There is no limitation for small duty cycles, because even at very low duty cycles, the switching frequency is reduced as needed to always ensure a proper regulation.

If the output voltage level comes close to the input voltage, the device enters 100% mode. While the high-side switch is constantly turned on, the low-side switch is switched off. The difference between VIN and VOUT is determined by the voltage drop across the high-side FET and the dc resistance of the inductor. The minimum VIN that is needed to maintain a specific VOUT value is estimated as:

Equation 3.

where

• V_IN,MIN = Minimum input voltage to maintain an output voltage
• I_OUT,MAX = Maximum output current
• R_DS(on) = High-side FET ON-resistance
• R_L = Inductor ohmic resistance (DCR)