The input to output voltage conversion ratio of the TPS55340-EP is limited by the worst-case maximum duty cycle of 89% and the minimum duty cycle, which is determined by the minimum on-time of 77 ns and the switching frequency. The minimum duty cycle can be estimated with Equation 7. With a 600-kHz switching frequency the minimum duty cycle is 4%.

The duty cycle at which the converter operates depends on the mode in which the converter is running. If the converter is running in discontinuous conduction mode (DCM), where the inductor current ramps to 0 at the end of each cycle, the duty cycle varies with changes of the load much more than it does when running in continuous conduction mode (CCM). In CCM, where the inductor maintains a minimum DC current, the duty cycle is related primarily to the input and output voltages as calculated in Equation 8. Assume a 0.5-V drop V_D across the Schottky rectifier. At the minimum input of 5 V, the duty cycle is 80%. At the maximum input of 12 V, the duty cycle is 51%.

Equation 8.

At light loads, the converter operates in DCM. In this case, the duty cycle is a function of the load, input and output voltages, inductance, and switching frequency as calculated in Equation 9. This can be calculated only after an inductance is chosen in the following section. While operating in DCM with very-light load conditions, the duty cycle demand forces the TPS55340-EP to operate with the minimum on-time. The converter then begins pulse skipping, which can increase the output ripple.

Equation 9.

All converters using a diode as the freewheeling or catch component have a load current level at which they transit from DCM to CCM. This is the point where the inductor current just falls to 0 during the off-time of the power switch. At higher load currents, the inductor current does not fall to 0, and diode and switch current assume a trapezoidal wave shape as opposed to a triangular wave shape. The load current boundary between discontinuous conduction and continuous conduction can be found for a set of converter parameters as follows.

Equation 10.

For loads higher than the result of Equation 10, the duty cycle is given by Equation 8. For loads less than the results of Equation 10, the duty cycle is given Equation 9. For Equation 7 through Equation 10, the variable definitions are as follows.

• V_OUT is the output voltage of the converter in V
• V_D is the forward conduction voltage drop across the rectifier or catch diode in V
• V_IN is the input voltage to the converter in V
• I_OUT is the output current of the converter in A
• L is the inductor value in H
• ƒ_SW is the switching frequency in Hz