Hi,
I am trying to build a simple control-loop to build a current-source. Control-loop will be controlling a buck SMPS, Fs=300 kHz. Control-loop is Type I.
I'm using single-supply op-amp, Vref attached to "+" node of amp, feedback through 80k ohm resistor to "-" input. A 0.1 uF cap is attached from "-" to op-amp output. 0 dB open-loop gain with above = ~20 Hz.
I can't seem to get the PSPICE / Micro-cap step-response to match Matlab's closed-loop step response. Matlab shows 125/s step-response settles with-in 45 ms (very acceptable), SPICE shows the system settles with-in 10 us!
I simulate closed-loop by having op-amp output drive a 10 uH / 47 uF L/C filter, output of filter goes to input side of 80k resistor. I apply a step to Vref at 100 ns to go from 0 to 1V. Output of op-amp responds with ~10-15 us delay to step with nearly 0 overshoot.
I would expect in "closed-loop" my op-amp output would reach 1V output in ~50 ms. Oddly my AC simulation of my op-amp circuit matches Matlab's open-loop gain, open-loop crosses 0 dB @ 20 Hz, with DC gain of ~42 dB. I can change the .1 uF cap to 100 uF and the SPICE step-response is the same.
What is a "failsafe" way of simulating my closed-loop op-amp operation? I'm taking the SMPS out of the picture even, just limiting it to the op- amp alone.
Regards. Rob