The buck-boost converter generates the negative voltage to turn off the direct-drive GaN device. While it is controlled internally, it requires an external power inductor and output capacitor. The converter is designed to use a 4.7-µH inductor and a 2.2-µF output capacitor.
As the peak current of the buck-boost is subject to two different peak current limits which are 0.4 A and 1 A for low and high frequencies (see Section 9.3.3), the inductor must have a saturation current well above the rated peak current limit. Once the higher limit is established at higher switching frequency, the current limit will not go down again even when GaN device is back to lower switching frequency. Therefore, it is critical to select the inductor according to maximum GaN switching frequency.
The buck-boost converter uses a peak current hysteretic control. As shown in Figure 10-2, the inductor current increases at the beginning of a switching cycle until the inductor reaches the peak current limit. The inductor current will go down to zero. The idle time between each current pulse is determined automatically by the output current, and can be reduced to zero. Therefore, the maximum output current happens when the idle time is zero, and is decided by the peak current but irrelevant to the inductor value.
A minimum inductance value of 3 µH is preferred for the buck-boost converter so that the di/dt across the inductor is not too high. This leaves enough margin for the control loop to respond. As a result, the maximum di/dt of the inductor is limited to 6 A/µs. On the other hand, large inductance also limits the transient response for stable output voltage, and it is preferred to have inductors less than 10 µH.