The TPS92643-Q1 contains both high-side and low-side N-channel MOSFETs. The high-side gate driver works in conjunction with an internal bootstrap diode and an external bootstrap capacitor, C_BST. During the on-time of the low-side MOSFET, the SW pin voltage is approximately 0 V and C_BST is charged from the VCC supply through the internal diode and external R_BST resistor. TI recommends a 0.1-µF to 2.2-µF capacitor and 2.2-Ω to 10-Ω resistor connected in series between the BST and SW pins.

A larger capacitor is required to prevent a bootstrap undervoltage fault when operating at low PWM dimming frequencies. Noise due to stored charge is reduced by the R_BST. In addition, the R_BST resistor allows optimization of EMI with respect to efficiency. A larger RBST resistor results in lower SW node rise time and allows energy in SW node harmonics to roll off near 100-MHz frequency. Switching with slower slew rate also decreases the efficiency.