The current limit threshold can be changed dynamically by altering the resistance going from the current limit pin to the ground of the device on the fly. This alteration allows the system to have a different current limit for start-up, when there can be significant inrush current, and during normal operation. The way this is commonly done is by putting two resistors in parallel on the ILIM pin and having a switch to enable or disable one of the resistors. This set-up can be seen in Figure 8-2. Alternatively, a digital potentiometer can be used to adjust the impedance on the ILIM pin on the fly. Care must be taken so that the capacitance on the ILIM pin is below approximately 100pF to keep the current regulation loop stable. The most common application where this feature is useful is capacitive loads.

Figure 8-2 Dynamic Changing Current Limit Setup

In a capacitive charging case, the initial current to charge the capacitor is the inrush current. Depending on the system requirements, dynamically changing the current limit can help either charge up a capacitor faster or charge up a larger capacitor. To allow a higher inrush level of current through in the beginning, the switch can be closed making the current limit be according to the equation below.

When the inrush event is over and the output voltage is charged up, the switch opens and the current limit is just the R_ILIM1 equivalent level. This timing can be seen in Figure 8-3.

Figure 8-3 Capacitive Charging Changing Current Limit

Alternatively, if the switch is open, the current limit starts out at a lower value and then the switch can be closed when the capacitance gets charged up. This lower current limit level allows higher value capacitance to be charged up. The timing diagram can be seen in Figure 8-4.

Figure 8-4 Large Capacitive Charging Changing Current Limit