7.3.9 Soft Start

The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turnon, after V_CC reaches the undervoltage threshold, an 8 µA internal current source charges up an external capacitor C_SS connecting to the SS pin. The ramping voltage at the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the output voltage V_OUT in a controlled manner.

The soft start time duration to reach steady-state operation is given by Equation 7:

This equation can be rearranged as follows:

Use of a 4.7-nF capacitor results in a 0.5-ms soft-start duration. This is a recommended value. Note that high values of C_SS capacitance cause more output voltage drop when a load transient goes across the DCM-CCM boundary. If a fast load transient response is desired for steps between DCM and CCM mode the softstart capacitor value should be less than 18 nF (which corresponds to a soft-start time of 1.8 ms).

An internal switch grounds the SS pin if any of the following three cases happens: (i) V_CC is below the UVLO threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the output voltage can be shut off by connecting the SS pin to ground using an external switch. Releasing the switch allows the SS pin to ramp up and the output voltage to return to normal. The shutdown configuration is shown in Figure 19.

Figure 19. Alternate Shutdown Implementation