Mp2155 inductor calculation

DC-DC Power Converters
1

I’m trying to calcluate the inductor value and it’s peak current needed. (also based updon the evaluation board) and I have difficulty why 3.3uH was chosen with such high current rating.

eg. Vin is between 2.5V and 5V ; Vout = 3.3V ; Load = 1 amp ; ripple at 20% ; I end up with values Lmin_buck = 5.61uH and Lmin_boost at 3.03uH

If I want to reduct the ripple to 10%, I end up with 11,22uH and 6,06 uH, so even for the evaluation board I never end up with 3,3uH as you have to choose the max value of these minimum L values.

Also the peak currents are much lower than the coils proposed in the BOM (they can handle 8amps or 10.1 amps and I get max currents of 4amps. So why this overkill in amp rating for the reference design?

Is my calculation wrong and if so why?

thanks

2

Sometimes you pick a larger inductor to avoid saturation, so for instance your app may only consume 2A but if the chip has a current limit of 6A you might chose a 7A inductor to avoid saturation on say a load transient. Sometimes you pick a larger inductor for efficiencies sake. Coilcraft has a nice design tool called a DC DC optimizer that calculates the loss for a bunch of their inductors in a circuit. Sometimes you chose an inductor because you are lazy and that is what you have.