SNAS879 December 2024 LMR60420
PRODMIX
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. After this limit is reached as shown in Figure 7-10 if clock frequency was to be maintained, the output voltage falls. Instead of allowing the output voltage to drop, the LMR60420 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 after peak inductor current is achieved or after a predetermined maximum on time of approximately 10µs passes. As a result, after 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. After the input voltage increases beyond the output voltage setpoint, the output voltage increases as though in soft start as described in Section 7.3.2.