a) Rapid changes in load current cause changes in output voltage due to limited control loop speed. Sometimes the internal regulation loop fails to respond to rapid changes in current (due to time delays), resulting in an undershoot/overshoot that is typically on the order of tens of millivolts (mV).
b) Rapid changes in the input voltage (usually caused by the output voltage ripple of the DC-DC converter) cannot be completely filtered by the control loop, so the changes in the input voltage will be reflected in the output voltage to a certain extent, this parameter is called Power Supply Rejection Ratio (PSRR), and usually a frequency-dependent parameter. Some manufacturers label the PSRR as a negative number and some as a positive number. In general, the higher the absolute value of PSRR, the less transmitted interference signals from input to output. Typically, the disturbed input voltage is transmitted to the output at the unit level of mV or less. Similarly, rapid changes in input voltage (ie, "line transient response") can occur at the LDO output.
c) The semiconductor structure itself generates inherent noise, mainly caused by free atoms colliding with the crystal structure of the base material. Since inherent noise is a physical phenomenon in semiconductors related to the principle of current conduction, it can be suppressed by some techniques, but it is impossible to completely remove it. The output noise of modern LDOs can be in the hundreds of microvolts (uV) or less, but top LDOs produce noise in the microvolt (uV) unit.
d) Other effects include a slow change in input voltage and its effect on line regulation, a slow change in load current and its effect on load regulation, thermal conductivity, and long-term stability.
In the real world, all these effects and their effects must be considered together to achieve a stable and accurate output voltage. Therefore, it is necessary to carefully consider that the above situation may be relevant to a specific application. For example, for camera applications that require the best image quality, the dynamic response of the LDO to load current changes is most important. When the noise value is below 100 uVrms and the PSRR value is normal (above 50 dB), the impact on image quality is negligible.
