A capacitor connected to this pin reduces the low frequency noise of the converter and sets the soft-start time. The larger the capacitor, the lower the noise and the longer the start-up time of the converter. A 470-nF capacitor is typically connected to this pin for a start-up time of 5 ms, although longer and shorter start-up times can be used. During soft start with a light load, the device skips switching pulses as needed to not discharge the output voltage. The device can start into a pre-biased output voltage.
The device achieves low noise by adding an R-C filter to the reference voltage, as shown in Section 7.2. During start-up, the NR/SS capacitor is charged with a constant current of 75 µA (typ) to 0.8 V. Larger NR/SS capacitors provide for lower low frequency noise, as shown in Figure 6-29. The maximum NR/SS cap is 3.3 µF for a start-up time of 35 ms. The minimum start-up time is set internally to 0.7 ms, which occurs when there is a small NR/SS capacitor or no NR/SS capacitor.