An internal buck-boost converter generates the regulated negative VEE voltage. The buck-boost converter operation is determined by the sensed VEE voltage on FBVEE pin. With an internal 90k resistor and a 4.5V reference voltage, VEE voltage can be programed and regulated between -2V to -8V.

The buck-boost converter is controlled by an integrated hysteresis voltage feedback loop for COM-to-VEE voltage regulation and an integrated current control loop for cycle-to-cycle current limit. When FBVEE voltage stays below the turn-off threshold, the buck-boost converter operates with peak current mode control. The inductor current increases at the beginning of a switching cycle until it reaches the peak current limit, then returns to zero. In normal operation, the converter operates in boundary conduction mode, but can enter continuous conduction mode during start-up. As the peak current of the buck-boost is limited to less than I_LIM, the chosen inductor must have a saturation current above I_LIM. As shown in the Electrical Characteristics table, the peak current limit, I_LIM, is implemented with a feedforward control based on VDD voltage, so that the maximum inductor current will not exceed I_LIM, considering the overshoot due to the control loop delay at different VDD voltages. A higher VDD voltage leads to a bigger overshoot, and thus results in a lower I_LIM value for compensation. The recommended inductor selection is between 3.0μH and 10.0μH. The typical switching frequency is f_{SW_VEE} under the conditions listed in the Electrical Characteristics.

With the higher buck-boost peak current limit (I_LIM)of UCC35131-Q1. It is recommended to connect a low parasitic inductance schottky diode between VEE and BSW to reduce internal body diode loss and help improve overall efficiency.

After the FBVEE voltage reaches the turn-off threshold, the buck-boost converter turns off. After the FBVEE voltage drops below the turn-on threshold due to the load current, the buck-boost converter is turned on again. With the accurate voltage reference and hysteresis control, the VEE output voltage can be regulated with ≤5% accuracy.

Figure 7-4 COM-VEE Voltage Regulation Functional Block Diagram

Figure 7-5 Concept of COM-VEE Regulation Scheme