Buck converter - at what current should inductor be calculated?

[shadewind]

Hello,

I'm designing a buck converter using the NSC LM2738X regulator. I'm trying to calculate the inductor value to use and the formulas are very clear to me, that's not the problem. From what I can gather, you want approximately 33% of the output current as ripple current. But at what output current should I calculate this? I'm guessing I'll average about 50 mA of output current but I want to have some headroom up to about 0.5A. So how should I set the ripple current for this?

I'm assuming that you never want the converter to go into discontinuous conduction mode at the average load current?

[Afrotechmods]

You calculate it as a percentage of the maximum output current. So 0.33 * 0.5A = a max of 165mA ripple current.

[shadewind]

But then it wil go into DCM at my median load of 50 mA.

[Afrotechmods]

Discontinuous mode may or may not be a big deal depending on how your controller handles light loads. If there is a minimum on time and no ability to pulse skip, then you may have to put a minimum load on your converter at all times and/or increase the output capacitance to dampen the unwanted extra pulses and/or look for a controller better suited for lighter loads. (50mA average for a buck converter is unusually low.) If your controller has pulse skipping or similar features that kick in at low loads then discontinuous mode shouldn't be a problem.