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 LM61440 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, t_{ON_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 t_{ON_MAX}, output voltage drops to slightly below the input voltage, V_DROP1. For additional information on recovery from dropout, reference Figure 8-9.

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, input voltage tracks output voltage.

Figure 8-20 Frequency and Output Voltage in Dropout

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 8-21 Dropout Waveforms