Hi Fujiwara-san,
Can customer set IFS above recommended operating condition? In particular, DRV8885 and DRV8886 can set IFS=2.1A?
The DRV8886 may operate at 2.1A. From section 7.1 of the datasheet "The actual full-scale and rms current depends on the ambient temperature, supply voltage, and PCB thermal capability."
My understanding is that IFS of recommended operating condition is defined full-scale current which is an actual current flow through motor and device. Is my understanding correct?
Yes. Please note the Microstepping Current Waveform on the first page of the datasheet. Full scale current is the 100% setting. RMS current is .707 * the full scale current.
If so, is there any limitation for an external resistor value?
If you trying to lower the resistor to achieve more current, the limit will depend on the factors listed above: ambient temperature, voltage, PCB thermals, and Rdson variation. For the DRV8886, the lower limit is 10kOhm because overcurrent can activate at 3A. In reality, the thermals will probably cause problems before then.
If you trying to increase the resistor to lower current, the limit will depend on the voltage, motor inductance, and decay mode.These factors will determine how well the steps are defined when microstepping.