Hi tg,

You are right in all you said. We would like to force DCM mode under light load conditions. The goal is to improve efficiency of DCDC converter. As soon as current begins to invert in NMOS transistor (flow to ground instead of from ground), efficiency begins to decrease in CCM mode.

In CCM mode D=Vout/Vin (D duty cycle). If you have a very ligth load, as DeltaI in inductor (Delta of current in inductor between NMOS and PMOS commutation) is always the same whichever load current you choose (in first approximation), Current have to go below zero.

As a good drawing is better than a long text, I have drawn inductor current in three cases for Vout=Vin/4:
- CCM, high load.
- CCM, light load.
- DCM, light load.

I hope this answer your question...

Matthieu

Yes, I should have forgotten to attach the file  8-)

If you want, maxim-ic website has a lot of interesting and hand on information about DCDC converters.
(http://www.maxim-ic.com/appnotes.cfm/appnote_number/2031/ for example)

Matthieu