Hi Sasaki-san,
The ILIM pin is a multiplexed pin for OCP setting and low-side gate drive.
To save to OCP setting on ILIM pin, the voltage on ILIM pin is recorded internally in a digital format when the part is initially powered on. Later on, the ILIM is used for low-side gate drive. Due to the resolution of the analog-to-digital conversion for saving the OCP setting information, when changing the ROCSET from 620ohm to 750ohm, the OCP setting saved in the digital format does not change, hence the OCP detection level does not change. That's the reason you see step-by-step change by changing the ROCSET.
Since the TPS56121 is a device for 15-A full load current, it is not intentionlly designed for OCP level below 10A. So we suggest using ROCSET value above 1kohm to get relative accurate OCP tripping point.
Regards,
Na