The duty cycle of the buck converter is given by D = VOUT / VIN and increases as the input voltage comes close to the output voltage. The minimum off-time is about 80 ns. When the minimum off-time is reached, TPS62147, TPS62148 scale down the switching frequency while they approach 100% mode. In 100% mode the high-side switch is on continuously. The high-side switch stays turned on as long as the output voltage is below the internal set point. This allows the conversion of small input to output voltage differences, for example for longest operation time of battery-powered applications. In 100% duty-cycle mode, the low-side FET is switched off.

The minimum input voltage to maintain output voltage regulation, depending on the load current and the output voltage level, can be calculated as:

Equation 7.

where

• IOUT is the output current,
• R_DS(on) is the on-state resistance of the high-side FET and
• R_L is the dc resistance of the inductor used.