SMPS Output Capacitor Selection For Ripple Current (SEPIC)

[alexturner]

I'm designing a SEPIC converter based on a wide input voltage range SMPS controller (LTC3758). The requirement is for a wide input voltage (10-60V) with a fixed output of 28V @ 1.5A. I'm trying to determine Cout selection which per the data sheet, is the same for a SEPIC converter as it is a Boost converter.

The equation for ripple current across the output capacitor is the square root of the ratio of the max duty cycle multiplied by output current. However the max duty cycle is a factor of voltage only and doesn't consider frequency or capacitance.

This current calc yields an output ripple of 2.4A which is challenging to achieve with SMD capacitors and doesn't seem correct. How does one best determine the output capacitors for a SEPIC circuit as such and what is the effect of increased capacitance and frequency on this selection (if any)? Surely the derivative would be a better determination here, is this just a bad datasheet?

[T3sl4co1l]

Show the equation.

Was F, C, etc. not factored out in a previous step?

Tim