Hi Chuck,
The feedforard feature is based on the assumption that VDD=VIN. The PWM ramp slope is proportional to VDD, not directly to VIN. If VDD is tied to aux supply instead of VIN, the PWM ramp slope will not be proportional to VIN, hence losing the feedforward feature.
The short-circuit protection is implemented on high-side MOSFET by comparing the ILIM voltage to the SW. The ILIM voltage is set to (VDD- ILIM sink current x RLIM) while SW voltage is (VIN - ISCP x Rds,on). If VDD is significantly smaller than VIN, it equivalently disables the short-circuit protection.
Regards,
Na