Can't recall or find in available reference material the basis for the need that the VRM crossover freq. (0 dB) be << frequency of the VRM switching, Fswitch, and less than half of it. I believe it's due to any unipolar synchronous noise at 0.5*Fswitch leading to instability. Correct?
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Essentially. A VRM is a sampled data systems and so Nyquist holds that the bandwidth must be less than half the sample rate (switching frequency). In reality I would limit to less than 1/6 the switching frequency.