A linear regulator that can maintain a low voltage difference between the input and output of the power supply is often called a low voltage drop (LDO) regulator. Its basic feature is that it can maintain a constant output voltage regardless of how the output current, input voltage, thermal drift or working life (aging) changes. These are ideal conditions, but things are a little different in the real world.

Voltage regulators are critical in applications that want to obtain a stable supply voltage from unstable or variable power supplies. This type of power supply includes gradually discharging batteries or rectified AC voltages. Applications that are sensitive to noise or residual AC ripple from switching regulators, including RF transceivers, Wi-Fi modules and optical image sensors, use linear regulators to minimize errors and errors throughout the system.

A linear regulator that can maintain a low voltage difference between the input and output of the power supply is often called a low voltage drop (LDO) regulator. Its basic feature is that it can maintain a constant output voltage regardless of how the output current, input voltage, thermal drift or working life (aging) changes. These are ideal conditions, but things are a little different in the real world. Since the LDO output voltage is not stable, it mainly affects the following operating functions:

1, due to the limited speed of the control loop, the rapid change of the load current will lead to a change in the output voltage. Sometimes the internal regulating circuit is unable to respond to rapid changes in current (due to time delays), resulting in a downshoot/overshoot, usually around tens of millivolts (mV).

2, the rapid change in the input voltage (usually caused by the output voltage ripple of the DC-DC converter) can not be completely filtered through the control loop, so the change in the input voltage will be reflected in the output voltage to a certain extent, the parameter is called the power supply rejection ratio (PSRR), and is usually a frequency variable parameter. Some manufacturers label the PSRR as negative, while others are positive. In general, the higher the PSRR value, the less interference signals are transmitted 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 the input voltage (i.e., "line transient response") can occur at the LDO output.

3, the semiconductor structure itself will produce inherent noise, mainly caused by the collision of free atoms with the crystal structure of the underlying material. Since inherent noise is a physical phenomenon related to the principle of current conduction in semiconductors, it can be suppressed by some techniques, but it is impossible to completely remove it. The output noise of modern Ldos can reach hundreds of microvolts (uV) or less, but the noise produced by Ldos can reach microvolts (uV) units.

4, other effects include a slow change in the input voltage and its effect on the line adjustment rate, a slow change in the load current and its effect on the load adjustment rate, thermal conductivity and long-term stability.

In the real world, all these effects and their effects must be considered in order to achieve the stability of the output voltage. Therefore, it is necessary to carefully consider that the above situation may be relevant to a particular application. For example, for camera applications requiring good image quality, the dynamic response of LDO to load current changes is particularly important. When the noise value is below 100 uVrms and the PSRR value is the normal level (above 50 dB), the impact on image quality is negligible.