The NR/SS pin has the dual function of controlling the soft-start time and reducing the noise generated by the internal band-gap reference and the external resistor R_REF. The NR/SS capacitor (C_NR/SS) reduces the output noise to very low levels and sets the output ramp rate to limit inrush current.

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 its set value (the set output voltage). The V_NR/SS reference voltage is set by the R_REF resistor and, during start up, the device uses a fast charging current (I_{FAST_SS}), as shown in Figure 8-4, to charge the C_NR/SS capacitor.

Figure 8-4 Simplified Soft-Start Circuit

The 200-μA (typical) I_NR/SS current quickly charges C_NR/SS until its voltage reaches approximately 97% of the set output voltage, then the I_SS current turns off, the switch between REF and NR/SS closes, and only the I_REF current continues to charge C_NR/SS to its set output voltage level.

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

The I_SS current is provided in the Section 6.6 section and has a value of 200 μA (typical). The I_REF current has a value of 50 μA (typical). The remaining 3% of the start-up time is determined by the R_REF × C_NR/SS time constant. Figure 8-5 shows the PG threshold at start up.

Figure 8-5 PG Threshold During Start-Up

The output voltage noise can be lowered significantly by increasing the C_NR/SS capacitor. The C_NR/SS capacitor and R_REF resistor form a low-pass filter (LPF) that filters out noise from the V_REF 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 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.

Figure 8-6 and Figure 8-7 show the impact of the C_NR/SS capacitor on the LDO output voltage noise.

CIN = 4.7 μF, COUT = 22 μF, VCP_EN = VIN, VIN = 5.3 V, VOUT = 5 V, IOUT = 5 A

Figure 8-6 Output Voltage Noise Density vs C_NR/SS With Charge Pump Enabled

CIN = 4.7 μF, COUT = 22 μF, VIN = 5.3 V, VOUT = 5 V, VBIAS = 11 V

Figure 8-7 Output Voltage Noise Density vs C_NR/SS With Charge Pump Disabled