7.4.3 Fault Timer and Restart

When the current limit or power limit threshold is reached during turnon or as a result of a fault condition, the gate-to-source voltage of Q1 is modulated to regulate the load current and power dissipation. When either limiting function is activated, an 85-µA fault timer current source charges the external capacitor (C_T) at the TIMER pin as shown in Figure 25 (fault timeout period). If the fault condition subsides during the fault timeout period before the TIMER pin reaches 4 V, the LM5069 returns to the normal operating mode and C_T is discharged by the 2.5-µA current sink. If the TIMER pin reaches 4 V during the fault timeout period, Q1 is switched off by a 2-mA pulldown current at the GATE pin. The subsequent restart procedure then depends on which version of the LM5069 is in use.

The LM5069-1 latches the GATE pin low at the end of the fault timeout period. C_T is then discharged to ground by the 2.5-µA fault current sink. The GATE pin is held low by the 2-mA pulldown current until a power-up sequence is externally initiated by cycling the input voltage (V_SYS), or momentarily pulling the UVLO pin below 2.5 V with an open-collector or open-drain device as shown in Figure 24. The voltage at the TIMER pin must be <0.3 V for the restart procedure to be effective.

The LM5069-2 provides an automatic restart sequence which consists of the TIMER pin cycling between 4 V and 1.25 V seven times after the fault timeout period, as shown in Figure 25. The period of each cycle is determined by the 85-µA charging current, and the 2.5-µA discharge current, and the value of the capacitor C_T. When the TIMER pin reaches 0.3 V during the eighth high-to-low ramp, the 16-µA current source at the GATE pin turns on Q1. If the fault condition is still present, the fault timeout period and the restart cycle repeat.