Fig. 1. Constant on-time control (V2 type).
type constant on-time control is a particular implementation for current mode control, which uses the ESR as the sensing resistor for inductor current. Along with inductor current information, capacitor ripple is also fed back to the modulator. Previously CPES proposed a model for V2 control based on describing function. Although the model is pretty accurate, it cannot reveal the relationship between current mode control and corresponding V2 control. The new modeling method tries to separate current feedback information with capacitor ripple feedback. In this way we can directly use the existing model for current mode control.
The concept is to separate the inner voltage loop as a inductor current feedback loop and a capacitor voltage feedback loop. For the inductor current feedback loop, as the information is harmonic-rich, we use previous result based on describing function. For the capacitor voltage feedback, due to the low pass characteristic of this feedback loop, average modeling method is used.
Fig. 2. Voltage ripple information.
Fig. 3. Separated feedback structure.
Fig. 4. Equivalent circuit for V2 control.