8.4.3 Dropout

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 is reduced to 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 falls out of regulation. To avoid this problem, the LMR36520 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 the 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.

Figure 11. Overall Dropout Characteristic
V_OUT = 5 V

Figure 12. Frequency Dropout Characteristics
ƒ_SW = 400 kHz