Hi, I am trying to find a good-quality linear regulator. I am not sure how they define PSRR(power-supply rejection ratio). The PSRR related to ripple is important parameter. **TPS72325DBVR is 48dB at 10kHz, and ADP7182ACPZN-2.5R7 is 66dB at 10kHz. Which is good with noiseless?
PSRR describes the ratio between an AC ripple component at the regulator’s input and the portion of that signal that appears at the output. Larger values indicate a greater degree of noise suppression, though the test frequency is also important since not all regulators perform equally well as the input ripple frequency increases.
It can be difficult to compare the PSRR specs between different regulators, as you not only have to look at the PSRR numbers themselves, but also the test conditions under which those specifications are measured.
One must consider not only the test frequencies, but also the input and output voltages used (both absolute value and difference between the two), the input and output capacitor values, and the test load current. All of these can affect the PSRR values measured. So, when trying to compare different parts, try to compare them at the conditions you are most likely to be using them in your intended circuit.
The following are general guidelines for how various conditions affect measured PSRR values, although there may be exceptions for individual regulators to these rules.
How PSRR measurements affected by test conditions:
- Greater test frequency → Reduces PSRR
- Greater (Vin - Vout) differential → Increases PSRR
- Greater Vout → Increases PSRR
- Greater Cin → Increases PSRR
- Greater Cout → Increases PSRR
- Greater Iout → Reduces PSRR
In most cases above, the rate of change in PSRR is most pronounced near the lower threshold of the respective variable. So, a small increase in the variable will have a greater effect on PSRR near the lower end of the range than that same increase would have on the higher end.
You can gain a little more insight by looking at the relevant graphs presented in the respective datasheets (page 10 for the TPS72325DBVR and pages 16-18 for the ADP7182ACPZN-2.5R7). Take note of the conditions for each graph and also that in the TPS72325DBVR graph, they use a positive scale where a more positive value means greater suppression, whereas in the ADP7182ACPZN-2.5R7 graph, they show PSRR on a negative scale, where more negative means greater suppression.
TPS72325DBVR PSSR graph:
ADP7182ACPZN-2.5R7 PSRR graph:
Thanks for your reply.
Thanks a lot.