Buck converter operation is impacted by minimum on-time, minimum off-time, and minimum peak-to-peak inductor ripple limitations. The converter reaches the minimum on-time of 96 ns (typical) when operating with high input voltage and low-output voltage. In this control scheme, the off-time continues to increase and the switching frequency reduces to regulate the inductor current and LED current to the desired value.

The converter reaches the minimum off-time of 91 ns (typical) when operating in dropout (low input voltage and high output voltage). As the on-time and off-time are fixed, the duty cycle is constant and the buck converter operates in open-loop mode. The inductor current and LED current are not in regulation.

The behavior and response of valley comparator is dependent on sensed peak-to-peak voltage ripple, ΔV_(CSP-CSN), and is a function of current sense resistor, R_CS, and peak-to-peak inductor current ripple, Δi_L(PK-PK). To ensure periodic switching, the sensed peak-to-peak ripple must exceed the minimum value. At high (near 100%) or low (near 0%) duty cycles, the inductor current ripple may not be sufficient to ensure periodic switching. Under such operating conditions, the converter transitions from periodic switching to a burst sequence, forcing multiple on-time and off-time cycles at a rate higher than the programmed frequency. Although the converter may not operate in a periodic manner, the closed-loop control continues regulating the average LED current with a larger ripple value corresponding to higher peak-to-peak inductor ripple. TI recommends choosing an inductor, output capacitor, and switching frequency to ensure minimum sensed peak-to-peak ripple voltage under nominal operating condition is greater than 8 mV. The Application and Implementation section summarizes the detailed design procedure.