For a successful design, the junction temperature of device must be kept below the absolute maximum rating during both dynamic (start-up) and steady-state conditions. Dynamic power stresses often are an order of magnitude greater than the static stresses, so it is important to to determine that power dissipation is below a certain limit to avoid thermal shutdown during start-up.
Slew rate is 5 V/ms typically for TPS25961. The inrush current can be calculated as:
Equation 9.
The average power dissipation inside the part during inrush can be calculated as:
Equation 10.
For the given power dissipation, the thermal shutdown time of the device must be greater than the ramp-up time tR to avoid start-up failure. Figure 8-2 shows the thermal shutdown limit, for 0.03 W of power, the shutdown time is very large as compared to tR = 2.4 ms. Therefore this application will have successful startup.
Figure 8-2 Time to Thermal Shutdown vs Power Dissipation