I want to use a buck converter to step down a 20VDC Input to 15V to use for a charging IC (BQ24193). The reason for this is because the charging IC I'm using won't operate with a voltage input higher than 17V. I'm designing a Buck Converter on a PCB, and I'm trying to figure out what inductor size I need. I'm able to calculate the ripple current of the inductor and compare it to the Average Inductor current when the charging IC is in CC mode. There, the Buck Converter will operate in Continuous Conduction Mode since the Average Inductor Current is greater than the ripple current. The issue is I'm facing is when the charging IC reaches CV mode. There, the output current is going to decrease, and the Buck Converter may end up in the Discontinuous Conduction Mode since the ripple current may become greater than the Average Inductor Current. So my question is if my Buck Converter operates in the DCM, how will that effect the charging IC? Will it just still operate like normal since the output Voltage on the Buck Converter will have minimal ripple (under 100mV)? Or will it turn on and off as the inductor current rises from and falls to 0A? Thanks.
There will be no problem when you are in DCM.
Yes your inductor current will go to zero for a certain amount of time each switching cycle. But this doesn't mean there is zero current flowing between the buck converter and the battery charger! Instead, the current is flowing from the buck converter's output capacitor during this time. The buck converter is monitoring the capacitor's voltage (which is just the buck's output voltage) and once it falls below a certain threshold, the buck converter will start another switching cycle.
Comment if anything is unclear!
There's no issue with the converter entering DCM at lighter loads. You should easily be able to design for output voltage ripple less than 100mV from zero to full load. The control to output transfer function is different in the two modes, so you should check stability, but it's rare for that to be a problem if you have plenty of phase margin in CCM.