In this article, we focus on two key application-oriented parameters of the linear regulator: Power Supply Ripple Rejection (PSRR) and output noise. These parameters have been discussed before (see References). This article provides a practical and straightforward approach that is useful to design engineers of many experience and education levels.
Before moving forward, it is necessary to review of the five main types of linear regulators. These include the standard NPN, NPN pass transistor, PNP pass transistor, P-channel FET, and N-channel FET regulators. All linear regulators consist of three important elements:
- A pass element
- A bandgap voltage reference (bandgap)
- An error amplifier
PSRR is defined as the linear regulator’s ability to eliminate output ripple caused by input variations. High PSRR values are desirable over the frequency range that is critical to the linear regulator, typically 10 Hz to 10 MHz. Mathematically, PSRR is the reverse gain of the output ripple over the input ripple at a particular frequency. For LDOs, specifically, the PSRR is given as:
PSRR = 20log(|AV /AVO|), expressed in dB (Equation 1)
*where AV is the open-loop gain of the feedback loop, and AVO is the gain from VIN to VOUT. when feedback loop is open.
The PSRR of a linear regulator is determined by three factors:
- The PSRR of the bandgap voltage reference, whose output ripple affects the output ripple of the linear regulator up to the roll-off frequency of the bandgap.
- The open-loop gain of the feedback loop (AV), which is roughly equal to the open-loop gain of the error amplifier, up to the 0dB (unity gain) crossover frequency of the linear regulator (Equation 1).
- The open-loop gain from VIN to V<> (AVO), which is determined by the parasitic parameters of the pass element and values of external components (especially the output capacitor) along the VIN to VOUT path, starting from the 0dB crossover frequency (Equation 1).
The PSRR of the bandgap is a critical contributor to the PSRR of the linear regulator up to the bandgap’s roll-off frequency, since any bandgap output ripple is amplified by the error amplifier, which in turn propagates the amplified ripple through the pass element to VOUT
. In practice, bandgap PSRR values for all five types of linear regulators are comparable.
In addition to having a bandgap with high intrinsic PSRR, it is a common practice to “bypass” its output ripple to the ground through a low-pass filter before it reaches the error amplifier input. The low-pass filter is typically constructed with an internal resistor and an external capacitor. Its roll-off frequency should be as low as possible (100Hz typical), which enables it to filter out high frequency output ripple. This means the value of the RC should be high. Typically, the internal resistor has a very high value -- in the hundreds of K range. The external capacitor usually has a relatively low value -- in the nF or even hundreds of pF range. This is because too large a capacitor negatively impacts the start-up response of the linear regulator (Figure 1, using a P-channel FET regulator as example).
Figure 1. P-channel FET regulator with a low-pass filter and output capacitor