9.2.2.5 Inrush Current

Use Equation 9 to determine how much inrush current is caused by the C_L capacitor.

Equation 9.

where

• I_INRUSH is the amount of inrush caused by C_L
• C_L is the capacitance on VOUT
• dt is the output voltage rise time during the ramp up of VOUT when the device is enabled
• dV_OUT is the change in V_OUT during the ramp up of VOUT when the device is enabled

The appropriate rise time can be calculated using the design requirements and the inrush current equation. As the rise time (measured from 10% to 90% of V_OUT) is calculated, this is accounted in the dV_OUT parameter (80% of V_OUT = 4 V) as shown in Equation 10.

To ensure an inrush current of less than 400 mA, choose a CT value that yields a rise time of more than 220 μs. Refering to the Table 2 at V_IN = 5 V, CT = 220 μF provides a typical rise time of 650 μs. Adding this rise time and voltage into Equation 9, yields Equation 12.

This inrush current can be seen in the Application Curves section. An appropriate C_L value must be placed on VOUT such that the I_MAX and I_PLS specifications of the device are not violated.