9.2.2.3 Buck-Boost Converter Design

The Buck-boost converter generates the negative voltage necessary to turn off the direct-drive GaN FET. While it is controlled internally, it requires an external power inductor and output capacitor. The converter is designed to use a 22 µH inductor and a 2.2 µF output capacitor. As the peak current of the buck-boost is limited to less than 350 mA, the inductor chosen must have a saturation current above 350 mA. A Taiyo-Yuden BRC2518T220K 22 µH SMT inductor in a 0806 package is recommended. This inductor is connected between the BBSW pin and ground. A 2.2 µF, 25V 0805 bypass capacitor is required between V_NEG and ground. Due to the voltage coefficient of X7R capacitors, a 2.2 µF capacitor will provide the required minimum 1.0 µF capacitance when operating.