In a synchronous buck or other converter where the low-side switch occasionally operates in third-quadrant, the bootstrap supply charges through a path that includes the third-quadrant voltage drop of the low-side LMG352xR030-Q1 during the dead time as shown in Figure 11-1. This third-quadrant drop can be large, which may over-charge the bootstrap supply in certain conditions. The VDD supply of LMG352xR030-Q1 must be kept below 18 V.
As shown in Figure 11-2, the recommended bootstrap supply includes a bootstrap diode, a series resistor, and a 16-V TVS or zener diode in parallel with the VDD bypass capacitor to prevent damaging the high-side LMG352xR030-Q1. The series resistor limits the charging current at start-up. and when the low-side device is operating in third-quadrant mode. This resistor must be selected to allow sufficient current to power the LMG352xR030-Q1 at the desired operating frequency. At 100-kHz operation, a value of approximately 2 Ω is recommended. At higher frequencies, this resistor value should be reduced or the resistor omitted entirely to ensure sufficient supply current.