SBVS311A November 2019 – March 2020 TPS7A54
Improve the ultra-low noise floor and PSRR of the device by careful selection of:
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 a zero-pole 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 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 midband frequency noise. However, a large feed-forward capacitor can create new issues that are discussed in the Pros and Cons of Using a Feed-Forward Capacitor with a Low Dropout Regulator application report.
Use a large output capacitor to reduce high-frequency output voltage noise. Additionally, a bias rail or higher input voltage improves 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 4 A. The 5.0-V output is used because this output is the worst-case condition for output voltage noise.
|OUTPUT VOLTAGE NOISE
|11.7||10||10||47 || 10 || 10|
|7.7||100||10||47 || 10 || 10|
|6||100||100||47 || 10 || 10|