The first step to understanding POWER SUPPLIES ICs! | ⑥ To create a clean POWER SUPPLIES: What is the ripple rejection ratio?

The ripple rejection ratio is the ratio at which the input voltage ripple (Δvin) is rejected (attenuated) by the output voltage.
PSRR (Power Supply Ripple Rejection, Power Supply Rejection Ratio) is also known as POWER SUPPLIES voltage fluctuation rejection ratio or ripple rejection rate.
It is generally expressed in logarithms and the calculation formula is as follows:

Ripple rejection ratio = 20 × log (ΔVin/ΔVout)

20dB:1/10 40dB:1/100 60dB:1/1000

For example, if the ripple rejection ratio is 40dB, it is 1/100, so if the input ripple is 100mV, the output ripple will be reduced to 1mV.

Figure 1 Ripple rejection ratio

Since the control (amplifier) of a linear regulator has frequency characteristics, the ripple rejection ratio also depends on the frequency.
Figure 2 shows an example of the frequency characteristics of the ripple rejection ratio.
As the frequency increases, the ripple rejection ratio decreases, and then increases at a certain frequency.
This is because the linear regulator control cannot respond above a certain frequency, and the ON resistance of the output driver becomes fixed, resulting in an RC FILTERS between the ON resistance and output capacitance.
When checking the ripple rejection ratio in the specification table of a data sheet, you need to pay attention to the frequency specifications.
The characteristics also change depending on the input/output voltage difference and output current, and the ripple rejection ratio deteriorates when the input/output voltage difference approaches the dropout voltage.

Figure 2 Ripple rejection ratio vs. frequency

Generally, a good ripple rejection rate results in good transient response (less fluctuation in output voltage due to input and load fluctuations).
Furthermore, linear regulators with good ripple rejection rates tend to consume more current, so there is a trade-off between ripple rejection rate (responsiveness) and current consumption.
(There are also linear regulators with circuit innovations that consume less current, have less ripple, and have good response.)