8.2.1.2.4 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 × V_DS < P_LIM) the maximum start time can be computed with Equation 11.

Equation 11.

For most designs (including this example), I_LIM × V_DS > P_LIM, so the hot swap starts in power limit and transition into current limit. In that case, the estimated start time can be computed with Equation 12.

Equation 12.

Note that the above start-time assumes constant, typical current limit and power limit values. The actual startup time is slightly longer, as the power limit is a function of Vds and decreases as the output voltage increases. To ensure that the timer never times out during start-up, TI recommends setting the minimum fault time (t_flt) to be greater than the start time (t_start) by adding an additional margin of 50% of the fault time. This accounts for the variation in power limit, timer current, and timer capacitance. Thus C_TIMER can be computed with Equation 13.

Equation 13.

The next largest available C_TIMER is chosen as 150 nf. Once the C_TIMER is chosen the actual programmed fault time can be computed with Equation 14.

Equation 14.

This is the typical time that the LM5069 shuts off the CSD19536KTT MOSFET.