How to determine sensing resistor(pin 1 / 15) for L298N? Is there any formula can be used? thanks for guidance.

 

The maximum sense voltage is 2 volts. By sizing the resistor to develop a 2V drop at the maximum current that you want to deliver to the load, the device can be made to auto-shutdown.

hgmjr

 

Here is an application note for the L6506 and L298N that presents the basic operation of the current sense function.

hgmjr

 

If you're not going to use a chopper driver like the L297, there's not much point in using sense resistors.

Here's a datasheet for the L297:
http://sigma.octopart.com/148273/datasheet/STMicroelectronics-L297/1.pdf
Note that in the sample application circuit on page 1, they use 0.5 Ohm resistors for Rs1 and Rs2. Note that these resistors must be the non-inductive type; wirewound must not be used.

You then set up Vref for the L297 to be equal to the maximum voltage drop you want across the sense resistors. If you wanted a maximum current of 1A, then the voltage drop across the resistors would be 1A * 0.5 Ohms = 0.5V. Establishing Vref at 0.5v would then cause the L297 to chop the current drive to the bridge in an attempt to maintain the 1A current as closely as possible. This enables you to use double the voltage that a stepper motor is rated for; you'll be able to start/stop and step the motor much faster than using it at the rated voltage.