Implementing adjustable Slew rate conrol

[lukaev3]

Hello EEV Forum


I am currently working on an electronic Load. I have figured a lot of stuff out and tested a lot. Currently, I want to implement slew rate control of the current.
As my current is controlled by a voltage input at an opamp, I figured, I could control the slew rate of the voltage to limit the one of the current.
I was thinking about a capacitor-resistor combination, but that wouldn't be a proper straight slope, because the current would follow the charge/discharge function of a capacitor.
My googling hat brought me little to no information, that I am looking for.
There are circuits that can limit the slew rate. An integrator does just that. But I need an adjustable one.
My thinking was from 0.01V-1V per us but I don't know how big of a challenge that is.
Edit: It needs to be on-the-fly adjustable!

I thought about a small constant current charging/discharging a capacitor, of course, that would need to be bidirectional...

Are there any Chips, which limit the slew rate of an input Voltage?
Is there a circuit that I can implement?
Or am I missing something completely?


Greetings from Switzerland and Thanks in advance!!

[moffy]

This should help: https://www.ti.com/lit/an/sboa218a/sboa218a.pdf?ts=1699470931599&ref_url=https%253A%252F%252Fwww.bing.com%252F

[Marco]

Could that do a V/us? U1 has to go to the rails rather fast and have fast recovery from overdrive.

[moffy]

Could that do a V/us? U1 has to go to the rails rather fast and have fast recovery from overdrive.

It is meant for slower slew rates than mentioned, guess a modified scheme would be needed.

[lukaev3]

Thank you for the Document. I have looked into it. One thing I forgot to mention, it needs to be adjustable on the fly....

[moffy]

Thank you for the Document. I have looked into it. One thing I forgot to mention, it needs to be adjustable on the fly....

What do you mean by 'on the fly', by a potentiometer?

[lukaev3]

Yes. preferably by a voltage/current from a dac. But I guess a variable resistor would also work.
I don't know if it is even possible, but I think I would need an opamp with a constant current output, that is variable. But I don't have any luck finding one that has that feature...
That constant current would charge/discharge the capacitor linearly until I reach the desired voltage. But then again, opamps aren't meant to drive capacitive loads.

[moffy]

The LM13700 is a variable transconductance amplifier i.e. voltage in, current out: https://www.ti.com/lit/ds/symlink/lm13700.pdf

[lukaev3]

Thank you. I'll look into that. It looks promising! The current can also be varied by the Iabc.
Hopefully, I can figure it out.

[Marco]

Do you want a linear amplifier with adjustable slew rate limit, or is it enough to just have a square wave with adjustable slew? Cause the latter is easier.

[lukaev3]

The first solution would be the proper one, but I can get away with the second I think. How is the circuit named?
I would be happy to try it!

[Marco]

Well, easiest is to just use an arbitrary waveform generator :p

[Doctorandus_P]

The first (if that is sboa218a) is called an integrator. The opamp integrates the voltage on it's input, and when the input voltage is a constant, the output will be a linear ramp (until it saturates).

[donlisms]

As I understand it, a more interesting way to think about the integrator is that the input sets the current, not a voltage, and the opamp passes that current through the capacitor.  Since dv/dt = I/C, you change ramp slope by changing the current.  And, fortunately for us, I = V/R, so the current can be controlled either by changing the voltage, or the resistance it passes through, whichever is more convenient. 

[Marco]

The first opamp has to slew all the way to the rails, significantly faster than the final slew, it will also be overdriven. You're probably looking at 100+ MHz opamps here and because of the long feedback loop, stability is probably going to be a problem.

[David Hess]

The easiest way is probably to insert an operational transconductance amplifier between the output and feedback loop of an operational amplifier.  Then the adjustable transconductance controls the slew rate.

Instead of an operational transconductance amplifier, a diode bridge can be driven with a positive and negative current for asymmetric slew rate control.  Some function generators use this method.

[Marco]

Entirely forgot about the diode bridge, a little more quiescent current but who cares. Is going to be more stable than the dual opamp approach.

[David Hess]

The diode bridge is really good for controlling nanosecond slew rates, but it works just as well at very slow slew rates.