SLUSAE5G August 2011 – April 2021 TPS53355
The external components selection is a simple process when using organic semiconductors or special polymer output capacitors.
Select operation mode and soft-start time using Table 7-3.
Select the switching frequency from 250 kHz to 1 MHz using Table 7-1.
The inductance value should be determined to give the ripple current of approximately 1/4 to 1/2 of maximum output current. Larger ripple current increases output ripple voltage and improves signal-to-noise ratio and helps ensure stable operation, but increases inductor core loss. Using 1/3 ripple current to maximum output current ratio, the inductance can be determined by Equation 8.
The inductor requires a low DCR to achieve good efficiency. It also requires enough room above peak inductor current before saturation. The peak inductor current can be estimated in Equation 9.
Refer to Section 18.104.22.168.3 to select external components because ceramic output capacitors are used in this design.
The overcurrent setting resistor, RTRIP, can be determined by Equation 10.
Use an RDS(on) value of 1.5 mΩ for an overcurrent level of approximately 30 A. Use an RDS(on) value of 1.7 mΩ for overcurrent level of approximately 10 A.
The recommended BST resistor value is 2 Ω and anything larger than 5.1 Ω is not recommended. Note that when the gate drive turns on, the voltage on the boot-strap capacitor splits between the internal pull-up resistance and the boot-strap resistance, with the internal circuits only seeing the portion across the internal pull-up resistance. Therefore, when the external resistor gets larger than the pull-up resistance, it crashes the head-room of the SW to BOOT logic, which can cause logic issues with the high-side gate driver.