The dropout performance of any buck converter is affected by the R_DSON of the power MOSFETs, the DC resistance of the inductor, and the maximum duty cycle that the controller can achieve. As the input voltage level approaches the output voltage, the off time of the high-side MOSFET starts to approach the minimum value. Beyond this point, the switching frequency becomes erratic and the output voltage can fall out of regulation. To avoid this problem, the TPS563300 automatically reduces the switching frequency (on-time extension function) to increase the effective duty cycle and maintain in regulation until the switching frequency reach to the lowest limit of about 140 kHz, the period is equal to (t_{ON_MAX} + t_{OFF_MIN}) (typically 7.14 μS). In this condition, the difference voltage between V_IN and V_OUT is defined as dropout voltage. The typical overall dropout characteristics can be found as Figure 7-6.