For a Li-ion charger, you want a sort of constant-current, constant-voltage supply (plus temperature measurement, timers, and maybe balancing circuitry. Although perhaps some of that is in other parts of your battery system.)
I think most Buck Converter chips are designed for one or the other (CV for most of them, CC for LED drivers.) Or, there are dedicated battery-charging ICs. (heh - Deja Vu - for my Senior design project I had planned on using an R/2R ladder and a comparator to implement an ADC, and my advisor advised "just buy an ADC chip.")
You might want to do a bit of searching for the base circuit for those cheap "lab" power supplies that have show up relatively recently, like https://www.amazon.com/Converter-Regulator-Adjustable-Regulated-Laboratory/dp/B0978T3JKH
They seem to feature microcontroller control, and have had some favorable reviews.