1 What is PSRR

PSRR Power Supply Rejection Ratio, the English name is Power Supply Rejection Ratio, or PSRR for short, which describes the circuit's ability to suppress any power supply changes transmitted to its output signal. It is usually measured in dB and is used to describe the impact of the output signal on the power supply. It is most commonly associated with the use of operational amplifiers (op amps), DC-DC converters, linear regulators, and low dropout regulators (LDO).

For op amps, power supply rejection ratio describes the amplifier's ability to maintain its output voltage when its DC supply voltage changes. Power supply rejection ratio, meanwhile, quantifies the ability to reject ripple voltage from the input source in power conversion applications.

The PSRR of an ideal op amp is zero. However, the PSRR of a real op amp is frequency dependent. The higher the signal frequency, the lower the PSRR. PSRR is often measured based on inputs, but there is no industry standard.

For example, when specified in terms of input, PSRR=10log (ΔVsupply2Av2 )/ΔVout2 ), where Av=voltage gain. The larger the PSRR, the less the output signal is affected by the power supply, so under the same circumstances, it is hoped that the larger the PSRR of the op amp, the better.

In low-noise and precision applications, PSRR is very important because the output of the op amp is a function of the power supply as well as the input value. If the amplifier stage is too sensitive to the signal input through the power supply, unwanted oscillations may occur. Additionally, poor PSRR results in reduced efficiency. Therefore, good PSRR is very important in precision automotive, industrial and medical designs that require minimum power consumption.

2 PSRR in Op Amp

It is known that PSRR is critical in design, so designers should measure its value independently of the data sheet. The data given by the manufacturer is indeed very good, but different design needs have different PSRR requirements. Especially at higher frequencies (as shown below), measuring PSRR often uses balanced signals at +Vcc and -Vee to prevent common-mode effects from affecting the PSRR measurement.

However, balanced signals may not always exist in actual circuit designs, so this will affect PSRR. Additionally, PSRR is related to signal amplitude. In many application circuits, the ripple and noise on the power lines will be much lower than the ripple and noise used for the test data used in the development datasheet. In actual circuit implementation, circuit gain and PCB parasitic parameters also affect PSRR.

Comparison of the PSSR of the op amp in the datasheet (left) and the measured value (right)

3 PSRR in LDO

In LDOs, PSRR is sometimes called supply voltage rejection ratio (SVRR) and is a measure of the LDO's ability to reject input noise at various frequencies (as shown in the figure below).

PSRR can be a critical parameter in audio, radio frequency (RF), as well as medical and other sensor applications. Input ripple can come from a variety of sources, including 50/60Hz interference from the power supply, output ripple from the upstream DC/DC switching converter, or ripple coupling from different circuit blocks on the PCB. PSRR is typically measured over a frequency range of 10Hz to 1MHz or higher. For power conversion applications, PSRR=20 log(ripple input/ripple output).

High PSRR enables the LDO to act as a noise filter to prevent ripple and noise on the input from coupling to the output

LDO PSRR consists of three frequency regions (as shown in the figure below). In the lower frequency region, up to the roll-off frequency of the bandgap filter, PSRR results from the combination of the open-loop gain and the bandgap factor. Above the bandgap filter's roll-off frequency, up to unity gain frequency, PSRR is primarily a result of the LDO's open-loop gain.

Above unity gain frequency, in Region 3 of the figure below, PSRR is dominated by the output capacitance and is less affected by the parasitic effects between Vin and Vout. Using a larger output capacitor with a lower equivalent series resistance can improve the PSRR in Region 3, but may also affect the PSRR at lower frequencies. Although LDOs with higher PSRR can be used in place of larger output capacitors, LDOs with high PSRR ratings tend to require higher supply current and are more susceptible to oscillation.

As mentioned above, PSRR in Region 2 is primarily determined by the gain of the feedback loop, and anything that affects gain will also affect PSRR. For example, higher load currents tend to reduce the LDO's output impedance, thereby reducing gain. Additionally, higher load current moves the output pole to a higher frequency, thereby increasing the bandwidth of the feedback loop. Therefore, the combined effect can reduce PSRR at lower frequencies and increase PSRR at higher frequencies.

4 Cooperative application of LDO and DC-DC converter

In some cases, a combination of LDOs and switch-mode DC-DC converters can provide the best power for noise-sensitive devices. When noise is not a major concern, such as when powering digital circuits, a DC-DC only solution can provide higher efficiency (for the same size and cost).

On the other hand, using only LDOs provides a small, simple, and cost-effective solution, but is less efficient.

Using a high PSRR LDO with a DC-DC converter solves the problem of low efficiency compared to the LDO-only approach and results in lower noise than using a DC-DC converter without an LDO.

In fact, it produces as low noise as an LDO-only solution. For example, in a distributed power architecture with a 12Vdc power bus, a DC-DC converter can be used to step down the bulk distribution voltage to 3.6VDC and feed a high PSRR LDO to provide 3.3Vdc to power noise-sensitive circuit elements.

As shown in the figure below, the ripple of the LDO output is in the range of tens of millivolts, which is much lower than the output ripple of the DC-DC converter.

(DC-DC converter (top) combined with high PSRR LDO (bottom) provides good end-to-end efficiency and low ripple)

Simply put, Power Supply Rejection Ratio (PSRR) is measured in dB and quantifies the ability of a circuit to suppress any power supply changes passed to its output signal. It is mainly used to describe the performance of op amps and LDOs in a variety of applications. Important, including audio, RF, instrumentation and sensors.

In addition, in applications that are particularly sensitive to PSRR, it is best to measure its value under actual operating conditions, because it may differ from the value in the data sheet, thus affecting the performance of the device.

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