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Variable Slope External Ramp to Improve the Transient Performance in Constant On-Time Current Mode Control
Fig. 1. Proposed variable ramp COTCM structure by adding dVc/dt with conventional ext. ramp.
In Fig 1, the proposed structure where the derivative of Vc to increase the ramp slope in transient, is added with the conventional COTCM structure. In conventional COTCM in Fig 2a, we can see that with the large external ramp, the Vc and Isum distance is very large in the steady state. Therefore, at the load transient it becomes very difficult for Vc and Isum to cross each other, which prohibits the duty cycle from saturating in order to increase the inductor current quickly. In Fig 2b, we can see that when a heavy load transient occurs, the dVc/dt is added with the slope of the external ramp, which is shown by a green circle in Fig 2b. As the slope becomes very high at transient, the duty cycle also becomes very high very quickly in order to increase the inductor current which reduces undershoot at output. Comparing Fig 2a and 2b, we can clearly see that, output voltage undershoot can be reduced by using the proposed method. The main advantages of this method are, since we have used dv/dt of Vc, the detection is fast and optimum which will reduce any chance of ring back. In this method there is no threshold detection and the small signal property is not affected by the proposed method. By using the proposed method, COTCM control can have both a high noise performance and a fast transient performance together.
Fig. 2. (a) Transient response of COTCM with fixed ramp. (b) Transient response of COTCM with proposed variable ramp at transient.
