweakable amount of gap-pad under the board, or above the amp? I >> >> >> suppose I could selectively drill out vias on the rev A board, but >> >> >> that's really ugly. >> >> >> >> >> >> I want a thermal conductivity trimpot. Or DAC. >> >> > >> >> >What's wrong with using FLAG? >> >> >> >> For what? We'll probably not connect it in real life. I guess it could >> >> light up a red LED or something. >> > >> >Didn't you say you wanted to know when it went into thermal cutoff so you could reduce the output current? That's for what. >> > >> >> >> >> > >> >> >Is that 1p for C1 right? You have stray that's 10x that. >> >> >> >> That's a placeholder for "maybe we don't need a cap there." I said >> >> that. >> >> >> >> Tweaking a bit more, it looks like C1 is a good idea: 1 uF. >> > >> >I'm not sure it will work so well since the cap shunts the feedback into the low impedance output of the opamp, when it cuts in. >> > >> >> The cap closes the feedback loop local to the opamp, at high >> frequencies, so the extra pole of the big output cap doen't make the >> closed loop unstable. That's pretty common. Win discusses that config >> in AoE3 p 264. > >He does? The feedback resistor from the output usually joins to the OA IN(-) and not the inner feedback R+C zero.As noted, the OPA552 is not unity-gain stable, so it makes sense to jam in the fast feedback before the gain-set divider.>And you're not compensating for the OA since your gain is large enough. You're compensating for that dominant pole on the output.That's just a lot of words. I designed it to work, and it does.> >> I tweaked that concept a bit to let me use the >> undercomp amp that we have in stock; it could oscillate if the HF >> feedback had unity gain. >> >> It simulates nicely. Spice sims are usually spot on for simple stuff >> like this, but I didn't expect the oscillation in slewing current >> limit. Maybe it's a limit-cycle thing. > >I wouldn't believe the sim. Are you using the TI model of the 552?Yes. One of my kids imported it into LT Spice for me. He used the auto-generated symbol for the opamp, and that's ugly. I've done this sort of opamp rail supply many times. It always works. Actually, I usually do the compensation and parts values in my head, and that always works. All you need is a big safe distance between the poles, and a giant output cap to hold up the load. We often need goofy-voltage bypassed rails, more for Vcm references and such than actual power. This topology works fine. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●March 3, 20192019-03-03
On Sunday, March 3, 2019 at 3:43:31 PM UTC-5, John Larkin wrote:> On Sun, 3 Mar 2019 11:46:45 -0800 (PST), > bloggs.fredbloggs.fred@gmail.com wrote: > > >On Sunday, March 3, 2019 at 1:42:31 PM UTC-5, John Larkin wrote: > >> On Sun, 3 Mar 2019 09:20:23 -0800 (PST), > >> bloggs.fredbloggs.fred@gmail.com wrote: > >> > >> >On Saturday, March 2, 2019 at 3:13:18 PM UTC-5, John Larkin wrote: > >> >> On Sat, 2 Mar 2019 11:38:17 -0800 (PST), > >> >> bloggs.fredbloggs.fred@gmail.com wrote: > >> >> > >> >> >On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, John Larkin wrote: > >> >> >> I need some programmable power supply rails, maybe -5 to +40 volts or > >> >> >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a > >> >> >> decomp, and it will be driving a big capacitive load, so there is an > >> >> >> oscillation hazard. One of my guys got the model from TI and kluged it > >> >> >> into LT Spice. The symbol is ugly but it seems to work. We don't know > >> >> >> how to make it look like a real opamp. > >> >> >> > >> >> >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 > >> >> >> > >> >> >> This looks fine, even without C1. Maybe I'll include C1 on the pcb > >> >> >> layout just to have another knob to turn. > >> >> >> > >> >> >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish > >> >> >> current limit, but I'd have to tune the pcb thermal resistance somehow > >> >> >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a > >> >> >> tweakable amount of gap-pad under the board, or above the amp? I > >> >> >> suppose I could selectively drill out vias on the rev A board, but > >> >> >> that's really ugly. > >> >> >> > >> >> >> I want a thermal conductivity trimpot. Or DAC. > >> >> > > >> >> >What's wrong with using FLAG? > >> >> > >> >> For what? We'll probably not connect it in real life. I guess it could > >> >> light up a red LED or something. > >> > > >> >Didn't you say you wanted to know when it went into thermal cutoff so you could reduce the output current? That's for what. > >> > > >> >> > >> >> > > >> >> >Is that 1p for C1 right? You have stray that's 10x that. > >> >> > >> >> That's a placeholder for "maybe we don't need a cap