I'd like to design a buck converter using MCP19035, I've gone through the datasheet of this component and and the design procedure using it. In the datasheet, it says that the component provides Two options for the frequencies (300kHz or 600kHz), but nothing says who to select one of two frequencies, which is a critical parameter for power train component selection. Can you please go through the datasheet and help me to understand this component ??
For each particular chip the frequency is not selectable by any means. You have to order proper part, and design components based on your selection.
The frequency is selectable based on part order suffix, MCP19035T-Axxxx is for 300 kHz, and MCP19035T-Bxxxx is for 600 kHz, see page 41 of the datasheet.
If the question is not how to select one of the two frequencies but rather why you would select one over the other the answer is as follows:
Lower switching frequency will have lower switching and AC losses (core, proximity, AC wire), and therefore higher efficiency. It will also mean a larger inductor for the same rippler current. It could possibly have lower EMI as well.
A higher switching frequency will typically allow a smaller solution size.
You should make the choice based on your design requirements.