8.2.2.4 Set Fault Timer

The fault timer runs when the hot swap is in power limit or current limit, which is the case during start-up. Thus the timer has to be sized large enough to prevent a time-out during start-up. If the part starts directly into current limit (I_LIM x V_DS < P_LIM) the maximum start time can be computed with Equation 17:

Equation 17.

For most designs (including this example) I_LIM x V_DS > P_LIM so the hot swap will start in power limit and transition into current limit. In that case the start time can be computed as follows:

Equation 18.

The actual startup time is slightly longer, as the power limit is a function of V_DS and decreases as the output voltage increases. To ensure that the timer never times out during start-up, TI recommends to set the fault time (t_flt) to be 1.75 x t_start or 5.11ms. This accounts for the variation in power limit, timer current, and timer capacitance. Thus C_TIMER can be computed as follows:

Equation 19.

The next largest available C_TIMER is chosen as 33 nF. Once the C_TIMER is chosen the actual programmed fault time can be computed as follows:

Equation 20.