Using a bootstrap supply for the high-side LMG341xR070 places additional constraints on the startup of the circuit. Before the high-side LMG341xR070 functions correctly, its VDD, LDO5V and VNEG power supplies must start up and be functional. Prior to the device powering up, the GaN device operates in cascode mode with reduced performance. In particular, under high drain slew rate (dv/dt), the transistor can conduct to a small extent and cause additional power dissipation. The correct startup procedure for a bootstrap-supplied half-bridge depends on the circuit used.
In a buck converter without pre-bias, where the initial output voltage is zero, the startup procedure is straightforward. In this case, before switching begins, turn on the low-side device to allow the high-side bootstrap transistor to charge up. When the FAULT signal goes high, the high-side device has powered up completely, and normal switching can begin.
In a boost converter or a buck converter with a pre-biased output, it is necessary to operate the circuit in switching PWM mode while the high-side LMG341xR070 is powering up. With a boost converter, if the low-side device is held on, the power inductor current will likely run away and the inductor will saturate. To start up a boost converter, the duty cycle has to be very low and gradually increase to charge the output to the desired value without the inductor current reaching saturation. This pulse sequence can be performed open-loop or using a current-mode controller. This startup mode is standard for boost-type converters.
However, with the LMG341xR070, during the boost converter startup, significant shoot-through current can occur for high drain slew rates while starting up. This shoot-through current is approximately 1.25 µC per switching event at 50 V/ns, and is comparable to a reverse-recovery event. If this shoot-through current is undesirable, the drain slew rate of the low-side device must be reduced during startup. In Figure 14, the FAULT output from the high-side device is used to gate MOSFET Q1. When FAULT from the high-side is high, once the device is powered up, Q1 turns on and reduces the effective resistance connected to RDRV on the low-side LMG341xR070. With this circuit, the dv/dt of the low-side device can be held low to reduce power dissipation and reduce ringing during high-side startup, but then increase to reduce switching loss during normal operation.