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Using OpAmps as Voltage Regulators
Using OpAmps as Voltage Regulators
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Old 22nd March 2020, 12:35 PM   #11
Tfive is offline Tfive  Germany
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I didn't find the original reference, but here's one that might help:
Simulation Shows How Real Op Amps Can Dri - Maxim Integrated

Note the plots to the end of the document. Given that the capacitance is >50uF here and that the opamp has an internal open loop output impedance in the order of at least several ohms the pole that is formed by this RC network is so low in frequency that the circuit cannot oscillate. At least that's how I understood it.
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Old 22nd March 2020, 01:11 PM   #12
MorbidFractal is offline MorbidFractal
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The 1622 also has a higher output drive current than the 1611, x5, and what looks like 1/5 the GBW. If you can find the reference then... but 4-5 orders of magnitude, 1E4 - 1E5, has to be a pipe-dream. Perhaps you are refering to an application where the device drives a 10-100uF DC blocking capacitor but the circuit includes series resistance in the coupled stage.

In terms of output current into a virtual ground I think the virtual ground is referenced to AVVC/2 so +/-3.86mA. However what is drawn from the supply does not necessarily follow what is delivered to the output.


...Sorry missed your last post. I'll have a look.

Last edited by MorbidFractal; 22nd March 2020 at 01:37 PM.
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Old 22nd March 2020, 02:36 PM   #13
MorbidFractal is offline MorbidFractal
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As you can see from the reference you give the opamp still needs the isolating resistor if feedback is taken from the output of the opamp. Don't confuse Rout with the additional resistor, R3. The additional feedback only helps at low frequencies. Again don't confuse Rout with R3 but yes the additional feedback works because its contribution to loop gain rolls off before the loop hits zero.
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Old 22nd March 2020, 02:55 PM   #14
Tfive is offline Tfive  Germany
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OK, I have to admit that my math knowledge is hardly any better than what I got from college (gymnasium in Germany). So could you please explain, why the circuit in the ESS app note (http://www.esstech.com/files/4514/40...PCB_Layout.pdf - page 4) does work in the real world with no additional resistor? Also other opamps like the OPA1611 or the LME49720 seem to cope well with this config.

I'm eager to fully understand this topic!
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Old 22nd March 2020, 03:09 PM   #15
soekris is offline soekris  Denmark
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Using OpAmps as Voltage Regulators
Most opamps work just fine and stable with a large enough capacitance on its output, use a large polymer, like 1000 uF 6V3, that what I do.... To increase stability, add a small resistor before the cap, like 10 mOhm, can be a PCB trace. Good idea to sim it.
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Old 22nd March 2020, 05:27 PM   #16
MorbidFractal is offline MorbidFractal
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Ouch! Some opamps don't like it but if you slug them hard enough they submit. Shades of let's just throw dominant pole compensation out of the window by introducing a bigger dominant pole. You may even find that slew rate limiting of the output stage comes into play.

Opamps of the non esoteric kind generally use something called dominant pole compensation. It's something applied to the input stage such that irrespective of what the rest of the circuit does the AC response is dominated by that pole for all frequencies up to the zero gain crossover. Everything is nice and first order with a 90 degree phase shift up until that point. Something like the first picture. Above about 1MHz other things come into play, including pole splitting, but otherwise Cdom dominates the response.


As you have noted the intrinsic output impedance, not the added resistor, of the opamp forms an additional pole at some frequency with a load capacitor and it is possible for that to render the loop second order. Your Maxim note gives a hint of this where they add an emitter follower and bias it into Class A. That's tied in with the intrinsic emitter resistance which is set by collector current. Second picture shows the result. This time the loop goes second order and crosses zero gain with 0 degrees. When you apply feedback to that it will be unstable.

Third picture shows what these animals are trying to do. You just whack up the value of the load capacitance to a level where it, in conjunction with the intrinsic emitter/output resistance, becomes the new dominant pole and you gain back some phase margin and stability. In this case I've dumped 1000uF on my model to do it and, of course, depending on the characteristics of the actual device you use things will be subject to variation.

Faced with something like this I would be inclined to revisit the original problem and ask some questions about what solution was being sought before this answer was chosen because at face value the fourth picture is just idiot speak for I don't know stuff but if I put a big enough capacitor on this most expensive part of the circuit I cripple it but that works.

When I first posted on this forum I went a bit ape at people building semi-discrete regulators suing opamps with no local feedback and paying no overall attention to the overall loop gain. This is a similar situation. Consider your 1162. It's got a Gain Bandwidth Product of 80 Freaking MEGA HERTZ and the first thing you do is throw that in the bin.
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Old 22nd March 2020, 05:58 PM   #17
Tfive is offline Tfive  Germany
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Thanks very much for simulating that! Things just got a tad clearer for me...

I'll take a step back though and explain why we arrived at the values for the caps that were chosen as of now. So what is it that we are seeking for? Well, the ideal voltage source which has zero output impedance over the needed frequency range. As zero is not possible, let's look at a more realistic approach.

The ESS has 0dB PSRR on the AVCC pin, it's a reference voltage. So to achieve a SNR of 125dB for example the noise voltage on the AVCC pin must not exceed 3.3V/125dB which equals 1.8uV. if we say the current swing demanded by the AVCC pin is 8mA the output impedance of the regulator must no exceed 0.2mOhms or otherwise the current will modulate the voltage by more than these 1.8uV (Hope there is no error in my calculation). Even if I would use an Opamp with no capacitance at all it will not be able to have such a low Zout over the whole frequency range that we need it to operate at. Keep in mind that this is a SD DAC which modulates in the MHz region. So we quickly came to the conclusion that there's no way we would get away without a cap across the AVCC supply. So we did quite a long search to find a suitable capacitor combination that a) does not induce resonances from the package inductances and will b) have a low impedance over a wide frequency range. That's why we ended up with a 1812 10uF MLCC in parallel with a four terminal feedthrough cap from TDK, 0603 package, 4.7uF.

So what are our options now? as far as I can see it's either no cap at all, let the opamp (or regulator for that matter) do it's thing, accept it's rising Zout at higher frequencies or try to make it stable with some carefully chosen capacitor values.

I'll throw one more question in, just for the fun of it:
If I add a ferrite bead in series with the output of the opamp, that would likely make it stable too, right? because the ferrite bead will show resistive behaviour from a few hundred kHz on. Of course ideally the resistance would only rise above a frequency where the caps already show very low impedance. In reality I didn't find a bead that shows really low impedance up into the MHz range. :/
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Last edited by Tfive; 22nd March 2020 at 06:12 PM. Reason: Wrong feedthrough cap mentioned
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Old 22nd March 2020, 06:13 PM   #18
chris719 is offline chris719  United States
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Using OpAmps as Voltage Regulators
It appears to me that LT304x meets most if not all of these specifications by itself.
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Old 22nd March 2020, 07:10 PM   #19
nigelwright7557 is offline nigelwright7557  United Kingdom
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I would have gone for the more traditional regulator and then decouple each IC as required. You have to decouple the IC's anyway.
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Old 22nd March 2020, 10:24 PM   #20
Tfive is offline Tfive  Germany
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@MorbidFractal do you think that I could model this with models of the actual OPA1611 or OPA1622? I have to admit that I have zero experience with SPICE. Any recommendations on how to get started with this quickly?
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