In many datasheets and application guides there is always an equation for ripple current, because it is fundamental to the operation of a buck or boost converter, but I never see a direct equation for ripple voltage.
I know the ripple comes from the finite capacitance that needs to discharge the load current during the off-part of the duty cycle, so I suspect the equation is a second order (since the derivative of the capacitors voltage depends on the derivative of the inductor current). I also know the capacitor’s ESR plays a role.
I ask out of experience. Once I built a boost converter and the output voltage ripple grew to over 100mV at max load, whereas another time I build a boost converter and the ripple was constant at 4mV across all loads. These were clearly satisfying very different power requirements, but what is the interplay between capacitance ESR and the pure inductive/capacitance relationship in determining output ripple? And when does it start to vary significantly across loads?