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?
The easiest way to test the LEDs is by the use of a properly selected SMU. The problem is that this may be a little expensive solution.So, to avoid the above, you are trying to do something else. But, to be really helped, some clarifications are needed. From the discussion it seems that there is a little confuse about the situation (there are proposals that are useful for msec, as they are simulated).
I will try to identify the situation from your description:
- there is a DC power supply for the load that is operated independently of everything else. It may be inactive, active for a period or continuously. Can it also operate in a pulsing mode?
- there is a current controller (sink) that is always active and waiting to be hit from the above power supply!
- the point, for the controller, is to guess when the hit will take place to react earlier!
This goal, I think, is not possible with the topology that is presented, because the action of the independent power supply is involved inside the control loop. In other words, the result (current) must be there, before the controller starts to adjust the current.
My proposal is a signal to be used, that will activate the current controller, after the output of the power supply. When the current is stabilized you can take the measurements.
This signal can one that may be provided from the power supply or can be created from the output of it.
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[I see now that you insist to have a controller that operates perfectly without knowing what it has to control!]