The TPS74901 features a programmable, monotonic, voltage-controlled soft-start that is set with an external capacitor (C_SS). This feature is important for many applications because power-up initialization problems are eliminated when powering FPGAs, DSPs, or other processors. The controlled voltage ramp of the output also reduces peak inrush current during start-up, minimizing start-up transient events to the input power bus.

To achieve a linear and monotonic soft-start, the TPS74901 error amplifier tracks the voltage ramp of the external soft-start capacitor until the voltage exceeds the internal reference. The soft-start ramp time is dependent on the soft-start charging current (I_SS), soft-start capacitance (C_SS), and the internal reference voltage (V_REF), and can be calculated using Equation 2.

Equation 2.

If large output capacitors are used, the device current limit (I_CL) and the output capacitor can set the start-up time. In this case, the start-up time is given by Equation 3:

Equation 3.

where:

• V_OUT(NOM) is the nominal set output voltage
• C_OUT is the output capacitance
• I_CL(MIN) is the minimum current limit for the device

In applications where monotonic start-up is required, the soft-start time given by Equation 2 must be set to be greater than Equation 3.

The maximum recommended soft-start capacitor is 0.015µF. Larger soft-start capacitors can be used and do not damage the device; however, the soft-start capacitor discharge circuit can possibly be unable to fully discharge the soft-start capacitor when enabled. Soft-start capacitors larger than 0.015µF can be a problem in applications where the enable pin must be rapidly pulsed while still requiring the device to soft-start from ground. C_SS must be low-leakage; X7R, X5R, or C0G dielectric materials are preferred. See Table 7-1 for suggested soft-start capacitor values.

Another option for setting the start-up rate is to use a feedforward capacitor. See the Pros and Cons of Using a Feedforward Capacitor with a Low-Dropout Regulator application note for more information.