As mentioned above, FCCM device doesn’t have the ZC logic. It means that the turn on time of high side MOSFET and the low side MOSFET is complemented, if deadtime is neglected. Figure 2-1 shows the basic logic block of adaptive constant on-time (D-CAP™) control [3], when analysis the FCCM device, the ZC part can be neglected.

From the operation logic of D-CAP structure, the device will compare the output feedback (FB) and reference voltage (Ref), the high side MOSFET will turn on when bias output voltage is lower than the target. If the bias output voltage is higher than target, high side will turn off and the lower side will turn on. The inductor current will ramp down, until it triggers negative operation current (NOC) protection. Because the logic priority of NOC is higher than the above compare logic, after triggering NOC, high side MOSFET will turn on, and the on time is same as the normal operation which is relative to input voltage V_in and output voltage V_out, as shown in the Equation 2.

Where F_sw=1/(R_on*C_on), shown in Figure 2-1.

Because high side will turn on and transfer negative current to V_in side. It is difficult to judge the final state of V_in by a direct logic, so the hypothetical backstepping logic can be used. Suppose V_in can be a stable and constant voltage finally. Because there is no load in V_in side, so if V_in can be stable, the average inductor current must be zero. It means the absolute value of peak inductor current and valley inductor current is same, as shown in Figure 2-2.

From Figure 2-2, the turn-off time of high side switch is show in following.

From the basic relationship between V_in and V_out, V_in is shown in following formula.

Substitute Equation 2, Equation 3 into Equation 4, Vin can be written as Equation 5.

From Equation 5, the stable value of V_in is related to I_NOC and inductor value L. If V_bias/F_sw<=2LI_NOC, it means V_in can be stable with a non-infinite constant value; if V_bias/F_sw>2LI_NOC, V_in can be a stable with a constant value, but it is determined by I_NOC and L. Generally, V_bias/F_sw-2LI_NOC is a small value if the converter is reasonably designed, so V_in may be very large in this condition. So there exists the risk of over voltage damage.

If the output voltage is lower than the target, the minimum off time of high-side switch will always be triggered. So, on time is same as Equation 3, off time is shown in Equation 6. In steady sate, the voltage-second balance is established, as shown in Equation 7. By substitute Equation 2, Equation 6 into Equation 7, V_in can be written as Equation 8. Generally, the minimum off time is much smaller than 1/F_sw, so V_in will be closed to V_bias. It is not a high value that can casue damage.

Although the input voltage will not be a high value if V_out <V_target. But if consider the regulation and fluctuation, the operation of FCCM device has relative high risk of damage on the whole. Because the input voltage may ramp up to a very high value. For FCCM device, it suggests to use Enable function to shut down itself, and keep the bias standby operation safety. Most of TI's power DC/DC device support the enable function, can control the operate and shut down of converter flexibility.