Opamp linear current regulator must come quickly out of positive saturation

[treez]

Hello,
We have a linear LED current regulator involving an opamp which drives into the base of a FET and regulates the current in a sense resistor in the source of the FET (thers a reference voltage into the noninv input of the opamp). At first the opamp is powered but there is no voltage to power the LEDs….therefore the opamp current regulator is “hard-on” with its outputs banged up to its positive rail (in positive saturatrion). Then suddenly the LED driving voltage comes on and so the opamp has to slam very quickly out of positive saturation and quickly bring the LED current into regulation without LED current overshoot.
..In order that the opamp can more quickly come out of positive saturation, and start regulating the LED current, should we add circuitry to prevent the opamp’s output going any closer than about 2V away from its positive rail whilst it is in positive saturation?

 

[KlausST]

Hi,

I see two solutions:
* The better one monitors the LED voltage, and as long as it is below a certain level it forces the noninverting input to GND.
This is the more clean solution, because after the LED power is available..the current starts from zero.

* The other solutionis to use a zener (or similar circuitry) from output to to inverting input. In when the LED power is absent, the the OPAMP drives it's output high until the zener gets active andpulls the inverting input as high as the noninverting input.
The benefit is, that it is a simple solution. The OPAMP is always in regulating state - non saturating - therefore it doesn't suffer from long time coming out of saturating state. Mind that some, even fast OPAMPs may need several 10ms for this.
You will see a little unavoidable overshot.

Klaus

 

[FvM]

I presume the controller behavior can be improved, but I prefer discussing these problems referring to circuits.

 

[treez]

Thanks , We really need absolute minimum LED current overshoot....the supply goes on and off regularly....we really need to limit or preferably get rid of any LED current overshoot.
The schem and offending led current overshoot waveform is as attached.

There is also an LTspice simulation attached.
This shows the exact situation that we have, but for copyright reasons, i unfortunately cannot tell or show the actual application.

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* The better one monitors the LED voltage, and as long as it is below a certain level it forces the noninverting input to GND.
This is the more clean solution, because after the LED power is available..the current starts from zero.

Thanks and that is an excellent solution, but for reasons of the actual application, we cannot do this.

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* The other solutionis to use a zener (or similar circuitry) from output to to inverting input. In when the LED power is absent, the the OPAMP drives it's output high until the zener gets active andpulls the inverting input as high as the noninverting input.
The benefit is, that it is a simple solution. The OPAMP is always in regulating state - non saturating - therefore it doesn't suffer from long time coming out of saturating state. Mind that some, even fast OPAMPs may need several 10ms for this.
You will see a little unavoidable overshot.

Thankyou very much Klaus, you are too quick for me.....what you describe is what we are doing now....it improves things, but we still get some overshoot, more than we would like.

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I just wondered if we choose an opamp with "rail-to-rail output", then would that help us?

 

[KlausST]

Hi,

The LT6220 should be fast enough.
But with C2 you slow it down.
--> reduce C2 to 10pF

--> a zener has a relatively high capacitance (increasing fall time of Opamp output), therefore you can speed it up with a 1N4148 in series with the zener

--> you may slow down LED current rise rate with an inductance in series with the LEDs. Don't forget to put a free wheeling diode across the inductance.

Are the device values like in your circuit? Especially the 1Ohm shunt?
What is the max input voltage

Please measure opamp output voltage with maximum setup current and minimum rated LED supply voltage.
This gives you the max. voltage when in regulation. Add some headroom....this is the limiting value.

Mind that in your case: opamp_limit_voltage = opamp_noninverting_input_voltage + zener_voltage.
Maybe you need to lower zenervoltage.

Klaus

 

[treez]

a zener has a relatively high capacitance (increasing fall time of Opamp output), therefore you can speed it up with a 1N4148 in series with the zener

Thank you Klaus, again you are ahead of me, i didnt show it here but we are already doing that...it improves it but we still need less overshoot.
Yes it is a 1Ohm shunt in the actual circuit.
I tried adding the inductances but it gave other problems and so had to stop this.

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Thankyou Klaus, like the idea of the lower zener voltage.....

 

[FvM]

Reduce R4 to a reasonable value, e.g. several 100 ohms.