JAJSDC3A June 2017 – August 2018 UCC27712
PRODUCTION DATA.
Resistor RBOOT is selected to limit the current in DBOOT and limit the ramp up slew rate of voltage of HB-HS to avoid the phenomenon shown in Figure 45. It is recommended when using the UCC27712 that RBOOT is between 2 Ω and 20 Ω. For this design we selected an RBOOT current limiting resistor of 2.2 Ω. The bootstrap diode current (IDBOOT(pk)) was limited to roughly 5.0 A.
The power dissipation capability of the bootstrap resistor is important. The bootstrap resistor must be able to withstand the short period of high power dissipation during the initial charging sequence of the boot-strap capacitor. This energy is equivalent to 1/2 × CBOOT × V2. This energy is dissipated during the charging time of the bootstrap capacitor (~3 × RBOOT × CBOOT). Special attention must be paid to use a bigger size RBOOT when a bigger value of CBOOT is chosen.