8.2.2.2 Selecting the Hot Swap FET(s)

It is critical to select the correct MOSFET for a hot swap design. The device must meet the following requirements:

• The V_DS rating should be sufficient to handle the maximum system voltage along with any ringing caused by transients. For most 12-V systems a 30-V FET is a good choice.
• The SOA of the FET should be sufficient to handle all usage cases: start-up, hot-short, start into short.
• R_DSON should be sufficiently low to maintain the junction and case temperature below the maximum rating of the FET. TI recommends to keep the steady state FET temperature below 125°C to allow margin to handle transients.
• Maximum continuous current rating should be above the maximum load current and the pulsed drain current must be greater than the current threshold of the circuit breaker. Most MOSFETs that pass the first three requirements also pass these two.
• A V_GS rating of ±20 V is required, because the TPS2490 can pull up the gate as high as 16 V above source.

For this design the CSD19532KTT was selected for its low R_DSON and good SOA. After selecting the MOSFET, the maximum steady state case temperature can be computed as follows:

Equation 9.

Note that the R_DSON is a strong function of junction temperature, which for most MOSFETs will be close to the case temperature. A few iterations of the above equations may be necessary to converge on the final R_DSON and T_C,MAX value. According to the CSD17552Q5B datasheet, its R_DSON is approximately 1.4x at 78°C. The Equation 10 uses this R_DSON value to compute the T_C,MAX.

Equation 10.

This maximum steady state case temperature indicates that a second MOSFET is not needed to reduce and distribute power dissipation during normal operation.

For reference, when using parallel MOSFETs, the maximum steady state case temperature can be computed as follows:

Equation 11.

Iterate until the computed T_C,MAX is using two parallel MOSFETs is less than to the junction temperature assumed for R_DSON. Then, no further iterations are necessary.