Ideally, the regulator would control the duty cycle over the full range of zero to one. However due to inherent delays in the circuitry, there are limits on both the maximum and minimum duty cycles that can be reliably controlled. This in turn places limits on the maximum and minimum input and output voltages that can be converted by the LM22678. A minimum on-time is imposed by the regulator to correctly measure the switch current during a current limit event. A minimum off-time is imposed in order the re-charge the bootstrap capacitor. Equation 8 can be used to determine the approximate maximum input voltage for a given output voltage.

Equation 8.

where

• F_sw is the switching frequency.
• T_ON is the minimum on-time.

Both values can be found in Section 6.6.

The worst case occurs at the lowest output voltage. If the input voltage found in Equation 8 is exceeded, the regulator will skip cycles; thus, effectively lowering the switching frequency. The consequences of this are higher output voltage ripple and a degradation of the output voltage accuracy.

The second limitation is the maximum duty cycle before the output voltage will "dropout" of regulation. Equation 9 can be used to approximate the minimum input voltage before dropout occurs.

Equation 9.

where

• The values of T_OFF and R_DS(ON) are found in Section 6.6.

The worst case here occurs at the highest load. In this equation, R_L is the dc inductor resistance. Of course, the lowest input voltage to the regulator must not be less than 4.5 V (typ.).