The duty cycle of a buck converter operated in PWM mode is given as D = VOUT / VIN. The duty cycle increases as the input voltage comes close to the output voltage and the off-time gets smaller. When the minimum off-time of typically 10 ns is reached, the TPS62851x skips switching cycles while it approaches 100% mode. In 100% mode, it keeps the high-side switch on continuously. The high-side switch stays turned on as long as the output voltage is below the target. In 100% mode, the low-side switch is turned off. The maximum dropout voltage in 100% mode is the product of the on-resistance of the high-side switch plus the series resistance of the inductor and the load current.