So we are building a 12 volts output buck converter with 17 volts input for our project. And i was wondering if using LM317 as regulators and NE 555 for the PWM generator a good idea.thanks.
The LM317 is a linear regulator with no correlation to a buck converter.
An NE555 is not a practical element for pulse width modulation required of a buck converter.
There are hundreds of special purpose ICs made specifically to be the heart of a buck converter. You should review the available application notes and choose one that satisfies your requirements.