SBVS233B January 2016 – June 2021 TPS7A84
The ultra-low noise floor and PSRR of the device can be improved by careful selection of:
A larger noise-reduction capacitor improves low-frequency PSRR by filtering any noise coupling from the input into the reference. The feed-forward capacitor can be optimized to place a pole-zero pair near the edge of the loop bandwidth and push out the loop bandwidth, thus improving mid-band PSRR. Larger output capacitors and various output capacitors can be used 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; see the Figure 6-2 curve. A bias rail also improves the 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 mid-band frequency noise. However, a large feed-forward capacitor can create some new issues that are discussed in the Pros and Cons of Using a Feed-Forward Capacitor with a Low Dropout Regulator application report.
A large output capacitor reduces 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 8-1 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 5.4 V, an R1 of 12.1 kΩ, and a load current of 3 A. The 5.0-V output is chosen because this output is the worst-case condition for output voltage noise.
|OUTPUT VOLTAGE NOISE (µVRMS)||CNR/SS (nF)||CFF (nF)||COUT (µF)|
|11.7||10||10||47 || 10 || 10|
|7.7||100||10||47 || 10 || 10|
|6||100||100||47 || 10 || 10|