Hi folks
I happen to have several ADR02 and noted in the datasheet that a Precision voltage regulator can be made using this chip plus an opamp and a mosfet.
The example with a 2N7002 mosfet gives an output for 50mA, which is a bit lower than I need, but it states that a larger mosfet can be used to get a higher output current. However, I don't know what parameter of the mosfet must be larger. If I wanted to create a regulator with say, 150mA output, how would I select the larger mosfet?
Also, the opamp used in an ADA4522, I take it because it is a zero drift type. Can a normal opamp be substituted as ADA4522 is rather expensive?
I happen to have several ADR02 and noted in the datasheet that a Precision voltage regulator can be made using this chip plus an opamp and a mosfet.
The example with a 2N7002 mosfet gives an output for 50mA, which is a bit lower than I need, but it states that a larger mosfet can be used to get a higher output current. However, I don't know what parameter of the mosfet must be larger. If I wanted to create a regulator with say, 150mA output, how would I select the larger mosfet?
Also, the opamp used in an ADA4522, I take it because it is a zero drift type. Can a normal opamp be substituted as ADA4522 is rather expensive?
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You can use any MOSFET up to millions of Ampere as theirs gate is the same. If fact no, because larger MOSFET's has larger capacity, but as a linear pass element isn't critical in this aspect.
Take into account that a MOSFET's gate needs about 3V over the source voltage, so a minimal of 8-9V you will need at the input, and some more room if notable levels of ripple are at the input.
Take into account that a MOSFET's gate needs about 3V over the source voltage, so a minimal of 8-9V you will need at the input, and some more room if notable levels of ripple are at the input.
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Hi Osvaldo. Obviously, the output is 5V, so I would need to feed the circuit at least 9V, if I understand you correctly. I would be using this regulator circuit in series with another linear regulated supply - the ADR02 circuit being used as a local regulator to something such as a DAC chip, so there shouldn't be a lot of ripple on the input.
So if I were to use a TO-220 MOSFET such as IRF610, it would work fine and be able to deliver a lot more current.
I'm only intending to use this circuit to provide power to DAC chips and some basic 74 series logic, so I don't need a lot of current.
I suppose I can build the circuit and use a socket for the opamp, then try it with a cheap opamp like the NE5534.
So if I were to use a TO-220 MOSFET such as IRF610, it would work fine and be able to deliver a lot more current.
I'm only intending to use this circuit to provide power to DAC chips and some basic 74 series logic, so I don't need a lot of current.
I suppose I can build the circuit and use a socket for the opamp, then try it with a cheap opamp like the NE5534.
Ok. I want to add, that if you have several voltages available, you can use (for example), a 12V low current for the amp, whilst the drain of the mosfet tied to, say, a ripple free but unregulated 6V higher current capable. It isn't mandatory that the mosfet's drain and the opamp supply be the same.
A variant of this scheme were used extensivelly in 486 motherboards and P1 when no need for large currents and 3.3V output was need for the ųpc. Then, the mosfet's drain was powered from the 5V line high current line whilst the regulator TL431 (or equivalent) and the gate were powered from the 12V line via a 6.8K resistor. I had used this topology several times in my own designings.
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A variant of this scheme were used extensivelly in 486 motherboards and P1 when no need for large currents and 3.3V output was need for the ųpc. Then, the mosfet's drain was powered from the 5V line high current line whilst the regulator TL431 (or equivalent) and the gate were powered from the 12V line via a 6.8K resistor. I had used this topology several times in my own designings.
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A very simple, no-cost improvement would be to supply the ref from Vout instead of Vin.
A second improvement would be to feed the opamp from Vout instead but you would need to use a depletion mode FET.
Also note that this a very slow regulator, with the 1000pF in the feedback loop, so don't expect good performance at higher frequencies. Especially with the slow opamp. This obviously is an app note for a ref for an ADC or DAC or something, but not as a dynamically stable regulator for audio loads.
Jan
A second improvement would be to feed the opamp from Vout instead but you would need to use a depletion mode FET.