there." I said > >> >> that. > >> >> > >> >> Tweaking a bit more, it looks like C1 is a good idea: 1 uF. > >> > > >> >I'm not sure it will work so well since the cap shunts the feedback into the low impedance output of the opamp, when it cuts in. > >> > > >> > >> The cap closes the feedback loop local to the opamp, at high > >> frequencies, so the extra pole of the big output cap doen't make the > >> closed loop unstable. That's pretty common. Win discusses that config > >> in AoE3 p 264. > > > >He does? The feedback resistor from the output usually joins to the OA IN(-) and not the inner feedback R+C zero. > > As noted, the OPA552 is not unity-gain stable, so it makes sense to > jam in the fast feedback before the gain-set divider. > > >And you're not compensating for the OA since your gain is large enough. You're compensating for that dominant pole on the output. > > That's just a lot of words. I designed it to work, and it does.That 20 + 20 u alone breaks at 2500 Hz, that's not a lot of words, and the output resistance of the OA probably makes it 1000Hz. The OA actually has a pretty clean phase/frequency, looks like a perfect integrator out to 1MHz before the higher frequency internals start cutting in. Did you run a gain/phase on the TI model to see if it resembles fig 2 of the datasheet?> > > > >> I tweaked that concept a bit to let me use the > >> undercomp amp that we have in stock; it could oscillate if the HF > >> feedback had unity gain. > >> > >> It simulates nicely. Spice sims are usually spot on for simple stuff > >> like this, but I didn't expect the oscillation in slewing current > >> limit. Maybe it's a limit-cycle thing. > > > >I wouldn't believe the sim. Are you using the TI model of the 552? > > Yes. One of my kids imported it into LT Spice for me. He used the > auto-generated symbol for the opamp, and that's ugly. > > I've done this sort of opamp rail supply many times. It always works. > Actually, I usually do the compensation and parts values in my head, > and that always works. All you need is a big safe distance between the > poles, and a giant output cap to hold up the load. > > We often need goofy-voltage bypassed rails, more for Vcm references > and such than actual power. This topology works fine. > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Reply by ●March 3, 20192019-03-03
On Sun, 3 Mar 2019 14:05:58 -0800 (PST), bloggs.fredbloggs.fred@gmail.com wrote:>On Sunday, March 3, 2019 at 3:43:31 PM UTC-5, John Larkin wrote: >> On Sun, 3 Mar 2019 11:46:45 -0800 (PST), >> bloggs.fredbloggs.fred@gmail.com wrote: >> >> >On Sunday, March 3, 2019 at 1:42:31 PM UTC-5, John Larkin wrote: >> >> On Sun, 3 Mar 2019 09:20:23 -0800 (PST), >> >> bloggs.fredbloggs.fred@gmail.com wrote: >> >> >> >> >On Saturday, March 2, 2019 at 3:13:18 PM UTC-5, John Larkin wrote: >> >> >> On Sat, 2 Mar 2019 11:38:17 -0800 (PST), >> >> >> bloggs.fredbloggs.fred@gmail.com wrote: >> >> >> >> >> >> >On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, John Larkin wrote: >> >> >> >> I need some programmable power supply rails, maybe -5 to +40 volts or >> >> >> >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a >> >> >> >> decomp, and it will be driving a big capacitive load, so there is an >> >> >> >> oscillation hazard. One of my guys got the model from TI and kluged it >> >> >> >> into LT Spice. The symbol is ugly but it seems to work. We don't know >> >> >> >> how to make it look like a real opamp. >> >> >> >> >> >> >> >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 >> >> >> >> >> >> >> >> This looks fine, even without C1. Maybe I'll include C1 on the pcb >> >> >> >> layout just to have another knob to turn. >> >> >> >> >> >> >> >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish >> >> >> >> current limit, but I'd have to tune the pcb thermal resistance somehow >> >> >> >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a >> >> >> >> tweakable amount of gap-pad under the board, or above the amp? I >> >> >> >> suppose I could selectively drill out vias on the rev A board, but >> >> >> >> that's really ugly. >> >> >> >> >> >> >> >> I want a thermal conductivity trimpot. Or DAC. >> >> >> > >> >> >> >What's wrong with using FLAG? >> >> >> >> >> >> For what? We'll probably not connect it in real life. I guess it could >> >> >> light up a red LED or something. >> >> > >> >> >Didn't you say you wanted to know when it went into thermal cutoff so you could reduce the output current? That's for what. >> >> > >> >> >> >> >> >> > >> >> >> >Is that 1p for C1 right? You have stray that's 10x that. >> >> >> >> >> >> That's a placeholder for "maybe we don't need a cap there." I said >> >> >> that. >> >> >> >> >> >> Tweaking a bit more, it looks like C1 