7.4 Device Functional Modes

The UCC28742 operates in different modes according to input voltage, VDD voltage, and output load conditions:

• At start-up, when VDD is less than the turn-on threshold, V_VDD(on) , the device is simply waiting for VDD to reach this threshold while the VDD capacitor is getting charged.
• When VDD exceeds V_VDD(on), the device starts switching to deliver power to the converter output. The initial 3 switching cycles control the primary-peak current to I_PP(min). This allows sensing any initial input or output faults with minimal power delivery. When confirmed with input voltage above predetermined level and no fault conditions, start up process proceeds and normal power conversion follows. The converter will remain in discontinuous current mode operation during charging of the output capacitor(s), maintaining a constant output current, I_OCC, until the output voltage reaches its regulation point. The maximum time duration when I_O stays on I_OCC can only last 120 ms, and when 120-ms timer out, the device will initiate shutdown. Therefore, a design needs to make sure the maximum time when output current reaches and stays on I_OCC does not exceed 120 ms during start. For more details refer to Constant Current Limit and Delayed Shutdown
• When operating with I_PP = I_PP(max), the UCC28742 operates continuously in the run state. In this state, the VDD bias current is always at I_RUN plus the average gate-drive current.
• When operating with I_PP < I_PP(max), the UCC28742 operates in the wait state between switching cycles and in the run state during a switching cycle. In the wait state, the VDD bias current is reduced to I_WAIT after demagnetizing time of each switching cycle to improve efficiency at light loads. This helps reduce no-load to medium-load power losses, particularly for achieving higher efficiency at 10%, 25% load conditions, and possible at < 50% load conditions, depending on a design.
• The device operation will stop if any events occur as listed below:
  • If VDD drops below the V_VDD(off) threshold, the device stops switching, its bias current consumption is lowered to I_START until VDD rises above the V_VDD(on) threshold. The device then resumes operation through start-up.
  • If a fault condition is detected, the device stops switching and its bias current consumption becomes I_FAULT. This current level discharges VDD to V_VDD(off) where the bias current changes from I_FAULT to I_START until VDD rises above the V_VDD(on) threshold.
• If a fault condition persists, the operation sequence described above in repeats until the fault condition or the input voltage is removed. Refer to Fault Protection for fault conditions and post-fault operation.