EEVblog #567 - Precision 1A Current Source

[EEVblog]

Dave breadboards a precision 1A current source circuit and finds that even the application reference circuit doesn't work as expected. Showing that you can't just blindly trust that datasheet application circuit will work first go.
LTC6655 http://cds.linear.com/docs/en/datasheet/6655fc.pdf

[pickle9000]

This is my favorite type of video. Very enjoyable.

[Wytnucls]

Now the 1000$ question is, what does Dave need a precision 1A source for?

[robrenz]

Maybe testing the µcurrents

[lewis]

Try a big electrolytic in parallel with the input bypass capacitor, about 100-470uF should do it. Inductance of the leads from your PSU to the breadboard can upset loop stability and 100nF/4.7uF (or whatever it was) is nowhere near enough to compensate with that much series inductance at any significant load.

Try substituting the ceramic caps for electrolytics, or add a very small series resistance (0.5-1R) in series with the ceramics. Sometimes their ESR is TOO LOW, forms an underdamped LC tank circuit with the PCB trace/wire/package/whatever inductance, causing ringing on transients and oscillation. Some stuff here: http://cds.linear.com/docs/en/application-note/an88f.pdf.

10-22pF between force and sense (very close to the IC) might help too, as may a 22R resistor in series with the pass transistor base.

[Rufus]

The emitter follower doesn't increase the loop gain but it does lower the output impedance by a factor of 100 or whatever so a capacitor stuck on the output needs to be 100 (or whatever) times bigger for the same roll off. A diode in series isn't helping either (the capacitor isn't there if the diode is not conducting so it might make it prone to low frequency motor boating type oscillation).

I think you will have to stick enough capacitance on the chip output to make it stable and then maybe some more on the emitter to improve output noise and high frequency output impedance.

Now the 1000$ question is, what does Dave need a precision 1A source for?

A uohm meter adapter probably.

[Frank]

In my old job we had a precision current source made with the Analog Devices's AD584 reference, using a topology very similar to this one. 
 

Maybe testing the µcurrents

+1 for the new test jig for the µcurrents 

[rolandpenplotter]

Now the 1000$ question is, what does Dave need a precision 1A source for?

To power the circuit that powers the relay that powers his editing computer that creates the "TV station teardown" video!

[Marco]

If the sense input was high impedance this IC would be a lot nicer for this application.

The emitter follower doesn't increase the loop gain but it does lower the output impedance by a factor of 100 or whatever so a capacitor stuck on the output needs to be 100 (or whatever) times bigger for the same roll off.

In a different part they recommend 2.7 uF minimum for normal use, so the 4.7 uF for use with the 2n2222 in the datasheet was just hopelessly too small then? Could you just put an additional capacitor across the base emitter junction to stabilize the loop instead? Regardless, AC impedance of the current source is going to be pretty poor when all is said and done ... the whole concept of a high side current source with a NPN output transistor is a bit alien to me.

[Rerouter]

its a glorified op amp, (that constantly sinks 2mA on its sense line  ) so perhaps add a 125 ohm resistance on the sense line, a 1K resistance between the VF pin and the transistor base, and a 1nF cap between the transistor base and the VS pin (after the resistor)

this will give mean you would need to change to a 1.5 ohm shunt, but give it a shot.

[TheGreatGooglyMoogly]

Say Dave,

I think the culprit may lay in the ESR of the output cap (just a wild guess based on the video) - the ESR is too low.
I would try using a capacitor with a 0.5 - 2.0-ohm ESR -or- use a small resistor in series with the ceramic.
I've run into stability issues before while working on regulators and references used in Nano-positioning controllers. A quick peek at this data sheet in the bypass section will give you an idea what I am on about.

http://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDF (page 9)

Great video as always! (I hate that regulator also :p )


EDIT: the data sheet for the LT6655 claims use a 0.1-ohm or less ESR cap. So my first guess is technically wrong, but I have my doubts still. The behavior looks exactly like a mis-bypassed NPC1117 regulator. Is it possible the breadboard leads have too much ESR or ESL? Or that this is one of those chips that never works? Puzzling indeed.

[Dave]

This video comes to mind. An LM399 is a bit of an overkill here, but you may get some ideas for your circuit.

[EEVblog]

this will give mean you would need to change to a 1.5 ohm shunt, but give it a shot.

I've go a couple of kick-arse 1.25ohm 4 terminal shunts on the way from Vishay, so need to stick with this value.

[Rerouter]

then try the base resistance as a starting point, i can understand when things are already ordered,

other than that the only solutions i can think of is to
A. find a better chip
B. measure the voltage on the shunt differentially through a low passed op amp with a slew rate limiting cap and drive the VS pin,  as opposed to floating the ground :/ yes i agree it may hurt absolute accuracy

[Marco]

This video comes to mind. An LM399 is a bit of an overkill here, but you may get some ideas for your circuit.

Seems like it could work fine with the LTC6655.

Could someone explain the trarlington to me, why is the extreme beta necessary?

