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 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, t_ON-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 t_{ON_MAX}, output voltage drops to slightly below input voltage, V_DROP1. See Section 7.

Output voltage and frequency versus input voltage: If there is little difference between input voltage and output voltage setting, the IC reduces frequency to maintain regulation. If input voltage is too low to provide the desired output voltage at approximately 110 kHz, output voltage tracks input voltage.

Figure 8-24 Frequency and Output Voltage in Dropout

This image shows the switching waveforms while in dropout. Inductor current takes longer than a normal clock to reach the desired peak value. As a result, frequency drops. This frequency drop is limited by t_{ON_MAX}.

Figure 8-25 Dropout Waveforms