Power Supply Ripple Rejection and Linear Regulators: What's all the noise about?

Linear regulators, among other things, are tasked with shielding sensitive analog blocks from noise in the supplies, making wide-spectrum power supply ripple rejection (PSRR) a vital specification parameter. In broad terms, the function of the regulator is to generate an independent supply voltage through a series-shunt feedback network. Figure 1 illustrates the basic architecture of a low dropout (LDO) linear regulator whose added function is to sustain low input-output voltage differentials, which is critical, for instance, in low voltage, battery-supplied applications. Shunt feedback is accomplished by sensing the output voltage through the R₁-R₂ feedback network, comparing it against a stable voltage reference, and generating a corresponding error-control signal to a pass device via an error amplifier. In essence, the feedback control signal will do whatever it takes to ensure the output voltage is close to the reference, and how close is determined by the loop gain (A_olβ) of the circuit.

Figure 1. Block diagram of a typical low dropout (LDO) linear regulator

Catching the wave of integration and the demand for point-of-load (POL) regulation, many load-specific LDOs are now being deployed on-chip with output currents in the range of 10 to 50 mA [7-9]. Many of these LDO circuits, unfortunately, do not enjoy the luxury of off-chip capacitors because of the pin- and board-limited nature of the densely integrated systems they serve. Historically, however, designers have relied on these large external passives to shunt away output noise. Today, IC designers are confronted with more noise and less capacitance with which to fight it, with on-chip capacitors ranging only up to 200 pF.