4) Power Supply Rejection Ratio (PSRR)
PSRR is an indication of the LDO’s ability to attenuate fluctuations in the output voltage caused by the input voltage, as expressed in Equation 3. While line regulation is only considered at DC, PSRR must be considered over a wide frequency range. Equation 3:
Considered a conventional closed-loop system, the small-signal output voltage, Ṽ out , can be expressed as shown in Equation 4:
Where Ṽ in is the small signal input voltage, G vg is the open-loop transfer function from input to output voltage, k v is the output voltage sensing gain, G c is the compensator’s transfer function, G oc is the open-loop transfer function from the control signal to the output voltage, and
kv x G c x G oc is the closed-loop transfer function, T (s).
As we can see in Equations 3 and 4, it is clear that the PSRR consists of the closed-loop gain, T (s) , and the inverse of the open-loop transfer function from input to output voltage, 1/G vg, as shown in Figure 4. While the closed-loop transfer function dominates at lower frequencies, the open-loop transfer function from input to output voltage dominates at higher frequencies.
Figure 4: PSRR vs. Frequency
This parameter normally refers to the noise on the output voltage generated by the LDO itself, which is an inherent characteristic of the bandgap voltage reference. Equation 4 shows the relation of the reference voltage to the output voltage. Unfortunately, the closed-loop transfer function is not effective at rejecting the noise from the reference voltage to the output voltage. Hence, most low-noise LDOs need an additional filter to prevent noise from entering the