Low dropout linear regulators (LDOs) are a simple way to regulate an output voltage that is powered from a higher voltage input. Though it is simple to operate, its self generated noise is most of the times confused with its Power Supply Rejection Ratio (PSRR). Many times the two are combined together and loosely called just “noise”. This is not correct. Noise is generated by the transistors and resistors in the LDO’s internal circuitry and by the external components. The type of noise may include thermal, flicker and shot noise. PSRR is a measure of circuit’s power supply rejection expressed as a ratio of output noise to noise at the power supply input. It provides a measure of how well a circuit rejects ripple at various frequencies injected from its input power supply. In the case of a LDO, it is a measure of the output ripple compared to the input ripple over a wide frequency range and is expressed in decibels (dB). The basic equation for PSRR is given in Equation 1:

Equation 1.

Figure 1-1 explains how noise and PSRR are different from each other. The noise is something which may be internal and external to the LDO whereas PSRR is an internal parameter of the LDO. LDO users generally concentrate on PSRR and not on the self-generated output noise. PSRR rejects noise coming from outside of LDO but there is always noise generated inside the LDO. So an LDO with high PSRR may not be better for noise rejection. The user should always think of both parameters.

Figure 1-1 PSRR and Noise in LDO

Noise is purely physical phenomenon that occurs with transistors and resistors. Transistors generate shot noise and flicker noise. The resistive element of MOSFETs also generates thermal noise like resistors. Thermal and shot noise is truly random in nature and its power is flat over frequency. It remains flat up to the bandwidth of the amplifier. Flicker noise is the noise due to trapped charges at the gate of the MOSFET. It follows Poisson’s Distribution with 1/f roll-off in power versus frequency hence it is higher at low frequencies. This noise dominates until it becomes smaller than thermal noise. (See Figure 2-1)

Figure 2-1 LDO Noise (types)