In the simulation that we perform to find the loop gain responses,

we already mentioned we do that for

four different values of the amplitude of artificial ramp.

If you look at magnitude responses, they're all fairly similar

with slight difference is in the magnitude responses at low frequencies and

some slight differences at high frequencies.

The crossover frequency in all cases is pretty close to around 100 kHz, but the phase

margin is very different depending on the amplitude of the artificial ramp.

It goes from 51 degrees for small value of artificial ramp to 21 degrees for

a very large value of the artificial ramp, equal to four times the slope m2.

The practical case that we will

recommend in most situations is the choice of ma equal to m2.

That results in a crossover frequency close to 100 kHz, and

close to 45 degrees of phase margin, close to but

slightly smaller than the 50 degrees that we have planned to have

in the design based on the simple model earlier in the lectures.

So corresponding to these

four choices the step responses examined for different values of Va.

This is now going back to transient simulation.

But looking at just the output voltage for four different values of

the compensation ramp. Notice that when the phase margin is relatively large,

in the first three cases, the responses look nice,

whereas in the case where our artificial ramp is too large, we have a small

phase margin which corresponds to a transient response upon

step-load that has ringing and overshoot in the opposite direction

and that ringing takes a considerable amount of time to settle.

So that's not very desirable,

so this choice here would show a too large value of the artificial ramp.