To allow optimization of EMI with respect to efficiency, the LM61435-Q1 is designed to allow a resistor to select the strength of the driver of the high-side FET during turn on. See Figure 8-7. The current drawn through the RBOOT pin (the dotted loop) is magnified and drawn through from CBOOT (the dashed line). This current is used to turn on the high-side power MOSEFT.

Figure 8-7 Simplified Circuit Showing How RBOOT Functions

With RBOOT short circuited to CBOOT, rise time is very fast. As a result, SW node harmonics do not "roll off" until above 150 MHz. A boot resistor of 100 Ω corresponds to approximately 2.7-ns SW node rise, and this 100-Ω boot resistor virtually eliminates SW node overshoot. The slower rise time allows energy in SW node harmonics to roll off near 100 MHz under most conditions. Rolling off harmonics eliminates the need for shielding and common mode chokes in many applications. Note that rise time increases with increasing input voltage. Noise due to stored charge is also greatly reduced with higher RBOOT resistance. Switching with slower slew rate also decreases the efficiency.