7.3.1 COT Control Circuit Overview

COT control is based on a comparator and a one-shot ON-timer, with the output voltage feedback (feeding to the FB pin) compared with an internal reference of 0.8 V. If the voltage of the FB pin is below the reference, the main MOSFET is turned on for a fixed ON-time determined by a programming resistor R_ON and the input voltage V_IN, upon which the ON-time varies inversely. Following the ON-time, the main MOSFET remains off for a minimum of 260 ns. Then, if the voltage of the FB pin is below the reference, the main MOSFET is turned on again for another ON-time period. The switching will continue to achieve regulation.

The regulator will operate in the discontinuous conduction mode (DCM) at a light load, and the continuous conduction mode (CCM) with a heavy load. In the DCM, the current through the inductor starts at zero and ramps up to a peak during the ON-time, and then ramps back to zero before the end of the OFF-time. It remains zero and the load current is supplied entirely by the output capacitor. The next ON-time period starts when the voltage at the FB pin falls below the internal reference. The operating frequency in the DCM is lower and varies larger with the load current as compared with the CCM. Conversion efficiency is maintained because conduction loss and switching loss are reduced with the reduction in the load and the switching frequency, respectively. The operating frequency in the DCM can be calculated approximately as follows:

Equation 1.

In the continuous conduction mode (CCM), the current flows through the inductor in the entire switching cycle, and never reaches zero during the OFF-time. The operating frequency remains relatively constant with load and line variations. The CCM operating frequency can be calculated approximately as follows:

Equation 2.

The output voltage is set by two external resistors R_FB1 and R_FB2. The regulated output voltage is