LDO noise is divided into inside LDO noise and outside LDO noise. The inside LDO noise comes from the bandgap reference of the internal circuit, the amplifier, and the transistor. Noise external to the LDO comes from the input. In the manual of the LDO, the PSRR (Power Supply Rejection Ratio / PowerSupply Ripple Rejection) is used to characterize the ability of the LDO to suppress external noise. The total output noise of the LDO is the parameter that characterizes the internal noise suppression of the LDO. Generally, it is expressed in the unit of μVRMS in the electrical characteristics table or on the noise spectral density graph.
Internal noise sources include noise VN (REF) from the bandgap reference, noise VN (AMP) from the error amplifier, noise VN (FET) from the FET, and noise VN (R1) and VN (R2) from the feedback resistor. In most cases, because the bandgap reference circuit is composed of many different resistors, transistors, and capacitors, the noise it generates will be much larger than the noise generated by the feedback resistor. And the bandgap reference is the input of the error amplifier, and the noise it generates will be controlled by the error amplifier to control the FET, so the noise generated by the error amplifier itself and the FET will be lower than the noise of the bandgap reference. It can be said that the largest noise source inside the LDO is the bandgap reference.
