By OnSemi & signal integrity
Translated by Hualink RF Sunny Li
Low voltage drop (LDO) regulator
Voltage regulators are essential for applications that want to obtain a stable supply voltage from an unstable or variable supply.
This type of power supply includes a gradually discharging battery or ac voltage after rectification.
Applications that are sensitive to noise or residual AC ripples 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 maintains a low voltage difference between the input and output of a power supply is often called a low voltage drop (LDO) regulator.
Its basic feature is to maintain a constant output voltage regardless of how the output current, input voltage, thermal drift or service life (aging) changes.
These are ideal conditions, but things are a little different in the real world.
Since the LDO output voltage is not absolutely stable, it mainly affects the following operation functions:
1. Due to the limited control loop speed, rapid changes in the load current will result in changes in the output voltage.
Sometimes the internal regulating circuit fails to respond to rapid changes in current (due to time delays), resulting in downshoots/overshoots that are typically in the tens of millivolts (mV) range.
2. Rapid changes in the input voltage (usually caused by the output voltage ripple of the DC-DC converter) cannot be fully filtered through the control loop, so changes in the input voltage are partly reflected in the output voltage. This parameter is called the power rejection ratio (PSRR) and is usually a frequency variable parameter.
Some manufacturers label PSRR as negative and some as positive.
In general, the higher the absolute value of PSRR, the less interference signals are transmitted from input to output.
Typically, the disturbed input voltage is transmitted to the output in units of mV or less.
Similarly, rapid changes in input voltage (the "line transient response") can occur at the LDO output.
3. Semiconductor structures generate inherent noise, mainly caused by collisions between free atoms and the crystal structure of the underlying material.
Because inherent noise is a physical phenomenon related to the principle of current conduction in semiconductor, it can be suppressed by some techniques, but it is impossible to completely remove it.
Modern Ldos produce output noise in the hundreds of microvolts (uV) or less, but the top ldos produce noise in microvolts (uV) units.
4. Other effects include a slow change in input voltage and its effect on line adjustment, a slow change in load current and its effect on load adjustment, thermal conductivity, and long-term stability.
In the real world, all these effects and their effects must be considered together to achieve stability and accuracy of the output voltage.
Therefore, it is necessary to carefully consider how the above situation may relate to a particular application.
For example, the dynamic response of LDO to load current changes is most important for camera applications that require optimal image quality.
When the noise value is lower than 100 uVrms and the PSRR value is at the normal level (higher than 50 dB), the effect on image quality is insignificant.
What is LDO noise?
Generally speaking, noise can be divided into two types, internal noise and external noise.
Among them, internal noise is inevitable, every electronic equipment will produce internal noise.
The LDO is powered by an ideal source, which means that it is unaffected by external influences and therefore has no external noise at the input (although THE LDO does have internal noise at the output).
External noise is a variety of noise generated by external influences (ripples at the input - the actual source), and the input ripple is related to the power rejection ratio (PSRR).
In addition, there are different types of noise such as heat and flicker.
Specifically, thermal noise is caused by the random thermal motion of particles, called diffusion, which exists without external voltage connections.
In contrast, scintillation noise is caused by a random change in the particle current, a motion called drift.
The drift is caused by the external voltage, which means that without the external voltage there can be no flicker noise.
In addition, noise can also be divided by spectrum.
We can use color to identify specific noise spectrum, such as white, pink, brown, gray, etc.
The first type of noise identified by color is white noise, which is smooth over the entire spectrum.
So how do we measure noise?
As mentioned earlier, internal noise is the noise produced by an LDO with an ideal source at the input.
In practical measurements, this ideal source may be the battery, which has lower internal noise than the LDO regulator.
This noise is frequency dependent and is represented by a parameter such as a spectral noise density curve or an integral noise value (the output noise voltage is expressed as microvolt - uVRMS over a specific frequency range, typically 10 Hz to 100 kHz).
What is spectral noise density?
If you measure the noise in time, you can only see the absolute amplitude value of the noise, but not the frequency of all the noise properties.
As the picture above shows, the noise is in the x10 range.
The spectral noise density curve is the sum of the noises generated, each of which is measured over a narrow frequency range, as you can see in the picture below.
Noise at the LDO output is also load dependent.
Load consumption current Iout, which is equal to resistance RLOAD.
Related to the load are multiple Rloads and output capacitance Couts.
A higher RLOAD value or a higher COUT value means that the portion of the curve associated with the load moves toward a lower frequency.
In the picture above, you can see that it is related to IOUT;
In the image below, you can see the association with COUT.
