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Home > Technology List > How to show the difference between reality and ideal of LDO regulator

How to show the difference between reality and ideal of LDO regulator by formula

Published time: 2018-06-22

In the process of using a LDO regulator, indeedly there is a difference between noise and residual AC ripple, both realistic and ideal, but how are these differences presented by the formula?The jotrin electronics engineer will explain to you about the knowledges

the process of using a LDO regulator 

Voltage regulators are essential when stable supply voltages come from unstable or variable power supplies. This source can be a gradually discharged battery or a rectified AC voltage. In applications that are sensitive to noise generated by switching regulators or residual AC ripple, linear regulators are used to minimize the overall system error. These applications include RF transceivers, Wi-Fi modules, and Optical image sensors.

Linear regulators can operate with low input and output voltage differences, commonly referred to as Low Drop-Out (LDO) Regulators.. Since the LDO output voltage is not absolutely stable, it will mainly affect the following operating functions:

Their basic feature is to maintain a constant output voltage regardless of the changes in output current, input voltage, thermal drift, or operating life (ageing). These conditions would be ideal, however in the real world the situation is different. Because the LDO output voltage is not absolutely stable,

a)) A rapid change in the load current will cause a change in the output voltage because of the limited speed of the control loop. Sometimes internal regulation loops cannot respond to rapid changes in current (due to time delays), resulting in overshoots or overshoots in the tens of millivolts (mV) range.

Dynamic load response to fast load change 

Example 1: Dynamic load response to fast load change

b) The rapid change of the input voltage (generally caused by the pulsation of the output voltage of the DC-DC converter) cannot be completely filtered out by the control circuit, and the change of the input voltage is partially reflected in the output voltage. This parameter is called power rejection rate (PSRR) and is usually related to frequency. Some manufacturers indicate that PSRR is negative and some are positive. In general, the higher the absolute PSRR value, the smaller the transmission of interference signals from input to output. Typically, the disturbing input voltage is transmitted to the output at a unit voltage of mV and a smaller voltage. Similarly, rapid changes in the input voltage at the output of the LDO are referred to as line transient responses.

 power supply ripple rejection characteristic 

c) The main reason why the semiconductor structure generates its own intrinsic noise is the collision of free atoms with the crystal structure of the substrate. There are techniques that can suppress its own noise, but because it is a physical phenomenon associated with the principle of current conduction in semiconductors, it will never completely eliminate it. The output noise of modern LDOs can reach hundreds of microvolts (uV) or less, but top-level LDOs produce noise at the uV cell level.

noise density spectrum 

d) Other effects include slow changes in the input voltage and its effects on line regulation, slow changes in load current, and their effects on load regulation, thermal coefficient, and long-term stability.

Finally ,Jotrin Electronics test lab measured the LDO device , The 1G high-frequency oscilloscope and the high-precision multi-meter are used to measure the voltage of the output function. The input voltage parts takes the power of the filter and combines the above formula to perform scientific measurement.


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