Soft-start refers to the ramp-up characteristic of the output voltage during LDO turn-on after EN and UVLO achieve threshold voltage. The soft start current is fixed for fixed output voltage versions.

Although the device does not have any sequencing requirement, following the sequencing order of BIAS, IN, and EN makes sure that the soft start starts from zero.

Figure 8-1 shows an example of the device behavior when the EN pin is enabled prior to having either power supply up. Under this condition, the output jumps from 0 V to approximately 0.3 V almost instantly when the IN voltage is sufficient to power the circuit.

Figure 8-1 Sequencing and Soft-Start Behavior

As shown in Figure 8-2, connecting EN to IN, in conjunction with a slow input or output voltage ramp, can have undesired behavior in the application. For a smooth IN and OUT ramp, consider using the EN pin separate from the IN voltage.

Figure 8-2 V_EN = V_IN Behavior

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, which is not recommended. However, Equation 5 can estimate this soft-start 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