One of the parameters that influences the dropout performance of a buck regulator is the minimum off time. 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 7.5). Beyond this point the switching may become erratic and/or the output voltage falls out of regulation. To avoid this problem, the LM53625/35-Q1 automatically reduces the switching frequency to increase the effective duty cycle. This results in two specifications regarding dropout voltage, as shown in Section 7.6. One specification indicates when the switching frequency drops to 1.85 MHz; avoiding the A.M. radio band. 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. The overall dropout characteristic for the 5-V option can be seen in Figure 8-11 and Figure 8-12. The SYNC input is ignored during frequency foldback in dropout. Additional dropout information is discussed in for 5-V output (Section 9.2.2.3 and for 3.3 V output (Section 9.2.3.3).

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

Figure 8-12 Frequency Dropout Characteristics (V_OUT = 5 V)