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 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.

Equation 5.

The converter reaches the minimum off-time 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 converter continues to switch unless disabled by the IADJx input.

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 is not 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 cannot 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 20 mV. The Application and Implementation section summarizes the detailed design procedure.