The duty cycle of the buck converter operated in PWM mode is given as D = VOUT/VIN. The duty cycle increases as the input voltage comes close to the output voltage and the off-time gets smaller. When the minimum off-time of typically 80ns is reached, TPS62135 scales down its switching frequency while it approaches 100% mode. In 100% mode it keeps the high-side switch 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 5.

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.