An internal inverting buck-boost converter generates a regulated negative rail for the turn-off supply of the GaN device. The buck-boost converter is controlled by a peak current mode, hysteretic controller. In normal operation, the converter remains in discontinuous-conduction mode, but can enter continuous-conduction mode during start-up and overload the conditions. The converter is controlled internally and requires only a single surface-mount inductor and output bypass capacitor.
The LMG342xR030 supports the GaN operation up to 2.2 MHz. As power consumption is very different in a wide switching frequency range enabled by the GaN device, two peak current limits are used to control the buck-boost converter. The two ranges are separated by IN positive-going threshold frequency. When switching frequency is in the lower range, the peak current is set to the lower value (around 0.4 A) so a smaller inductor can be selected. When switching frequency is in the higher range, the peak current is raised to the higher value (around 1 A) and requires a larger inductor. There is a filter on this frequency detection logic, therefore the LMG342xR030 requires five consecutive cycles at the higher frequency before it is set to the higher buck-boost peak current limit. The current limit does not go down again until power off after the higher limit is set. Even if the switching frequency returns to the lower range, the current limit does not decrease to the lower limit.