Calculation for input bulk capacitor of a buck converter: Needs help in understanding equations
I’m not sure, but maybe we can look at it this way: The (voltage) load step can only drop at the ESR resistor the moment it happens, because the input cap does not allow a fast voltage change. So the ESR ‘sees’ the remaining RLC (+Vhost) in series. So yes, the dynamic response is that of a series RLC circuit.