SNVSBY6 October 2021 LM61430-Q1
Dropout operation is defined as any input-to-output voltage ratio that requires frequency to drop to achieve the required duty cycle. At a given clock frequency, duty cycle is limited by minimum off time. Once this limit is reached, if clock frequency were maintained, output voltage would fall. Instead of allowing the output voltage to drop, the LM61430-Q1 extends on time past the end of the clock cycle until needed peak inductor current is achieved. The clock is allowed to start a new cycle once peak inductor current is achieved or once a pre-determined maximum on-time, tON_MAX, of approximately 9 µs passes. As a result, once the needed duty cycle cannot be achieved at the selected clock frequency due to the existence of a minimum off-time, frequency drops to maintain regulation. If input voltage is low enough so that output voltage cannot be regulated even with an on time of tON_MAX, output voltage drops to slightly below input voltage, VDROP1. For additional information on recovery from dropout, reference Figure 8-9.