8.1.3 Optimizing Noise and PSRR

Improve the ultra-low noise floor and PSRR of the device by careful selection of:

• C_NR/SS for the low-frequency range
• C_FF in the midband frequency range
• C_OUT for the high-frequency range
• V_IN – V_OUT for all frequencies, and
• V_BIAS at lower input voltages

A larger noise-reduction capacitor improves low-frequency PSRR by filtering any noise coupling from the input into the reference. To improve midband PSRR, use the feed-forward capacitor to place pole-zero pair near the edge of the loop bandwidth and push out the loop bandwidth. Use larger output capacitors to improve high-frequency PSRR.

A higher input voltage improves the PSRR by giving the device more headroom to respond to noise on the input. A bias rail also improves PSRR at lower input voltages because greater headroom is provided for the internal circuits.

The noise-reduction capacitor filters out low-frequency noise from the reference, and the feed-forward capacitor reduces output voltage noise by filtering out the midband frequency noise. However, a large feed-forward capacitor can create new issues that are discussed in Pros and Cons of Using a Feed-Forward Capacitor with a Low Dropout Regulator.

Use a large output capacitor to reduce high-frequency output voltage noise. Additionally, a bias rail or higher input voltage improves the noise because greater headroom is provided for the internal circuits.

Table 4 lists the output voltage noise for the 10-Hz to 100-kHz band at a 5.0-V output for a variety of conditions with an input voltage of 5.5 V, an R₁ of 12.1 kΩ, and a load current of 2 A. The 5.0-V output was used because this output is the worst-case condition for output voltage noise.