[Rufus]

In a different part they recommend 2.7 uF minimum for normal use, so the 4.7 uF for use with the 2n2222 in the datasheet was just hopelessly too small then?

The output impedance (open loop) and the output capacitor need to form a dominant pole which reduces the open loop gain to less than unity before other circuit phase shifts add up to 90 degrees and the -ve feedback becomes +ve. The output impedance is likely quite low which is why you need a big capacitor with low ESR. It is a crappy way to form a dominant pole except for voltage regulators and references where a big capacitor on the output is a good thing reducing noise and improving high frequency transient response. The emitter follower still has some output impedance (re/hfe ?) so how hopelessly too small depends on how that compares with the chip output impedance.

[Rufus]

Could someone explain the trarlington to me, why is the extreme beta necessary?

Because the circuit regulates current in the sense resistor while the load gets that current less any base drive current for the pass transistor. With a tralington? gain of 1 million the load gets 1uA less than it should.

[Jay_Diddy_B]

Hi Dave,

This circuit will oscillate even in the simplest simulation:



The 4.7u capacitor current flows through the output of the op-amp A2.

If I remove the capacitor and put a 10uH inductor in series with the load, the circuit will also oscillate:



It doesn't matter if the inductance is in series with the supply or the load resistor, because they are in series, I have a two terminal current limiter. This circuit will also oscillate:



Adding a RC damping network will stabilize the circuit for inductance in the load circuit:



To stabilize for input inductance, an electrolytic should be added on the input:




I have attached a zip file with the zip models if you want to play along. There is no LTspice model for the LTC6655.

Jay_Diddy_B

[ScribblesOnNapkins]

This is crying out to be constructed dead bug style on a sheet of copper clad.

[EEVblog]

This circuit will oscillate even in the simplest simulation:

Thanks, but I haven't even gotten there yet, the basic app note circuit oscillates without the ground biasing opamp.
I haven't tried the PNP solution yet either.

[owiecc]

Regarding your initial circuit. I would remove the diode and the opamp from the circuit if you can have the output floating. Then you need to move the sense resistor to the current return path. Your LT is then ground referenced.

[Sarowie]

What I do not like about that "typical application circuit" is, that  the LTC6655 can source and sink.
With the NPN Transitors Base-Emiter Junction in Series to the forced output, that doesn't make sense:
If the sense pin is ever so slightly above the set point, the LTC6655 will try to sink current over the forced out put. But with the Base-Emiter junction in series, the LTC6655 is just not able to do so.
The forced output will then go more and more toward the Ground, until the sense pins becomes to low due to the load (which will be at least 2mA from the sense voltage divider). Then the forced output has then to climp back up from whatever voltage it was to VRef + VBE (1.25V+0.6V = 1.85V). As the NPN Transistor has an finite speed, it will ever so slightly over shoot and guess what: The LTC6655 forced output tries again to sink current, sucks out all the charge out of the base-emiter junction and goes toward GND. After some time, the sense input becomes to low and the error amp starts to increase the forced output voltage and will again overshoot ever so slightly.

I do not get, why they recommend a circuit which has an issue with the sink capability of the voltage reference.
Beeing able to source and sink is the main difference between a voltage regulator and a voltage reference.

[jesuscf]

Hi Dave,

This circuit will oscillate even in the simplest simulation:
...
Jay_Diddy_B

Jay_Diddy_B,

I think the oscillations you observe in the simulations are not caused  by the circuit topology itself but by the choice of integration method. The problem shows when solving the differential equations for inductors and capacitors if the trapezoidal integration method is used.   In most SPICE based simulators the trapezoidal method is selected by default. I got rid of the oscillations in the first circuit you posted by changing the integration method from trapezoidal to Gear.

As for the oscillations in Dave's circuit, they have to be caused by some kind of positive feedback.  I would check the main DC power source first and then check what is the effect of the typical 50pF capacitance between breadboard pins.  I can not see clearly from the video, but adjacent breadboard pins may be a problem in some circuits when one is an input and the other is an output.

Jesus

[riconette]

imho the scopes display "error" is the result of the persistence feature.
i've fed a scrambled signal in my scope (2072 fw 00.01.01, hw 1.0, fully hacked) and observed the same behaviour. the life waveform shows (persistence-?)artifacts of old waveforms when pressing <stop>. they vanish when changing the timebase of the stopped signal (forth->back->forth for example), OR when changing the waveform display from vectors to dots (and back).
so everytime the scope has to repaint the screen it resorts to the saved waveform data and it dumps the persistence.

might it be the persistence buffer is in fact the video memory where everytime before a new waveform is painted the video-mem content is dimmed by x% ?

whatever… it's no show stopper

[nitro2k01]

Maybe testing the µcurrents

Or maybe he's testing the new and completely revolutionary no-prefix-current, which can measure currents in the range 1 mA - 10 A. Just like with the µCurrent, the user can plug in a test current on one side and get a voltage reading on the two other  terminals, with the scale 1 V/A. I hear the no-prefix-current features a passive design, which has a heavily reduced complexity compared to the µCurrent and doesn't even require a battery.