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 as described in theSection 8.2.2.2 section. With an internal 45k resistor and a 4.5V reference voltage, VEE voltage can be programed and regulated from 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. The peak current limit, I_LIM, is optimized based on VDD voltage, so that considering the overshoot due to the control loop delay at different VDD voltages, the maximum inductor current does not exceed the maximum I_LIM at the condition without VDD feedforward. A higher VDD voltage leads to a bigger overshoot, and thus results in a lower I_LIM value with VDD feedforward control 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 table.

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.