The NR/SS pin is the input to the inverting terminal of the error amplifier, see the Functional Block Diagram. A resistor connected from this pin to GND sets the output voltage by the pin internal reference current I_NR/SS, V_OUT = I_NR/SS × R_NR/SS. Connecting a capacitor from this pin to GND significantly reduces the output noise, limits the input inrush-current, and soft-starts the output voltage. Use the minimum value or larger capacitor from NR/SS to ground as listed in the Electrical CharacteristicsElectrical CharacteristicsElectrical CharacteristicsElectrical CharacteristicsElectrical CharacteristicsElectrical CharacteristicsElectrical Characteristics table and place the NR/SS capacitor as close to the NR/SS and GND pins of the device as possible.

The device features a programmable, monotonic, voltage-controlled, soft-start circuit that is set to work with an external capacitor (C_NR/SS). In addition to the soft-start feature, the C_NR/SS capacitor also lowers the output voltage noise of the LDO. The soft-start feature can be used to eliminate power-up initialization problems. The controlled output voltage ramp also reduces peak inrush current during start up, minimizing start-up transients to the input power bus.

To achieve a monotonic start up, the device output voltage tracks the V_NR/SS reference voltage until this reference reaches the set value (the set output voltage). The V_NR/SS reference voltage is set by the R_NR/SS resistor and, during start up, using a fast charging current (I_{FAST_SS}) in addition to the I_NR/SS current, as shown in Figure 8-10, to charge the C_NR/SS capacitor.

Figure 8-10 Simplified Soft-Start Circuit

The 2.1-mA (typical) I_{FAST_SS} current and 150 μA (typical) I_NR/SS current quickly charge C_NR/SS until the voltage reaches approximately 93% of the set output voltage, then the I_{FAST_SS} current disengages and only the I_NR/SS current continues to charge C_NR/SS to the set output voltage level. If there is any error during start up or the output overshoot prevention circuit is triggered, the NR/SS discharge FET turns on, thus discharging the C_NR/SS capacitor to protect both the LDO and the load.

The soft-start ramp time depends on the fast start-up (I_{FAST_SS}) charging current, the reference current (I_NR/SS), C_NR/SS capacitor value, and the set (targeted) output voltage (V_OUT(target)). Equation 3 calculates the soft-start ramp time.

The I_NR/SS current is provided in the Electrical CharacteristicsElectrical CharacteristicsElectrical CharacteristicsElectrical CharacteristicsElectrical CharacteristicsElectrical CharacteristicsElectrical Characteristics table and has a value of 150 μA (typical). The I_{FAST_SS} current has a value of 2 mA (typical) for V_IN > 2.5 V. Figure 8-11 and Figure 8-12 depict the I_NR/SS and I_{FAST_SS} current versus V_IN and temperature.

Figure 8-11 I_NR/SS Reference vs Input Voltage and Temperature for V_OUT = 3.3 V

Figure 8-12 I_{FAST_SS} Reference vs Input Voltage and Temperature for V_OUT = 3.3 V

Because the error amplifier is always operating in unity-gain configuration, the output voltage noise can only be adjusted by increasing the C_NR/SS capacitor. The C_NR/SS capacitor and R_NR/SS resistor form a low-pass filter (LPF) that filters out the noise from the V_NR/SS voltage reference, thereby reducing the device noise floor. The LPF is a single-pole filter and Equation 4 calculates the LPF cutoff frequency. Increasing the C_NR/SS capacitor can significantly lower output voltage noise; however, doing so greatly lengthens start-up time. For low-noise applications, use a 4.7-μF C_NR/SS for optimal noise and start-up time trade off.

The Typical Characteristics section illustrates the impact of the C_NR/SS capacitor on the LDO output voltage noise.

Figure 8-13 illustrates the relationship, timing, and output voltage value during the start-up phase.

Figure 8-13 Relationship Between Threshold Voltage, Output Voltage, I_{FAST_SS}, and I_NR/SS During Start-Up