Also note that this a very slow regulator, with the 1000pF in the feedback loop, so don't expect good performance at higher frequencies. Especially with the slow opamp. This obviously is an app note for a ref for an ADC or DAC or something, but not as a dynamically stable regulator for audio loads.
Jan
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I have four ADR02 and my idea was to use them to create four regulators to power the analogue and digital sides of a pair of DAC chips.
Is it a case of needing to select the right mosfet & opamp for this application or is the idea just not a good one and it would be better to simply use a good regulator such as the LT1084?
Is it a case of needing to select the right mosfet & opamp for this application or is the idea just not a good one and it would be better to simply use a good regulator such as the LT1084?
The concept is pretty universal, you need a reference, an error amp to compare the ref with (a sample of) the output and adjust the series pass device.
But the shown circuit is more a basic concept and is far from optimized.
Check out the superreg in the diyaudio store for an audio-optimized application. (It is shown as sold out but I know that Jason has a batch delivered; but anyway there's a writeup on it).
Jan
But the shown circuit is more a basic concept and is far from optimized.
Check out the superreg in the diyaudio store for an audio-optimized application. (It is shown as sold out but I know that Jason has a batch delivered; but anyway there's a writeup on it).
Jan
Cheers Jan, I shall study your articles on the superreg, but not tonight, I have a football match to watch. 🙂
The 1000pF works with 100 Ohms to form a 1.6MHz 0dB gain point. The op amp has a 3MHz unity gain crossover so the 1000pF cap doesn't reduce the circuit unity gain crossover frequency much. LTSpice roughly agrees with this. The regulator should not be particularly slow.
That might be the case, because the opamp itself has a very limited bandwidth. It is not a very good choice if you want to build a regulator for audio. But as said before this app looks more like for a constant load reference for a DAC or ADC and there it probably is sufficient. For such an app you are probably more interested in low noise and good temp stability than wide band low output impedance.
Jan
Jan
Finely balanced at half time!
So, what I'd like to do is modify the circuit to make it ideal for powering a DAC chip. I suppose the most important thing is the selection of the opamp?
So, what I'd like to do is modify the circuit to make it ideal for powering a DAC chip. I suppose the most important thing is the selection of the opamp?
Yes, a high gain fast opamp is good. And a BJT as pass devices for higher Gm than a MOSFET, and lower dropout.
Actually the reference is less critical. You don't care whether the output changes from 14.9 to 15.1 over an hour with temperature (which would be disaster for a DAC or ADC), but the ref should be low noise. In the superreg the ref has an extra RC noise filter.
Most important spec of the reg. is very low Zout over the audio frequency, there are some graphs with the diyaudio store article.
The superreg has remote sensing so you can actually regulate the voltage at the load, not at the reg output.
Also important is the PCB layout and separating sensitive tracks from high current tracks.
I am trying to compress 20 years of finetuning 😎
Jan
Actually the reference is less critical. You don't care whether the output changes from 14.9 to 15.1 over an hour with temperature (which would be disaster for a DAC or ADC), but the ref should be low noise. In the superreg the ref has an extra RC noise filter.
Most important spec of the reg. is very low Zout over the audio frequency, there are some graphs with the diyaudio store article.
The superreg has remote sensing so you can actually regulate the voltage at the load, not at the reg output.
Also important is the PCB layout and separating sensitive tracks from high current tracks.
I am trying to compress 20 years of finetuning 😎
Jan
Thanks for your help Jan, I'm very much a novice so it is much appreciated.
The simple circuit in the datasheet had me thinking it was a simple, straightforward matter to build a 5v regulator around the ADR02, but I see now that it's just not that simple.
I'm just wondering now if I should put the ADR02s to one side for a possible future use and look at a different solution to providing a nice clean 5V supply for DAC chips etc.
The simple circuit in the datasheet had me thinking it was a simple, straightforward matter to build a 5v regulator around the ADR02, but I see now that it's just not that simple.
I'm just wondering now if I should put the ADR02s to one side for a possible future use and look at a different solution to providing a nice clean 5V supply for DAC chips etc.