The COT regulator does not require loop compensation, but it provides excellent transient performance. The COT can be controlled by the output voltage Ripple using a small inductor of 1/2 or even 1/4 of a conventional fixed-frequency DC-DC system. But why is the COT-controlled Buck unstable under PART2 CCM? This article explains the stability conditions of the COT-controlled Buck.
Figure 1 is an overall block diagram of a COT-controlled Buck.
Its basic control logic is shown in Figure 2. When the feedback voltage is lower than the reference voltage, the upper switch tube is opened, the SW point becomes high, and the energy is output, so that VOUT stops falling and rises; after the Ton time, the upper switch tube is closed, the lower switch tube is opened, and the SW point is low. Let VOUT stop rising and falling. When VOUT drops to a certain level, that is, the feedback voltage is lower than the reference voltage, the SW point is turned high again, and so on.
This basic control logic is incomplete. After the SW goes high, the Ton time, if the output is not high enough, the feedback voltage is still lower than the reference voltage. So you need to introduce an extra logic:
When the upper switch is turned off (after SW goes high ton time), if the feedback voltage is still lower than the reference voltage, the upper power transistor is turned off after the Toffmin time is turned off.
In fact this extra logic does not do this, but instead:
When the SW goes high for Ton time, the upper switch tube forcibly turns off the Toffmin time. After the Toffmin time, if the feedback voltage is still lower than the reference voltage, the upper switch tube is turned back on.
(Can you compare the difference between the two?)
This is the control logic of the bulb controlled by the COT. If you want to compare with other control modes, simply think about what the conditions for the upper switch to open and close are. For example, when the fixed-frequency voltage mode control Buck, when does the upper power tube open? When the clock comes on (or the edge of the sawtooth fast falling); when is the upper power tube off? Off when the feedback voltage is higher than the reference voltage.
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