[MUSIC PLAYING] In today's video, we'll be going over Power Supply Rejection Ratio, otherwise known as PSRR, and why the ability to attenuate volt ripple generated by switch mode power supplies is one of the most touted benefits of low dropout regulators, or LDOs. 

Let's begin. So what exactly is PSRR? PSRR is the degree to which an AC element, usually a voltage generated from a switch mode power supply of a certain frequency, is attenuated from the input to the output of the LDO. In other words, it determines how much noise from the input couples into the output. 

This ratio can be expressed as the following equation. PSRR is equal to 20 multiplied by the log of the input voltage ripple over the output voltage ripple. This equation also tells you that the higher the attenuation, the higher the PSRR value is in decibel. 

Using the following high-level block diagram as an example, the DC to DC converter introduces switching noise, which is conducted via the line. There may also be radiated noise that is being conducted at the input as well. When following the arrows, we see that the LDO will then reject some of the noise that is generated before adding some internal radiated noise. The resultant signal will then be a clean DC signal with minimal ripple. 

We will now go over how to determine PSRR in your application. In this example, we will use Figure 1, which shows a DC to DC converter that is regulating 4.3 volts from a 12-volt rail. For the LDO, TPS717, a high-PSRR LDO, will follow the DC to DC converter by regulating a 3.3-volt rail. The goal of having a high PSRR on an LDO is to get flat DC signal at the output of the TPS717. 

The ripple being generated is 50 millivolts on the 4.3 rail on the output of the DC to DC converter. The PSRR of the LDO will determine the amount of ripple remaining at the output of the TPS717. 

In order to determine the degree of attenuation, you first need to know at which frequency the ripple is occurring. For this example, let's assume 1 megahertz. It is common to find the PSRR value specified on the data sheet under an electrical characteristics table. As you can see, the value for the PSRR at 1 megahertz is 45. It is important to note that these values only apply under these conditions, which means the PSRR values may be different under different conditions. 

Another way to determine the PSRR is to consult the PSRR versus frequency plot that can be found on the data sheet. This particular graph is for the TPS717, with a delta between V-in and V-out of 1 volt. Now, assuming that these conditions match your own, your PSRR value will be 45 decibels. 

Now using Equation 1, you can then solve for your attenuation factor, which in this case would be 178. This means that you can expect the 50 millivolt ripple at the input to be reduced down to 281 microvolts at the output. 

But let's say that you changed the conditions and decided to reduce your V-in to V-out delta to 250 millivolts, in order to regulate more efficiently. We would then have to consult Figure 3. 

You can see that if we hold all other conditions constant, the PSRR at 1 megahertz is reduced to 23 decibels, or an attenuation factor of 14. This is due to the fact that the CMOS pass element is entering the triode, or linear region. Or in other words, as the V-in to V-out delta approaches the dropout voltage, PSRR begins to degrade. 

Now, please bear in mind that dropout voltage is a function of output current, among other factors. Hence, a lower output current decreases the dropout voltage and helps improve PSRR. 

Changing the output capacitor will also have implications as well, as shown in Figure 4. By increasing the output capacitor from 1 microfarad to 10 microfarad, the PSRR at 1 megahertz increases to 42 decibels, despite the V-in to V-out delta remaining at 250 millivolts. 

When we compare the two plots, you can see that the high-frequency hump in the curve has shifted to the left. This is due to the impedance characteristics of the output capacitors. Now, by sizing the output capacitor appropriately, you can tune the attenuation to coincide with a particular switching noise frequency. 

Now we've seen that by adjusting the V-in to V-out delta and the output capacitance, we can improve PSRR for a particular application. These are by no means the only variables affecting PSRR. 

As you can see, the table shown outlines other various factors that come into play. For instance, using the last example, changing the output capacitance will have little to no effect at lower frequencies, but has an impact at higher frequencies, whereas changing the noise reduction capacitor will have no effect on higher frequencies, but will have a major impact on lower frequencies. 

To learn more about PSRR parameters, such as feed-forward capacitors or PCB layout, please refer to our additional resources that refer to other blog posts and application notes that go into more detail regarding the various parameters that affect PSRR. 

So that does it for today's video. Be sure to check out our help at ti.com/ldo for the latest information. Stop by our Training and Support home page to watch the latest LDO Basics videos, or head over to our E2E forum and read what our experts have to say about LDOs. 

We look forward to seeing you in our next LDO Basics videos. Have a great day.