The duty cycle of a buck converter operated in PWM mode is given as D = V_OUT / V_IN. 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 30 ns (typical) is reached, the TPS6244x-Q1 skips switching cycles while it approaches 100% mode. In 100% mode, the device 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.