UCC28750 is a fixed frequency controller under normal operating conditions, but modulates the switching frequency and CS thresholds at higher and lighter loads, providing a momentary boost of power for high loads and increased efficiency at light loads.The frequency control law uses alternating amplitude and frequency modulating regions in conjunction with an amplitude modulated current sense threshold to achieve regulation at varying conditions.

There are five regions of operation, referred to as section A-E in Figure 7-9 .

Figure 7-9 UCC28750 Control Law

1. Region A: This region is the power boost region in which the frequency increased to the maximum of 130/200kHz (based on the device variant) and the current sense threshold is fixed to the V_CS(limit) of 900mV.
2. Region B: This is the normal operating region, where the switching frequency is fixed to 65/100kHz, but the current sensing threshold is amplitude modulated to provide the necessary power required by the application load.
3. Region C: This is the frequency foldback region, where the switching frequency and current sensing threshold are being lowered, to accommodate lower power requirements and lower switching losses.
4. Region D, E: These two regions are the burst mode operation of the controller. When the FB voltage is lower than 1.2V,V3Burst, but higher than 1.1V, V4Stop, the controller continues to switch but at the lowest current sensing threshold and frequency. When the FB pin voltage is below V4Stop, the controller stops switching operation. Without switching the FB pin rises as the output voltage drops, and the controller resumes switching operation.