When the switch enables, the charge up the output capacitance from 0 V to the set value (1.8 V in this example). This charge arrives in the form of inrush current. Calculate inrush current using Equation 2.

Where:

• C_L is the output capacitance.
• dVOUT is the change in VOUT during the ramp up of the output voltage when device is enabled. Because rise time is 10% of VOUT to 90% of VOUT, this is 80% of the VIN value.
• dt is the rise time in VOUT during the ramp up of the output voltage when the device is enabled.

The TPS22997 offers an adjustable rise time for VOUT, allowing the user to control the inrush current during turn on. Calculate the appropriate rise time using the design requirements and the inrush current equation as shown below.

To ensure an inrush current of less than 1.5 A, a C_T setting that yields a rise time of more than 586 µs must be chosen. By using a 1000pF capacitance, a rise time of 819 μs is selected, limiting the inrush current to below 1.5 A.