This time constant occuring during the high-side off time explains the dependency on duty cycle. This duty cycle being constant, the bootstrap resistor and bootstrap capacitor should be tuned appropriately to achieve the desired start-up time. Increasing the bootstrap resistor values will increase the time constant leading to slower start-up time.

Additionally, the bootstrap resistor chosen must be able to withstand high power dissipation during the first charging sequence of the bootstrap capacitor. This energy can be estimated by Equation 7:

This energy is dissipated during the charging time of the bootstrap capacitor and can be estimated using Equation 8:

This resistor is essential in limiting the peak currents through the bootstrap diode at start-up and limiting the dv/dt of HB-HS (high-side floating supply to the return high-side floating supply). The peak current through this resistor can be calculated using Equation 9:

Figure 3-4 shows the fast ramp up on VDD (CH4) and HB-HS (CH1) when using a 0-Ohm resistor which leads to undesired change in voltage on LO(CH3) and HO(CH2).

Figure 3-4 VDD/HB-HS Fast Ramp Up (R_boot = 0Ohms)

Figure 3-5 shows how using slightly higher resistor value (R_boot = 2.2Ohms) solve this issue. It is important to note that the bias rising rate observed in Figure 3-5 does not apply to all drivers.

Figure 3-5 VDD/HB-HS Fast Ramp Up (R_boot = 2.2Ohms)