Схемы, платы, компоненты

Единственное, что там существенно ограничивает:  

BOOTSTRAP VOLTAGE (BOOT) The TPS54331 has an integrated boot regulator and requires a 0.1 F ceramic capacitor between the BOOT and PH pin to provide the gate drive voltage for the high-side MOSFET. A ceramic capacitor with an X7R or X5R grade dielectric is recommended because of the stable characteristics over temperature and voltage. To improve drop out, the TPS54331 is designed to operate at 100% duty cycle as long as the BOOT to PH pin voltage is greater than 2.1 V typically. OUTPUT VOLTAGE LIMITATIONS Due to the internal design of the TPS54331, there are both upper and lower output voltage limits for any given input voltage. The upper limit of the output voltage set point is constrained by the maximum duty cycle of 91% and is given by Equation 31: VOmax = 0.91 × ((VIN min ) - IO max × RDS(on) max + VD )- (IO max × RL ) - VD (31) Where: VIN min = Minimum input voltage IO max = Maximum load current VD = Catch diode forward voltage RL = Output inductor series resistance The equation assumes maximum on resistance for the internal high-side FET. The lower limit is constrained by the minimum controllable on time which may be as high as 130 ns. The approximate minimum output voltage for a given input voltage and minimum load current is given by Equation 32: VOmin = 0.089 × ((VIN max ) - IO min × RDS(on) min + VD )- (IO min × RL ) - VD (32) Where: VIN max = Maximum input voltage IO min = Minimum load current VD = Catch diode forward voltage RL = Output inductor series resistance This equation assumes nominal on-resistance for the high-side FET and accounts for worst case variation of operating frequency set point. Any design operating near the operational limits of the device should be carefully checked to assure proper functionality.

PS: при 30 вольтах входного это где-то минимум 3 вольта выходного получается. Что вполне приемлимо для вопрошающего. И 4.5 вольта входного вполне достаточно (если не выбрать особо выдающиеся индуктивность и диод) для получения выходного в 3.3в. Нужно расчитать порог UVLO, в зависимости от тока нагрузки, и соответствующие резисторы применить для отключения при напряжении 4.5в (а не 3.5 как по-умолчанию).