The TPS40170 is designed to operate with input voltages above 4.5 V. With voltages below 4.5 V if the EN pin is above its 600-mV turn-on threshold the VDD and VBP internal regulators are active. These regulators operate in drop out and output the highest voltage possible for the given VIN. The EN pin voltage must be below 100 mV to disable the VDD and VBP regulators. Switching is disabled while the VBP output voltage is below the VBP turn-on voltage of 4.4-V maximum. When there is sufficient VIN voltage to regulate the VBP voltage above 4.4 V the final condition for switching to begin is the UVLO pin voltage must be above its 900 mV typical threshold. After all three conditions are met the TPS40170 begins switching and the soft-start sequence is initiated. The device starts at the soft-start time determined by the external capacitance at the SS/TR pin. If a design requires operation near the minimum VIN voltage, due to lower VBP voltage when operating in dropout, lower gate threshold MOSFETs are recommended