SNVSB91C July 2019 – June 2020 LMR36506-Q1
PRODUCTION DATA.
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 as shown in Figure 29 if clock frequency was to be maintained, the output voltage would fall. Instead of allowing the output voltage to drop, the LMR36506-Q1 extends the high side switch on-time past the end of the clock cycle until the 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. As shown in Figure 28 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 the input voltage by VDROP. For additional information on recovery from dropout, refer back to Figure 17.