On Monday, March 4, 2019 at 11:20:27 PM UTC-5, John Larkin wrote:
> On Mon, 4 Mar 2019 15:18:24 -0800 (PST),
> bloggs.fredbloggs.fred@gmail.com wrote:
>
> >On Monday, March 4, 2019 at 11:31:14 AM UTC-5, John Larkin wrote:
> >> On Mon, 4 Mar 2019 07:45:10 -0800 (PST),
> >> bloggs.fredbloggs.fred@gmail.com wrote:
> >>
> >> >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.
> >>
> >> C1 fixes the loop stability problem. My current version has C1=1uF,
> >> but I was surprised that it behaves well at 1 pF, which is really a
> >> placeholder for zero.
> >>
> >> https://www.dropbox.com/s/3ecb7qtn9ilgqfa/ADG_Vhi_2.jpg?dl=0
> >>
> >> Step response is clean, so it's stable.
> >>
> >> TI doesn't give a schematic of the amp internals. Some opamps bury
> >> their dominant pole deep inside, so get unstable in circuits like this
> >> where an external pole is added to the loop. But some hang one end of
> >> the comp cap on the output pin, so capacitive loads change that
> >> internal rolloff, and they tolerate c-loads. A few opamps, like
> >> LM8261, tolerate any capacitive load.
> >
> >LOL- 0.1s rise and fall times won't be exciting any instabilities.
> >
>
> I limited the DAC slew rate so as to not current limit big swings into
> 100 uF. I don't expect people to want to change the output voltage
> very fast.
>
> Any loop, no matter its rise or fall times, could oscillate.
Yours won't, you have 70o phase margin judging from fig2. of the datasheet.
Probe the top of R5 for feedback gain/phase to input:
Version 4
SHEET 1 880 680
WIRE 0 32 -64 32
WIRE 144 32 80 32
WIRE 208 32 144 32
WIRE 448 32 288 32
WIRE 144 64 144 32
WIRE -64 160 -64 32
WIRE 144 160 144 128
WIRE 208 160 144 160
WIRE 352 160 288 160
WIRE 448 160 448 32
WIRE 448 160 352 160
WIRE 144 176 144 160
WIRE -64 288 -64 240
WIRE 144 288 144 256
WIRE 144 288 -64 288
WIRE 352 288 352 240
WIRE 352 288 144 288
WIRE 448 288 448 224
WIRE 448 288 352 288
WIRE 144 336 144 288
FLAG 144 336 0
SYMBOL res 192 176 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R1
SYMATTR Value 10
SYMBOL res 336 144 R0
SYMATTR InstName R2
SYMATTR Value 200
SYMBOL res 192 48 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL res -16 48 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R4
SYMATTR Value 8k
SYMBOL res -48 144 M0
SYMATTR InstName R5
SYMATTR Value 1k
SYMBOL cap 128 64 R0
SYMATTR InstName C1
SYMATTR Value 1µ
SYMBOL cap 432 160 R0
SYMATTR InstName C2
SYMATTR Value 20µ
SYMBOL voltage 144 160 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value AC 1
TEXT 224 320 Left 2 !.ac dec 100 10 100k
Really flat feedback gain at -20dB and non existent to negligible phase shift.
>
>
> --
>
> John Larkin Highland Technology, Inc
>
> lunatic fringe electronics