SNVS723G October 2011 – July 2018 LMZ10500
In addition to cycle by cycle current limit, the LMZ10500 features a second level of short circuit protection. If the load pulls the output voltage down and the feedback voltage falls to 0.375 V, the output short circuit protection will engage. In this mode the internal PFET switch is turned OFF after the current limit comparator trips and the beginning of the next cycle is inhibited for approximately 230 µs. This forces the inductor current to ramp down and limits excessive current draw from the input supply when the output of the regulator is shorted. The synchronous rectifier is always OFF in this mode. After 230 µs of non-switching a new startup sequence is initiated. During this new startup sequence the current limit is gradually stepped up to the nominal value as illustrated in the Start-up Behavior and Soft Start section. After the startup sequence is completed again, the feedback voltage is monitored for output short circuit. If the short circuit is still persistent after the new startup sequence, switching will be stopped again and there will be another 230 µs off period. A persistent output short condition results in a hiccup behavior where the LMZ10500 goes through the normal startup sequence, then detects the output short at the end of startup, terminates switching for 230 µs, and repeats this cycle until the output short is released. This behavior is illustrated in Figure 11.
Because the output current is limited during normal startup by the softstart function, the current charging the output capacitor is also limited. This results in a smooth VOUT ramp up to nominal voltage. However, using excessively large output capacitance or VCON capacitance under normal conditions can prevent the output voltage from reaching 0.375 V at the end of the startup sequence. In such cases the module will maintain the described above hiccup mode and the output voltage will not ramp up to final value. To cause this condition, one would have to use unnecessarily large output capacitance for 650mA load applications. See the Input and Output Capacitor Selection section for guidance on maximum capacitances for different output voltage settings.