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How to replace LT3083 with opamp+pass transistor
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AloyseTech:

--- Quote from: David Hess on September 20, 2018, 11:13:17 pm ---
--- Quote from: Hero999 on September 20, 2018, 06:11:34 pm ---My advice is don't bother with a voltage regulator IC. Make your own using a transistor and an op-amp.
--- End quote ---

Or keep the voltage regulator IC, not necessarily the LT3083, and insert it inside the feedback loop of an operational amplifier.

--- End quote ---

Could you elaborate on this solution please?


The OPA189 is not an instrumentation amplifier AFAIK. And I don’t need a 1uA precision on the 2A range.
I want to be able to measure the current even at low voltage. I haven’t found an high side current sense amp meeting my requirements so I started simulating my own based on the OPA189 + NPN transistor.
AloyseTech:
Is this what you mean by inserting the regulator in the feedback loop of an opamp?

Zero999:
To answer your questions:
1)  Refer to the link below for more information about the op-amp+pass transistor configuration.
https://www.petervis.com/electronics%20guides/voltage-regulator-using-op-amp-and-transistor/voltage-regulator-using-op-amp-and-transistor.html

2) Voltage headroom is the drop-out voltage, i.e. what's the minimum difference between the input and output voltages.

3) Both P and N channel devices may be needed to bypass the shunt. This is because a good 5V or so between he gate and source, is required to turn a MOSFET on. Using an N-channel device alone, would mean its gate voltage would need to be considerably above the output voltage for it to turn on enough, which might not be possible, when the regulator is set to higher output voltages. Using a P-channel devices alone, would mean its gate voltage would have to go negative, when the regulator is set to output voltages, lower than its gate threshold voltage. You need to build your own analogue switch to bypass the shunt.
http://www.analog.com/en/analog-dialogue/articles/switch-and-multiplexer-design-for-hostile-environ.html

In your circuit, the op-amp is configured as an instrumentation amplifier.

Yes, that's the correct way to put an IC regulator in the feedback loop. It has the advantage of safe operating area, thermal and over-current protection, which you don't get with a plain transistor.
AloyseTech:
Thanks, I'll read these papers.

Since I have a tracking regulator as an input which is configured to be 3V higher than the output, I guess I have 3V minimum of headroom, right?

I thought an instrumentation amplifier has some interconnection between two input buffer op amp and an output opamp? I see more or less why this is could be a instumentation configuration you're right. What exactly is wong about using this kind of configuration for my application, and what would be better? In the schematic I provided the "instrumentation" opamp is not used for the precision current sensing, only for compensating the dropout. But I'm quite sure this is not the best solution and will probably not be using it anyway :)

I have some basis in analog electronic but I miss some experience and tips and tricks so thanks for your help!
David Hess:

--- Quote from: AloyseTech on September 21, 2018, 07:42:52 am ---
--- Quote from: David Hess on September 20, 2018, 11:13:17 pm ---Or keep the voltage regulator IC, not necessarily the LT3083, and insert it inside the feedback loop of an operational amplifier.
--- End quote ---

Could you elaborate on this solution please?
--- End quote ---

Below is the example I commonly use.  This sort of circuit can achieve a load regulation so good that it takes a 7 or more digit multimeter to even see it.  Some care is needed in frequency compensation if the operational amplifier is high bandwidth which might be desirable for low noise.

Essentially the integrated regulator becomes an overload protected power pass element to replace a power transistor.  The low noise reference and operational amplifier *reduce* the integrated regulator's output noise within the closed loop bandwidth all the way down to DC so that includes drift.
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