Noise can be generally defined as any unwanted signal combining with the desired signal (such as the regulated LDO output) that results in degraded power-supply source quality. Noise can be easily noticed in audio as a hissing or popping sound. Extrinsic and intrinsic are the two basic groups that noise can be categorized into. Noise produced from an external circuit or natural phenomena such as 50 to 60 hertz power-line noise (spikes), along with its harmonics, is an excellent representative of extrinsic noise. Intrinsic noise is produced by components within the device circuitry such as resistors and transistors. For this device, the two dominating sources of intrinsic noise are the error amplifier and the internal reference voltage (V_REF). Another term that sometimes combines with extrinsic noise is PSRR, which refers to the ability of the circuit or device to reject or filter out input supply noise and is expressed as a ratio of output voltage noise ripple to input voltage noise ripple.

Optimize the device intrinsic noise and PSRR by carefully selecting:

• C_NR/SS for the low-frequency range up to the device bandwidth
• C_OUT for the high-frequency range close to and higher than the device bandwidth
• Operating headroom, V_IN – V_OUT (V_OpHr), mainly for the low-frequency range up to the device bandwidth, but also for higher frequencies to a less effect

The device noise performance can be significantly improved by using a larger C_NR/SS capacitor to filter out noise coupling from the input into the device V_REF reference. This coupling is especially apparent from low frequencies up to the device bandwidth. The low-pass filter formed by C_NR/SS and R_REF can be designed to target low-frequency noise originating in the input supply. One downside of a larger C_NR/SS capacitor is a longer start-up time. The device unity-gain configuration eliminates the noise performance degradation that other LDOs suffer from because of their feedback network. Furthermore, increasing the device load current has little to no effect on the device noise performance.

Further improvement to the device noise at a higher frequency range than the device bandwidth can be achieved by using a larger C_OUT capacitor. However, a larger C_OUT increases inrush current and slows down the device transient response.

These behaviors are described in the Section 6.6 section. Figure 6-17 and Figure 6-19 list the measured 10-Hz to 100-kHz RMS noise for a 5-V device and a 0.5-V output voltage with a 300-mV headroom for different C_NR/SS and C_OUT conditions with a 5-A load current. Table 8-2 and Table 8-3 list the typical output noise for these capacitors.

Increasing the operational headroom between V_IN and V_OUT has little to no effect on improving noise performance. However, this increase does improve PSRR significantly for frequency ranges up to the device bandwidth. Higher headroom can also improve transient performance of the device as well. Although C_OUT has little to no affect on improving PSRR at low frequency, C_OUT can improve PSRR for higher frequencies beyond the device bandwidth. A larger C_OUT can also lengthen start-up time and increase start-up inrush current. A combination of capacitors, such as 470 μF || 22 μF is more effective because a combination provides lower ESR and ESL. This behavior is illustrated in Figure 6-12.