True slew-rate control

Despite the aforementioned techniques, in some designs high-frequency EMI (60 to 250 MHz) may still not fall under specified standard limits. One way to mitigate and improve the margin in order to pass industry standards is to use a resistor in series with the boot capacitor of the switching converter. Using a resistor reduces the switching-edge slew rates, which reduces EMI, but comes with the expected penalty of reduced efficiency.

Switching converters such as the LM61440-Q1 and LM62440-Q1 are designed such that a resistor can be used to select the strength of the high-side FET’s driver during turn-on. As shown in Figure 23, the current drawn through the RBOOT pin (the teal dotted loop) is multiplied and drawn through from CBOOT (the red dashed line) to turn on the high-side power MOSFET. By doing this, the resistor can control the slew rate, but not suffer the efficiency loss that happens when a series BOOT resistor runs the majority of the current. With RBOOT short-circuited to CBOOT, the rise time is rapid; switch-node harmonics will not roll off until above 150 MHz. If CBOOT and RBOOT remain connected through 700 Ω, the slewing time increases to 10 ns when converting 13.5 V to 5 V. This slow rise time enables the energy in switch-node harmonics to roll off near 50 MHz under most conditions.