The dropout performance of any buck regulator 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 can become erratic, and the output voltage can fall out of regulation. To avoid this problem, the LMR33640 automatically reduces the switching frequency to increase the effective duty cycle and maintain regulation. In this data sheet, the dropout voltage is defined as the difference between the input and output voltage when the output has dropped by 1% of its nominal value. Under this condition, the switching frequency has dropped to its minimum value of about 140 kHz. Note that the 0.4 V short circuit detection threshold is not activated when in dropout mode. Typical dropout characteristics can be found in Figure 8-10, Figure 8-11, and Figure 8-12.

Figure 8-10 Overall Dropout Characteristic V_OUT = 5 V, I_OUT = 4 A

Figure 8-12 Typical Switching Frequency in Dropout Mode V_OUT = 5 V, f_SW = 400 kHz

Figure 8-11 Typical Dropout Voltage vs Output Current in Frequency Fold-back ƒ_SW = 140 kHz