Effect of load changes on buck converter
For the continuous inductor current case, a buck will follow the equation Vout = (duty cycle)*Vin.  If there's no feedback the equation still holds, but the output impedance of the converter is much higher.  

You have the DCR of the inductor and FETs, trace impedance, source impedance, etc, adding up to an effective output impedance that will cause the output to droop with load current.