Table 8-1 summarizes the design parameters that must be known before designing a hotswap circuit. When charging the output capacitor through the hotswap MOSFET, the FET’s total energy dissipation equals the total energy stored in the output capacitor (1 / 2CV²). Thus, both the input voltage and output capacitance determine the stress experienced by the MOSFET during start-up. The maximum load current drives the current limit and sense resistor selection. Additionally, the maximum load current, maximum ambient temperature, and thermal properties of the PCB (R_θCA) influence the selection of the MOSFET, including the R_DSON and he number of MOSFETs used. R_θCA is a strong function of the layout and the amount of copper that is connected to the drain of the MOSFET. Note that the drain is not electrically connected to the ground plane; therefore, the ground plane cannot be used to aid in heat dissipation. This design example uses R_θCA = 25°C/W, which is similar to the LM5066H1 and LM5066H2 evaluation modules. It is a good practice to measure the R_θCA of a given design after the physical PCBs are available.

Finally, it is important to understand what test conditions the hotswap needs to pass. In general, a hotswap is designed to pass both a hot-short and a start into a short, which are described in the previous section. Also, TI recommends keeping the load OFF until the hotswap is fully powered up. Starting the load early causes unnecessary stress on the MOSFET and could lead to MOSFET failures or a failure to start up.