Inrush current is defined as the current into the LDO at the IN pin during start up. Inrush current then consists primarily of the sum of load current and the current used to charge the output capacitor. This current is difficult to measure because the input capacitor must be removed. Operating without an input capacitor is not recommended because this capacitor is required for stability. However, Equation 5 can be used to estimate this current.

Equation 5.

where:

• V_OUT(t) is the instantaneous output voltage of the turn-on ramp
• dV_OUT(t) / dt is the slope of the V_OUT ramp
• R_LOAD is the resistive load impedance

As illustrated in Figure 8-10, the external capacitor at the NR/SS pin (C_NR/SS) sets the output start-up time by setting the rise time of the V_NR/SS reference voltage.

Inrush current for a no-load condition is given in Figure 6-37 to Figure 6-40.