The LM5181 has a flexible and easy-to-use soft-start control pin, SS/BIAS. The soft-start feature prevents inrush current impacting the LM5181 and the input supply when power is first applied. This is achieved by controlling the voltage at the output of the internal error amplifier. Soft start is achieved by slowly ramping up the target regulation voltage when the device is first enabled or powered up. Selectable and adjustable start-up timing options include a 6-ms internally-fixed soft start and an externally-programmable soft start.

The simplest way to use the LM5181 is to leave SS/BIAS open. The LM5181 employs an internal soft-start control ramp and starts up to the regulated output voltage in 6 ms.

However, in applications with a large amount of output capacitance, higher V_OUT or other special requirements, the soft-start time can be extended by connecting an external capacitor C_SS from SS/BIAS to GND. A longer soft-start time further reduces the supply current needed to charge the output capacitors while sourcing the required load current. When the EN/UVLO voltage exceeds the UVLO rising threshold and a delay of 20 µs expires, an internal current source I_SS of 5 µA charges C_SS and generates a ramp to control the primary current amplitude. Calculate the soft-start capacitance for a desired soft-start time, t_SS, using Equation 12.

Equation 12.

C_SS is discharged by an internal FET when switching is disabled by EN/UVLO or thermal shutdown.