Power Supply Rejection, often abbreviated as PSR, refers to the ability of an electronic device or circuit to reject or attenuate unwanted variations or fluctuations in the power supply voltage. It is a measure of how well the device or circuit maintains its performance despite changes in the input power.
Electronic devices require a stable and clean power supply for proper operation. However, power supplies can introduce noise, voltage fluctuations, and other irregularities due to factors such as AC mains variations, switching noise, load changes, or external interference. These disturbances can adversely affect the performance of the device or circuit, leading to issues like distortion, reduced signal quality, increased noise, or improper functionality.
Power Supply Rejection is a measure of how effectively the device or circuit rejects these power supply variations. It is usually expressed in decibels (dB) and represents the ratio between the amplitude of the unwanted power supply variation and the resulting change in the device's output or performance.
Power Supply Rejection for LDO
Power supply rejection ratio (PSRR), also known as ripple rejection, measures the LDO
regulator’s ability to prevent the regulated output voltage fluctuating caused by input voltage
variations. The same relation for line regulation applies to PSRR except that the whole
frequency spectrum is considered
The ripple rejection is defined by
For example, supply rejection in the frequency band between 100 kHz and 1 MHz is especially
important in applications where the output of a dc/dc switch mode power supply (SMPS) is used
to power the linear regulator. The output ripple of the SMPS is typically in the aforementioned
frequency span. Thus, the figure above does not seem to show a good PSRR performance for
the SMPS applications over the frequency range (100 kHz to 1 MHz). The worst performance
(maximum point in the graph) occurs when RESR is large and Cb is low.
The control loop tends to be the dominant contributor of supply rejection. Low ESR value, a
large output capacitor, and added bypass capacitors improve the PSRR performance, provided
they meet the CSR requirement.
A high Power Supply Rejection Ratio (PSRR) indicates that the device or circuit is effective in rejecting power supply variations and maintaining its desired performance. Conversely, a low PSRR means that the device is more sensitive to changes in the power supply and may be more prone to performance degradation.
PSRR is an important specification for various electronic systems, including audio amplifiers, voltage regulators, analog-to-digital converters (ADCs), and sensitive analog circuits. Designers strive to achieve high PSRR to ensure stable and accurate operation of these systems, even in the presence of power supply disturbances.