Let’s talk about some principles and indexes of switching power supply and LDO power supply. The purpose is to analyze their advantages and disadvantages, so as to find out how to choose and use better in PCB design. Originally, I wanted to cut in from the selection of the types of DC power supply. After consulting a lot of data, I found that the classification of DC power supply is not very clear. According to the type, according to the circuit structure, and according to the topology, it is not the same. Some divide it into linear type, switching type, silicon controlled rectifier type and induction type; some divide it into chemical power supply, linear regulated power supply and switching regulated power supply; some simply divide it into two types, linear type and switching type.

Back to the PCB that we are familiar with, the big classification is relatively clear. There are mainly linear power supply and switching power supply. The linear power supply mainly uses LDO power supply, and the switching power supply is what we usually call DC-DC power supply. In fact, strictly speaking, linear power supply can not be equated with LDO power supply. LDO power supply is only one of the linear power supply, but it is named for its relatively low regulating tube pressure difference.

The previous article has mentioned some principles of switching power supply, so before talking about their differences, I think we should supplement the principle of LDO, and then we can make the following comparison.

LDO (low dropout regulator) is a low dropout linear regulator in Chinese. Its general internal structure is as follows:

The components used are also relatively simple: a series regulator VT, two divider resistors R1 and R2, amplifier A, and reference voltage Ref. then the input and output can be connected. The in-phase input voltage of the amplifier obtained from R1 and R2 is the sampling voltage, and the reverse input voltage of the amplifier is a reference voltage. The output of the amplifier is used to drive the regulator, The input and output of the regulator are connected to the input and output voltage.

Then, how to stabilize the voltage? We describe its working principle as follows:

When the output voltage uout decreases, the sampling voltage divided by R1 and R2 (i.e. the in-phase input voltage of the amplifier) decreases, so the output drive current of the amplifier increases, which leads to the decrease of the voltage drop of the series regulator, i.e. the UIN uout decreases, and finally the uout voltage increases. The same is true when the output voltage uout rises.

If you think the principle description of switching power supply and LDO is too complicated, I specially draw the following model for analogy (please forgive me for not looking good).

We compare the input voltage to a large faucet. Our purpose is to connect a small water flow (output voltage) from the large faucet. We have the following two ways to complete it.

The way on the left is that we add a water valve, open the water valve to the position we need, fix the position, and then let the faucet flow out of the water we need; or we can open the brain cavity a little larger, and use the method on the right, we can get continuous water flow by continuously opening and closing the faucet head, provided that the speed of opening and closing is the same It should be faster, otherwise the water flow will be cut off, and then the water flow can be controlled by the ratio of on-off time to off time. This is actually the main difference between LDO and DC-DC power supply.

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