Power Supply Rejection Ratio in LDO Regulators

Power Supply Rejection Ratio in LDO Regulators

ersaelectronics.com

Power supply rejection ratio, usually shortened to PSRR, describes how well a linear regulator prevents noise and ripple on its input from appearing on its output. It is one of the most important specifications for any noise-sensitive supply rail, yet it is also one of the most misread on a datasheet. A part that looks excellent on the bench with a clean input can perform poorly in a real system where the input carries ripple from a switching converter. This article explains what PSRR means, why it changes with frequency, how to read the published curves, and what a designer can do to improve real rejection.

1. What PSRR Actually Means

PSRR is the ratio, usually expressed in decibels, of a ripple signal injected at the input to the ripple that survives at the output. A PSRR of 60 dB means that an input ripple of 1 V appears as only about 1 mV at the output. Mathematically it is the inverse of the regulator's input-to-output transfer function, so high PSRR is good. PSRR is defined at a particular frequency and, usually, a particular load current and headroom. It is not a single number; it is a curve that rolls off as frequency rises, and that curve is the real specification a designer should study.

1. Why PSRR Falls With Frequency

At DC and low frequency, the control loop compares the output against the reference and corrects any error aggressively, so a disturbance on the input is rejected strongly, typically on the order of 60 to 80 dB or more. As frequency increases, the open-loop gain of the error amplifier falls, and the loop has less ability to correct. Once the disturbance rises above the loop bandwidth, the loop can no longer respond, and the pass element begins to pass the input ripple directly. The result is a PSRR curve that is flat at low frequency and then rolls off, often at 20 dB per decade, until at high frequency the rejection is determined only by the output capacitor and by parasitic coupling. This is why an LDO that rejects 100 kHz ripple well may do little for a 5 MHz spike.

1. How to Read the PSRR Curve

Datasheets plot PSRR against frequency on log-log axes. The designer should look at three features. First, the low-frequency plateau shows the best achievable rejection. Second, the corner frequency, where the curve starts to drop, indicates the loop bandwidth; a higher corner means useful rejection extends to higher frequencies. Third, the high-frequency tail shows how much rejection remains above the loop bandwidth, which is set by output capacitance and by internal feedthrough. Always check the load current and headroom conditions on the curve, because PSRR at 1 mA is better than at the full rated load, and PSRR near dropout collapses. A single PSRR number quoted at 120 Hz or 1 kHz is often misleading for a supply that feeds a switching converter.

1. The Role of the Output Capacitor

Above the loop bandwidth, the output capacitor becomes the primary means of keeping ripple off the rail. Its impedance at the disturbance frequency directly shunts noise to ground, so a larger capacitor, and one with low equivalent series inductance, improves high-frequency PSRR even though the regulator loop itself can no longer help. This is why choosing the right output capacitor is as important as choosing the right LDO when the input carries high-frequency ripple. Ceramic capacitors are usually preferred for their low inductance, placed close to the output pin.

1. Practical Ways to Improve PSRR

Several techniques are available when the published curve is not enough:

• Choose an LDO whose PSRR corner frequency is above the dominant disturbance. A part designed for RF or audio use often maintains rejection to several hundreds of kilohertz.
• Reduce the disturbance at the source. Adding a small LC filter or increasing the input capacitance before the LDO lowers the ripple the LDO has to reject.
• Keep adequate headroom. Operating near dropout destroys PSRR, so leave at least a few hundred millivolts of margin.
• Use the noise-reduction or bypass pin, where present, to lower reference noise and improve rejection in the audio band.
• Stage regulation. A switching converter feeds a pre-filter rail, which then feeds the LDO, splitting the rejection burden.
• Increase output capacitance at frequencies above the loop bandwidth, where the loop is helpless.

1. When PSRR Limits the Design

If the input ripple lies above the loop bandwidth and the output noise requirement is strict, no amount of PSRR will be enough by itself. In that case the correct answer is to reduce the ripple upstream, perhaps by changing the switching frequency or the input filter, rather than expecting the LDO to perform beyond its bandwidth. PSRR is a transfer function, and a designer should treat it as such: multiply the expected input ripple by the rejection at that frequency, and confirm the residual output noise is below the circuit's noise budget.

Summary

• PSRR measures how much input ripple the LDO removes, expressed in decibels as input amplitude divided by residual output amplitude.
• It is high at low frequency where the control loop works, and rolls off above the loop bandwidth.
• The datasheet curve, read at the right load and headroom, matters more than any single quoted number.
• Above the loop bandwidth, output capacitance and upstream filtering do the work, not the regulator itself.
• Real designs improve PSRR by selecting a suitable part, keeping headroom, filtering the input, and staging regulation.