The duty cycle increases as the input voltage comes close to the output voltage and the off-time of the high-side switch gets smaller. When the off-time reaches the minimum value, it could not decrease any more, and keeps the minimum value even if the input voltage continues to drop.

Figure 3-1 shows waveforms during normal operation. The on time begins when FB voltage is below internal reference and the off time begins when one-shot pulse comes after a constant on-time timer. The min off-time signal starts the timer once the off-time begins and holds for a constant minimum off-time value shown in the data sheet. If the off-time is longer than the min off-time, the signal won't works.

Figure 3-1 Normal Operation without Min off-time Triggered

Figure 3-2 Operation with Min off-time Triggered

Figure 3-2 indicates the operation of converter when the min off-time is triggered. After the off-time decided by the loop is shorter than minimum off-time, even though the FB voltage is less than reference voltage, the off-time no longer follows the V_FB comparator but just operates the constant min value while the on-time is still decided by the one-shot circuit and proportional to the V_OUT/V_IN. With such two limitation, the output voltage would finally fall under the rated reference value and reaches another balance.

Without on-time extension function, it is obvious that the duty cycle is fixed when min off-time is triggered. Based on Equation 3, since Toff has a minimum value, it follows that the duty cycle, D, has a maximum value. If the F_SW is fixed, a smaller T_OFF(min), means a larger duty cycle can be supported. Alternatively, if the T_OFF(min) is fixed, a larger duty cycle can be supported with a smaller F_SW.

Equation 3.