SLUSC60B December 2017 – October 2019 UCC28064A
The UCC28064A peak limit comparator senses the total input current and is used to protect the MOSFETs during inrush and over-load conditions. For reliability, the peak current limit (IPEAK) threshold in this design is set for 120% of the nominal maximum current that will be observed during power up, as shown in Equation 34.
A standard 15-mΩ metal-film current-sense resistor will be used for current sensing, as shown in Equation 35. The estimated power loss of the current-sense resistor (PRS) is less than 0.25 W during normal operation, as shown in Figure 8.
The most critical parameter in selecting a current-sense resistor is the surge rating. The resistor needs to withstand a short-circuit current larger than the current required to open the fuse (F1). I2t (ampere-squared- seconds) is a measure of thermal energy resulting from current flow required to melt the fuse, where I2t is equal to RMS current squared times the duration of the current flow in seconds. A 4-A fuse with an I2t of 14 A2s was chosen to protect the design from a short-circuit condition. To ensure the current-sense resistor has high-enough surge protection, a 15-mΩ, 500-mW, metal-strip resistor was chosen for the design. The resistor has a 2.5-W surge rating for 5 seconds. This result translates into 833 A2s and has a high-enough I2t rating to survive a short-circuit before the fuse opens, as described in Equation 37.