The LMR36503-Q1 uses the internal LDO output and the VCC pin for all internal power supply. The VCC pin draws power either from the VIN (in adjustable output variants) or the VOUT/BIAS (in fixed-output variants). In the fixed output variants, once the LMR36503-Q1 is active but has yet to regulate, the VCC rail will continue to draw power from the input voltage, VIN, until the VOUT/BIAS voltage reaches > 3.15 V (or when the device has reached steady-state regulation post the soft start). The VCC rail typically measures 3.15 V in both adjustable and fixed output variants. To prevent unsafe operation, VCC has an undervoltage lockout, which prevents switching if the internal voltage is too low. See V_VCC-UVLO and V_{VCC-UVLO-HYST} in Section 7.5. During start-up, VCC momentarily exceeds the normal operating voltage until V_VCC-UVLO is exceeded, then drops to the normal operating voltage. Note that these undervoltage lockout values, when combined with the LDO dropout, drives the minimum input voltage rising and falling thresholds.