You mean 3977, I think.
If I recall correctly, Itripmax is the current at which you want to turn off the coil drivers. i.e. max current your steppers should draw. When I designed an a3977 board, I used .2 for Rs with the expectation that I wouldn't drive any stepper coil over 2.5A.
by the way, they call it PWM but I don't think it is since the current rise in the coils is sensed via Rs and when the coil current reaches the trip current, it turns off the drivers. then it waits for a fixed time, turns on the drivers and repeats the process. it sort of looks like PWM but you don't control the duty cycle directly.