There are three light load operation levels in Fig.16. What's the meaning?
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The UCC28730 controller is targeted for applications that require extremely low input power during stand-by (no-load) conditions. In many situations it is possible to achieve the so-called "zero-Power" level which is < 5mW input. To achieve this low of input power yet still be able regulate the output voltage it is necessary to reduce the switching frequency to a very low level. The UCc28730 is capable of switching down to 32 Hz, if necessary.
In the design of the IC, it was found that it was not feasible to map the full frequency range of 83300 Hz to 32 Hz over the Vcl range from 4.85 V to 1.3 V directly. The control resolution is too fine. Instead, light load operation (where fSW < 1.92kHz) is split into three more operating levels and the Vcl voltage range between 2.2V and 1.3V are re-used for each range of the three levels.
Transitions between ranges happen when the Vcl (error voltage) falls to < 0.75V or rises > ~2.9V. There is some overlap between the levels to institute hysteresis so that steady-state operation can be stable in either a higher or lower level, and prevent minor variations from toggling between levels when operating near one of the level limits. Each light-load operating level regulates by adjusting the fSW up or down a bit with Vcl somewhere on the slope between 1.3V and 2.2V.
Regulation happens along the slopes of each level. If the load changes to force Vcl onto the horizonal portions at either end of a light-load level, no change of frequency will take place, so the error (Vcl) will continue to increase (or decrease) until it hits one of the trigger points (0.75V or 2.9V) to transition to a new level and Vcl then settles onto the slope of the new level. This can be seen as very small rises and dips in the output voltage as the light load changes.