How to make a constant current-pulse current charger for lithium ion?
To do CCCV charging with a DC2DC you need to provide it with 2 regulation loops, one for current, one for voltage. Thus when the battery is flat, it will operate in CC, when nearly full, CV - a common way of doing this is to add an op-amp or high-side current sense arrangement to sense current in the positive lead (e.g. TI's INA180) of a DC2DC (set for the correct voltage) and use the output of this op-amp to steal current (often through something like a current mirror/matched transistor pair) from the DC-DC converter's comp pin, thus robbing the output of current to prevent it exceeding a certain limit. If there isn't a COMP pin available it can sometimes be done through the FB node, but be VERY careful of stability due to the high gain here.
However, Analog devices(Linear tech) and other manufacturers offer several IC's with many of these features built in, for a price. They have entire categories of their web site on switching chargers and pulse chargers.
Unless you are very familiar with DC2DC converters and what makes them unstable, AND wish a low-cost highly customised solution, I recommend you rather use an existing part and follow an application note closely.
A lot also depends on the size of battery you are charging, and if there are more than one in series, as to which solution becomes practical, as a battery management IC will become necessary for more than 2 in series.
Another point to note is that DC-DC converters will add current limiting in the inductor current already - if you aren't averse to running your converters hot (ambient temperature is never high and charger is well ventilated), and don't care if the current limit is accurate, this can sometimes be used to limit current - however, read the datasheet with care if doing this.