The minimum off time influences the dropout performance of the buck regulator. As the input voltage is reduced, to near the output voltage, the off time of the high-side switch starts to approach the minimum value (see Section 7.6). Beyond this point the switching may become erratic or the output voltage falls out of regulation. To avoid this problem, the LMS36x5-Q1 automatically reduces the switching frequency to increase the effective duty cycle. This results in two specifications regarding dropout voltage, as shown in Section 7.7. One specification indicates when the switching frequency drops to 330 kHz. The other specification indicates when the output voltage has fallen to 3% of nominal. See the Section 9.2.2.3 for typical dropout values. Figure 8-10 and Figure 8-11 show the overall dropout characteristic for the 5-V option. Additional dropout information is discussed in Section 9.2.2.3 for 5-V output and in Section 9.2.3.3 for 3.3-V output.

Figure 8-10 Overall Dropout Characteristics (V_OUT = 5 V)

Figure 8-11 Frequency Dropout Characteristics (V_OUT = 5 V)