is a good idea: 1 uF. >> >> > >> >> >I'm not sure it will work so well since the cap shunts the feedback into the low impedance output of the opamp, when it cuts in. >> >> > >> >> >> >> The cap closes the feedback loop local to the opamp, at high >> >> frequencies, so the extra pole of the big output cap doen't make the >> >> closed loop unstable. That's pretty common. Win discusses that config >> >> in AoE3 p 264. >> > >> >He does? The feedback resistor from the output usually joins to the OA IN(-) and not the inner feedback R+C zero. >> >> As noted, the OPA552 is not unity-gain stable, so it makes sense to >> jam in the fast feedback before the gain-set divider. >> >> >And you're not compensating for the OA since your gain is large enough. You're compensating for that dominant pole on the output. >> >> That's just a lot of words. I designed it to work, and it does. > >That 20 + 20 u alone breaks at 2500 Hz,radians> that's not a lot of words, and the output resistance of the OA probably makes it 1000Hz. The OA actually has a pretty clean phase/frequency, looks like a perfect integrator out to 1MHz before the higher frequency internals start cutting in. Did you run a gain/phase on the TI model to see if it resembles fig 2 of the datasheet?My guy made an inverting -1 gain amp in LT Spice, and the frequency response peaked right where it should have. The model looks OK. If anything is wrong with the loop comp, which is unlikely, I could always change a couple part values. If the amp or the circuit topology or the thermals were wrong, that would be a lot more serious. We always try to sell rev A, but an ECO or two is no big deal. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●March 4, 20192019-03-04
On Sunday, 3 March 2019 13:52:09 UTC, Winfield Hill wrote:> tabbypurr wrote... > >On Sunday, 3 March 2019 02:55:54 UTC, Winfield Hill wrote: > >> tabbypurr wrote... > > > >> > Maybe there's room for a biz that buys untraced parts > >> > & tests & certifies them. > >> > >> I've had surprising good luck finding hidden stock of > >> un-obtainium parts in Asia, searching Alibaba sellers. > >> Put up a part number and ask for quotes on 250 pieces. > > > > Yes, lots is gettable that way. Could test & certify > > replace manufacturer traceability sometimes? Not for > > milspec normally ... > > I'll give an example, I needed a certain 600V MOSFET > that had been long been discontinued, placed a request > for 250 pieces on AliExpress. Got quote at $0.38 each, > ordered with DHL shipping. The parts quickly arrived. > Right packaging, right labeling. Sets of capacitance > measurements showed the part's unique values. Would > a counterfeiter go to the relabeling trouble for a $95 > order? Did he have the time? How could he spoof the > physical measurements? One has to ask, how are the > parts not going to be exactly what they appear to be? > > I'm amazed these parts hid for years, without Google > revealing any sign that they existed, but they indeed > had hidden, and the Alibaba RFQ system turned them up. > > As a followup, the instruments made with these parts > worked perfectly.The most common problem with parts from unknown sources in China is when they turn out to be salvaged parts remarked as something else. With your fets you could determine they were what they claimed, that's not always so easy. But yes, if you don't need traceability or a trustworthy supplier you can get a lot for a little. NT
Reply by ●March 4, 20192019-03-04
On Sunday, March 3, 2019 at 5:33:23 PM UTC-5, John Larkin wrote:> On Sun, 3 Mar 2019 14:05:58 -0800 (PST), > bloggs.fredbloggs.fred@gmail.com wrote: > > >On Sunday, March 3, 2019 at 3:43:31 PM UTC-5, John Larkin wrote: > >> On Sun, 3 Mar 2019 11:46:45 -0800 (PST), > >> bloggs.fredbloggs.fred@gmail.com wrote: > >> > >> >On Sunday, March 3, 2019 at 1:42:31 PM UTC-5, John Larkin wrote: > >> >> On Sun, 3 Mar 2019 09:20:23 -0800 (PST), > >> >> bloggs.fredbloggs.fred@gmail.com wrote: > >> >> > >> >> >On Saturday, March 2, 2019 at 3:13:18 PM UTC-5, John Larkin wrote: > >> >> >> On Sat, 2 Mar 2019 11:38:17 -0800 (PST), > >> >> >> bloggs.fredbloggs.fred@gmail.com wrote: > >> >> >> > >> >> >> >On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, John Larkin wrote: > >> >> >> >> I need some programmable power supply rails, maybe -5 to +40 volts or > >> >> >> >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a > >> >> >> >> decomp, and it will be driving a big capacitive load, so there is an > >> >> >> >> oscillation hazard. One of my guys got the model from TI and kluged it > >> >> >> >> into LT Spice. The symbol is ugly but it seems to work. We don't know > >> >> >> >> how to make it look like a real opamp. > >> >> >> >> > >> >> >> >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 > >> >> >> >> > >> >> >> >> This looks fine, even without C1. Maybe I'll include C1 on the pcb > >> >> >> >> layout just to have another knob to turn. > >> >> >> >> > >> >> >> >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish > >> >> >> >> current limit, but I'd have to tune the pcb thermal resistance somehow > >> >> >> >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a > >> >> >> >> tweakable amount of gap-pad under the board, or above the amp? I > >> >> >> >> suppose I could selectively drill out vias on the rev A board, but > >> >> >> >> that's really ugly. > >> >> >> >> > >> >> >> >> I want a thermal conductivity trimpot. Or DAC. > >> >> >> > > >> >> >> >What's wrong with using FLAG? > >> >> >> > >> >> >> For what? We'll probably not connect it in real life. I guess it could > >> >> >> light up a red LED or something. > >> >> > > >> >> >Didn't you say you wanted to know when it went into thermal cutoff so you could reduce the output current? That's for what. > >> >> > > >> >> >> > >> >> >> > > >> >> >> >Is that 1p for C1 right? You have stray that's 10x that. > >> >> >> > >> >> >> That's a placeholder for "maybe we don't need a cap there." I said > >> >> >> that. > >> >> >> > >> >> >> Tweaking a bit more, it looks like C1 is a good idea: 1 uF. > >> >> > > >> >> >I'm not sure it will work so well since the cap shunts the feedback into the low impedance output of the opamp, when it cuts in. > >> >> > > >> >> > >> >> The cap closes the feedback loop local to the opamp, at high > >> >> frequencies, so the extra pole of the big output cap doen't make the > >> >> closed loop unstable. That's pretty common. Win discusses that config > >> >> in AoE3 p 264. > >> > > >> >He does? The feedback resistor from the output usually joins to the OA IN(-) and not the inner feedback R+C zero. > >> > >> As noted, the OPA552 is not unity-gain stable, so it makes sense to > >> jam in the fast feedback before the gain-set divider. > >> > >> >And you're not compensating for the OA since your gain is large enough. You're compensating for that dominant pole on the output. > >> > >> That's just a lot of words. I designed it to work, and it does. > > > >That 20 + 20 u alone breaks at 2500 Hz, > > radians > > > that's not a lot of words, and the output resistance of the OA probably makes it 1000Hz. The OA actually has a pretty clean phase/frequency, looks like a perfect integrator out to 1MHz before the higher frequency internals start cutting in. Did you run a gain/phase on the TI model to see if it resembles fig 2 of the datasheet? > > My guy made an inverting -1 gain amp in LT Spice, and the frequency > response peaked right where it should have. The model looks OK.Something isn't right because if you don't compensate for that low frequency output pole, the entire sense of the amplifier is inverted. Does LTSpice do a noise gain analysis? You should run it.> > If anything is wrong with the loop comp, which is unlikely, I could > always change a couple part values. If the amp or the circuit topology > or the thermals were wrong, that would be a lot more serious. We > always try to sell rev A, but an ECO or two is no big deal. > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Reply by ●March 4, 20192019-03-04
On Sat, 2 Mar 2019 12:26:31 -0800 (PST), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>l�rdag den 2. marts 2019 kl. 19.25.29 UTC+1 skrev John Larkin: >> I need some programmable power supply rails, maybe -5 to +40 volts or >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a >> decomp, and it will be driving a big capacitive load, so there is an >> oscillation hazard. One of my guys got the model from TI and kluged it >> into LT Spice. The symbol is ugly but it seems to work. We don't know >> how to make it look like a real opamp. >> >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0 >> >> This looks fine, even without C1. Maybe I'll include C1 on the pcb >> layout just to have another knob to turn. >> >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish >> current limit, but I'd have to tune the pcb thermal resistance somehow >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a >> tweakable amount of gap-pad under the board, or above the amp? I >> suppose I could selectively drill out vias on the rev A board, but >> that's really ugly. >> >> I want a thermal conductivity trimpot. Or DAC. >> > >opa547 ?I could make the current limit a function of the supply voltage, or of the programmed output voltage. https://www.dropbox.com/s/j95x1n2jrwo7oda/OPA547_Ilims.JPG?dl=0 I could drive it from a DAC, but that would take more parts, and code. -- John Larkin Highland Technology, Inc lunatic fringe electronics
