SNVS723G October 2011 – July 2018 LMZ10500
The LMZ10500 features a current limit based soft-start circuit in order to prevent large in-rush current and output overshoot as VOUT is ramping up. This is achieved by gradually increasing the PFET current limit threshold to the final operating value as the output voltage ramps during startup. The maximum allowed current in the inductor is stepped up in a staircase profile for a fixed number of switching periods in each step. Additionally, the switching frequency in the first step is set at 450 kHz and is then increased for each of the following steps until it reaches 2MHz at the final step of current limiting. This current limiting behavior is illustrated in Figure 10 and allows for a smooth VOUT ramp up.
The soft start rate is also limited by the VCON ramp up rate. The VCON pin is discharged internally through a pull down device before startup occurs. This is done to deplete any residual charge on the VCON filter capacitor and allow the VCON voltage to ramp up from 0V when the part is started. The events that cause VCON discharge are thermal shutdown, UVLO, EN low, or output short circuit detection. The minimum recommended capacitance on VCON is 220 pF and the maximum is 1 nF. The duration of startup current limiting sequence takes approximately 75 µs. After the sequence is completed, the feedback voltage is monitored for output short circuit events.