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What is PSRR? - Part III

by  Petr Novobilsky  - 10-25-2018 

Previously in our blog, “What is Integral Noise? Part II” we discussed integral noise and what it means. Today, we will focus on Low Dropout Regulator parameters and features surrounding Power Supply Rejection Ratio (PSRR) and how it is impacted by the application´s condition.

PSRR describes a LDOs’ ability to block ripple voltage from an input source and can be expressed by the following formula.

An example of a PSRR curve can be seen in Figure 1. We can split this chart into two areas. The first region is marked as LDO active area and covers the frequency range where the LDO works as an active ripple blocker. This means that the control loop is able to compensate the input ripple through a pass device and keep a stable output voltage. In fact, the shape is almost the same as an operational amplifier´s gain characteristic. It is linear up to the point where the control loop is not able to keep gain at a desired level. In an ideal world, and without any output capacitor, the gain reduces until it is equal to one. This point is called the transition frequency. In the real world, an LDO needs some output capacitor to be stable. Its impedance together with the parasitic impedance form an filter which helps to improve the high-frequency PSRR characteristic.

Figure 1. Simplified PSRR Chart Frequency Region

In the second region (COUT +PCB parasitic area) the LDO does not suppress the input voltage ripple by a control loop but contributes only by the output stage impedance.

The LDOs PSRR performance is not only impacted by the regulation loop performance but also the performance of a few key internal control circuits. Voltage ripple from the power supply passes through various internal blocks and affects output performance. Figure 2 shows the basic LDO block diagram and possible ways where input voltage ripple can impact output voltage.

Figure 2. Simplified LDO Block Diagram

The first important part is the internal voltage reference block. It creates a stable and clean reference voltage for the error amplifier and other LDOs blocks. When any ripple voltage passes through the reference block to its output then the error amplifier copies this undesired voltage ripple to the LDO output. This is unwanted behavior therefore the reference voltage block should be as clean as possible to achieve good PSRR.

The second sensitive path is the error amplifier power supply. Regardless of reference voltage stability, without a clean supply voltage for the error amplifier the results will not be accurate. Coupled voltage ripple can impact gain stability and frequency compensation in the amplifier and lead to output voltage disturbances and decreasing PSRR.

The third path is coupling through the pass device to the output. The ripple is reduced through proper compensated of the regulator. This is the major contributor to output voltage ripple and a well-designed LDO should be able to suppress this in low and mid-frequency regions.  

The PSRR performance of an LDO is affected also by external applications conditions. The most important factors are load current, output capacitor and voltage headroom. Let´s take closer look on each of them.

Figure 3 illustrates the impact of regulator load current. We can see that higher currents have worse PSRR in high-frequency ranges.

Figure 3. Output Current vs. PSRR – NCP163

Figure 4 shows how output capacitor selection affects PSRR. We can see that higher capacitance improves PSRR significantly in the high-frequency region. It confirms our previous theory that output impedance and PCB parasitic impedance forms an LC filter to maintain high PSRR. It is useful for tuning the PSRR when an LDO is used as a post DC-DC regulator. An experienced engineer can shift the PSRR peak to exactly match the converter switching frequency. Maximum allowed COUT value should be maintained.

Figure 4. Output Capacitor Value vs. PSRR – NCP163

In Figure 5, we can see the often overlooked parameter of voltage headroom and its impact on PSRR.   Voltage headroom is the voltage difference between VIN-VOUT and not to be confused for LDO dropout voltage. In the example below the NCP163 offers a very low dropout voltage therefore a very small voltage headroom can be used to achieve quite good PSRR performance. We can see that a 100 mV voltage headroom is enough for reliable function but each additional millivolt increase improves PSRR significantly.  Eventually there is diminishing returns and there is no need to use a difference higher than 300 mV.

Figure 5. Voltage Headroom vs. PSRR – NCP163

Understanding PSRR and how input voltage ripple can get coupled into the LDO structures and impact its performance are important when it comes to LDO performance. Stay tuned for our next blog where we will discuss what PSRR values mean in real world applications. In the next blog we will show what PSRR values mean in real world applications. Stay tuned for some nice scope pictures and demonstration of good design practices. All measurements and charts were taken on our Ultra-High PSRR NCP163.

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