In a synchronous buck, totem-pole PFC, or other converter where the low-side switch occasionally operates in third-quadrant mode, it is important to consider the bootstrap supply. During the dead time, the bootstrap supply charges through a path that includes the third-quadrant voltage drop of the low-side LMG341xR070. This third-quadrant drop can be large, which may over-charge the bootstrap supply in certain conditions. The VDD supply of LMG341xR070 must not exceed 18 V in bootstrap operation.
The recommended bootstrap supply connection includes a bootstrap diode and a series resistor with an optional zener as shown in Figure 18. The series resistor limits the charging current at startup and when the low-side device is operating in third-quadrant mode. This resistor must be chosen to allow sufficient current to power the LMG341xR070 at the desired operating frequency. At 100 kHz operation, a value of approximately 5.1 ohms is recommended. At higher frequencies, this resistor value should be reduced or the resistor omitted entirely to ensure sufficient supply current.
Using a series resistor with the bootstrap supply will create a charging time constant in conjunction with the bypass capacitance on the order of a microsecond. When the dead time, or third-quadrant conduction time, is much lower than this time constant, the bootstrap voltage will be well-controlled and the optional zener clamp in Figure 18 will not be necessary. If a large deadtime is needed, a 14-V zener diode can be used in parallel with the VDD bypass capacitor to prevent damaging the high-side LMG341xR070.