Dropout operation is defined as any input-to-output voltage ratio that requires frequency to drop to achieve the needed duty factor. At a given clock frequency, duty factor is limited by minimum off-time. Once this limit is reached, if clock frequency is maintained, output voltage falls. Instead of allowing the output voltage to drop, the LM6x4xx-Q1 extends on-time past the end of the clock cycle until the required peak inductor current is achieved. The clock can 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 factor 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 that the output voltage cannot be regulated even with an on-time of tON_MAX, output voltage drops to slightly below input voltage, VDROP1. See Section 7.